Ixobrychus exilis - (Gmelin, 1789)
Least Bittern
Other English Common Names: least bittern
Other Common Names: Socoí-Vermelho
Taxonomic Status: Accepted
Related ITIS Name(s): Ixobrychus exilis (J. F. Gmelin, 1789) (TSN 174846)
French Common Names: Petit Blongios
Spanish Common Names: Avetoro Mínimo
Unique Identifier: ELEMENT_GLOBAL.2.106202
Element Code: ABNGA02010
Informal Taxonomy: Animals, Vertebrates - Birds - Wading Birds
Image 10521

© Bruce A. Sorrie

Kingdom Phylum Class Order Family Genus
Animalia Craniata Aves Pelecaniformes Ardeidae Ixobrychus
Genus Size: C - Small genus (6-20 species)
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Concept Reference
Concept Reference: American Ornithologists' Union (AOU). 1998. Check-list of North American birds. Seventh edition. American Ornithologists' Union, Washington, D.C. [as modified by subsequent supplements and corrections published in The Auk]. Also available online: http://www.aou.org/.
Concept Reference Code: B98AOU01NAUS
Name Used in Concept Reference: Ixobrychus exilis
Taxonomic Comments: Constitutes a superspecies with I. sinensis and I. minutus (including I. novaezelandiae) (AOU 1998). The New Zealand subspecies, novaezelandiae, now considered extinct, has been treated as a separate species (Marchant and Higgins 1990) (AOU 2010). Although some debate exists, studies indicate that there are no morphological differences between the eastern and western subspecies, thus hesperis is not recognized here as a valid subspecies.
Conservation Status

NatureServe Status

Global Status: G4G5
Global Status Last Reviewed: 29Dec2017
Global Status Last Changed: 29Dec2017
Ranking Methodology Used: Ranked by calculator
Rounded Global Status: G4 - Apparently Secure
Reasons: Has probably declined over the past century, primarily due to extensive marsh habitat loss; however, still widespread over very large range (southeastern Canada to South America) and common in many areas.
Nation: United States
National Status: N5B,N5N (19Mar1997)
Nation: Canada
National Status: N4B,N3M

U.S. & Canada State/Province Status
Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
United States Alabama (S2N,S4B), Arizona (S3), Arkansas (S2B,S2N), California (S2), Colorado (S2B), Connecticut (S2B), Delaware (S1B), District of Columbia (S1B,S2N), Florida (S4), Georgia (S3), Illinois (S2), Indiana (S3B), Iowa (S3B,S2N), Kansas (S2B), Kentucky (S1S2B), Louisiana (S5B), Maine (S2B), Maryland (S2S3B), Massachusetts (S1S2B), Michigan (S3), Minnesota (SNRB), Mississippi (S2B), Missouri (S3), Nebraska (S4), Nevada (S2B), New Hampshire (S1B), New Jersey (S3B,S3N), New Mexico (S3B,S3N), New York (S3B,S1N), North Carolina (S2S3B), North Dakota (SNRB), Ohio (S2), Oklahoma (S4?), Oregon (S1B), Pennsylvania (S1B), Rhode Island (S2B,S2N), South Carolina (SNRB,SNRN), South Dakota (S2B), Tennessee (S2B), Texas (S4B), Utah (SHB), Vermont (S2B), Virginia (S3B,S3N), West Virginia (S1B), Wisconsin (S2S3B)
Canada Manitoba (S2S3B), New Brunswick (S1S2B,S1S2M), Ontario (S4B), Quebec (S2S3)

Other Statuses

Canadian Species at Risk Act (SARA) Schedule 1/Annexe 1 Status: T (05Jun2003)
Committee on the Status of Endangered Wildlife in Canada (COSEWIC): Threatened (26Apr2009)
Comments on COSEWIC: Reason for designation: This diminutive member of the heron family has a preference for nesting near pools of open water in relatively large marshes that are dominated by cattail and other robust emergent plants. Its breeding range extends from southeastern Canada through much of the eastern U.S. Information on the population size and exact distribution of this secretive species is somewhat limited. Nevertheless, the best available evidence indicates that the population is small (about 3000 individuals) and declining (> 30% in the last 10 years), largely owing to the loss and degradation of high-quality marsh habitats across its range.

Status history: Designated Special Concern in April 1988. Status re-examined and confirmed in April 1999. Status re-examined and designated Threatened in November 2001 and in April 2009.

IUCN Red List Category: LC - Least concern

NatureServe Global Conservation Status Factors

Range Extent: >2,500,000 square km (greater than 1,000,000 square miles)
Range Extent Comments: BREEDING: in western North America in southern Oregon, interior and southern coastal California, central Baja California, and southern coastal Sonora; in eastern North America from southern Manitoba, north-central U.S., southeastern Canada (Ontario and Quebec), eastern Maine, and southern New Brunswick (few records) south to western and southern Texas, Gulf Coast, Florida, and Greater Antilles; west to central Montana, Utah (Great Salt Lake, formerly), eastern Colorado, and south-central New Mexico; Middle America in Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica, Panama, and elsewhere, especially Mexico; widely in South America in central Colombia, coastal Peru, and east of Andes from Venezuela and Guianas south to northern Argentina and southern Brazil (AOU 1983, Gibbs et al. 1992). NON-BREEDING: southern California, southern Texas, and northern Florida south through the breeding range in South America (AOU 1983). Largest numbers in southern Florida, Rio Grande valley, lower Colorado River between California and Arizona, Baja California, Greater Antilles, and Central America (Root 1988, Gibbs et al. 1992).

Number of Occurrences: 81 to >300
Number of Occurrences Comments: Because least bitterns are secretive their population status and trends are not well known (Sterling 2008). See Gibbs and Melvin (1992) for information on status in certain states in the northeastern U.S. See Hands et al. (1989) and Brewer et al. (1991) for information on status in the north-central U.S. In Quebec breeding bird atlas surveys (1984-1989), found in 40 atlas squares, but breeding confirmed in only 10 of these (Fragnier 1996). In Ontario breeding bird atlas surveys (1981-1985), found in 223 squares, but breeding was confirmed in only 46 (Sandilands and Campbell 1988, James 1999). See Sterling (2008) for status in California; remaining core population centers in California occur in the Sacramento Valley, the Salton Sink, and the lower Colorado River valley.

Population Size: 10,000 to >1,000,000 individuals
Population Size Comments: Across the North American range, breeding density estimates range from 0.4 to 15.0 territorial males or nests per ha (Gibbs et al. 1992). In Canada, Ontario population may be on the order of 1000 pairs, with all other provinces combined supporting probably fewer than 100 pairs (Sandilands and Campbell 1988).

Overall Threat Impact Comments: The primary threat to this species is the loss and degradation of its freshwater marsh habitat (Gibbs et al. 1992). Populations are jeopardized by draining, filling, and degradation of marshes and by environmental contaminants and unnaturally high densities of predators such as raccoons (Evers 1992). Climate change is a growing threat with increased frequency and intensity of droughts threatening availability of marsh habitat, and altering water levels and timing of food availability.

May be negatively impacted by high water levels in the Great Lakes (Sandilands and Campbell 1988). Palmer (1962) stated that "...unquestionably, parts of the range have been affected adversely in recent years by marsh drainage, pollution, spraying of insecticides, and other activities of man." Loss of wetland habitat is likely the greatest threat in the northeastern U.S. Palustrine emergent wetlands, including inland, freshwater and brackish wetlands, are among the most threatened wildlife habitats in the U.S., and over 4.75 million acres (1.92 million ha) were lost between the mid-1950s and mid- 1970s (Tiner 1984). Wetland losses in the northeastern states are primarily caused by draining, dredging, filling, pollution, acid rain, agricultural practices, siltation, and urbanization (Jorde et al. 1989). Pollution and environmental contaminants may impair reproductive capacity and predispose birds to disease in industrialized and agricultural portions of their range. Relatively high concentrations of dieldrin have been detected in their eggs in Louisiana (Causey and Graves 1969). Organochlorines, heavy metals, and PCBs have been found in many other species of herons, and some contaminants (DDE, dieldrin) have persisted in tissues of herons long after their use was banned in the early 1970s (Fleming et al. 1983). Although acid rain could potentially reduce food supplies, least bitterns usually occupy wetlands of circumneutral pH with dense growths of emergent vegetation that may provide chemical buffering against acidification. Siltation resulting from erosion of farmlands and run-off containing insecticides may degrade nesting habitats and reduce food supplies in agricultural areas. Threats to birds nesting in Atlantic coast tidal marshes include high mercury levels and DDT residues in marsh soils (Meanley 1985). Marshland invasion by purple loosestrife (Lythrium salicaria) and phragmites may alter and degrade habitats. Vulnerable to various generalist mammalian, avian, and reptilian predators. Sources of mortality of chicks and adults include predation by raptors, crows (Corvus spp.), raccoons (Procyon lotor), mink (Mustela vison), snakes, snapping turtles (Chelydra serpentina), and bullfrogs (Rana Catesbeina) (Bent 1926, Trautman 1940, Weller 1961, Hancock and Kushlan 1984). Little is known about the effects of disease and parasites on reproduction. Friend (1987) reported that wading birds were susceptible to the following diseases: oil toxicosis (moderate), type c botulism (occasional), avian cholera (infrequent), chlamydial infections (frequent), sarcocystis (rare), and aspergillosis (infrequent). Known to host trematode worms (Font et al. 1984), two species of lice, and one species of mite (Peters 1936). Populations were thought to be reduced by an unknown disease during the nesting season at an Iowa wetland (Kent 1951). A nematode parasite, Eustrongilides, contracted from small fish, can devastate wading bird populations and thrives in waters polluted with nutrients and silt; populations at wetlands and impoundments receiving stormwater and run-off from residential and agricultural areas may be particularly vulnerable to epizootics of Eustrongylidosis (P. Frederick, pers. comm.). Seems relatively resistant to human disturbance and may persist in highly urbanized areas if wetlands remain relatively undisturbed (Bent 1926, Palmer 1962). Because they fly low to the ground, collisions with motor vehicles, barbed wire fences, and transmission lines can be a mortality factor (Forbush 1925, Guillory 1973). Collisions with airboats also kill nesting and overwintering least bitterns in the Florida Everglades; 3% of 607 least bitterns flushed by airboats were struck (Frederick et al. 1990). COMPETITION: Although both the American (Botaurus lentiginosus) and Least Bittern are diurnal and often breed at the same wetland, competition may be minimal between the two species due to differences in microhabitat used for foraging (Gibbs and Melvin 1990, Reid 1989), prey preferences (Howell 1932, Cottam and Uhler 1945), nest-site requirements (Harrison 1978), and breeding phenology (Bent 1926). Generally speaking, least bitterns prefer more densely vegetated, deeper-water habitats than do American bitterns for foraging and nesting, and take smaller prey. Also, least bitterns arrive at nesting areas about a month later than American bitterns, perhaps because least bitterns overwinter in more southerly regions (see Palmer 1962). Because least bitterns are highly insectivorous, delayed breeding also may be related to the life cycles of aquatic prey. Emergences of aquatic insects in temperate zone wetlands begin in April but peak in June (Orians 1980) when food demands of parenting least bitterns probably are highest. See also Sterling (2008) for summaries of regional threats in California.

Short-term Trend: Decline of <30% to relatively stable
Short-term Trend Comments: Range-wide information not available. Has declined in many areas in the eastern U.S. and adjacent Canada (Ehrlich et al. 1992, Sandilands and Campbell 1988, James 1999). Main pattern in California has been population decline and range retraction in the Central Valley and local extirpation elsewhere, especially along the southern coast. The declines have been most severe in the San Joaquin Valley due to loss of freshwater marsh habitat since 1945 (Sterling 2008).

Long-term Trend: Decline of 10-50%
Long-term Trend Comments: In North America, loss of marsh habitat has likely caused a substantial decline from historical numbers. Historical records suggest that declines in breeding populations may have occurred in New York, Delaware, Maryland, and Virginia; however, the species probably is often overlooked and may be more common than reports indicate (see Gibbs and Melvin (1992) for status information for particular states in the northeastern U.S.). See Hands et al. (1989) and Brewer et al. (1991) for information on status in the north-central U.S., and Sterling (2008) for status and trends in California.

Other NatureServe Conservation Status Information

Inventory Needs: 1) Identify existing marsh habitat used for breeding, wintering, and dispersing; 2) Implement rangewide monitoring protocol to determine population status and monitor trends (Sterling 2008).

Protection Needs: 1) Preserve, protect, and restore shallow marsh habitat (larger than 10 ha) with dense emergent veg­etation; 2) Protect existing marsh habitat used for breeding, wintering, and dispersing (Gibbs et al. 1992, Sterling 2008).

Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) BREEDING: in western North America in southern Oregon, interior and southern coastal California, central Baja California, and southern coastal Sonora; in eastern North America from southern Manitoba, north-central U.S., southeastern Canada (Ontario and Quebec), eastern Maine, and southern New Brunswick (few records) south to western and southern Texas, Gulf Coast, Florida, and Greater Antilles; west to central Montana, Utah (Great Salt Lake, formerly), eastern Colorado, and south-central New Mexico; Middle America in Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica, Panama, and elsewhere, especially Mexico; widely in South America in central Colombia, coastal Peru, and east of Andes from Venezuela and Guianas south to northern Argentina and southern Brazil (AOU 1983, Gibbs et al. 1992). NON-BREEDING: southern California, southern Texas, and northern Florida south through the breeding range in South America (AOU 1983). Largest numbers in southern Florida, Rio Grande valley, lower Colorado River between California and Arizona, Baja California, Greater Antilles, and Central America (Root 1988, Gibbs et al. 1992).

U.S. States and Canadian Provinces

Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
Color legend for Distribution Map
NOTE: The maps for birds represent the breeding status by state and province. In some jurisdictions, the subnational statuses for common species have not been assessed and the status is shown as not-assessed (SNR). In some jurisdictions, the subnational status refers to the status as a non-breeder; these errors will be corrected in future versions of these maps. A species is not shown in a jurisdiction if it is not known to breed in the jurisdiction or if it occurs only accidentally or casually in the jurisdiction. Thus, the species may occur in a jurisdiction as a seasonal non-breeding resident or as a migratory transient but this will not be indicated on these maps. See other maps on this web site that depict the Western Hemisphere ranges of these species at all seasons of the year.
Endemism: occurs (regularly, as a native taxon) in multiple nations

U.S. & Canada State/Province Distribution
United States AL, AR, AZ, CA, CO, CT, DC, DE, FL, GA, IA, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MS, NC, ND, NE, NH, NJ, NM, NV, NY, OH, OK, OR, PA, RI, SC, SD, TN, TX, UT, VA, VT, WI, WV
Canada MB, NB, ON, QC

Range Map
Note: Range depicted for New World only. The scale of the maps may cause narrow coastal ranges or ranges on small islands not to appear. Not all vagrant or small disjunct occurrences are depicted. For migratory birds, some individuals occur outside of the passage migrant range depicted. For information on how to obtain shapefiles of species ranges see our Species Mapping pages at www.natureserve.org/conservation-tools/data-maps-tools.

Range Map Compilers: NatureServe, 2002; WILDSPACETM 2002

U.S. Distribution by County Help
State County Name (FIPS Code)
AR Arkansas (05001), Hempstead (05057), Jefferson (05069), Lonoke (05085)*, Pulaski (05119), Woodruff (05147)*
AZ La Paz (04012), Maricopa (04013), Mohave (04015), Pinal (04021), Yuma (04027)
CA Imperial (06025), Inyo (06027), Riverside (06065), San Diego (06073)
CT Fairfield (09001), Hartford (09003), Litchfield (09005), Middlesex (09007), New Haven (09009), New London (09011)
DE Kent (10001), New Castle (10003), Sussex (10005)
FL Alachua (12001), Bay (12005), Brevard (12009)*, Broward (12011), Charlotte (12015), DeSoto (12027), Duval (12031)*, Franklin (12037), Hardee (12049), Hillsborough (12057), Manatee (12081), Marion (12083), Miami-Dade (12086), Monroe (12087), Pasco (12101), Sarasota (12115)
GA Chattooga (13055), Floyd (13115), Greene (13133), Liberty (13179), Oconee (13219), Richmond (13245), Wilkes (13317)
ID Jefferson (16051)
IL Boone (17007), Brown (17009), Carroll (17015), Cass (17017), Champaign (17019)*, Coles (17029)*, Cook (17031), DuPage (17043), Effingham (17049), Fayette (17051)*, Fulton (17057), Grundy (17063)*, Jackson (17077), Jasper (17079), Kane (17089), Lake (17097), Lawrence (17101), Lee (17103)*, Madison (17119), Marion (17121), Massac (17127), Mchenry (17111), Mclean (17113), Monroe (17133), Peoria (17143), Perry (17145), Pulaski (17153)*, Putnam (17155), Saline (17165)*, Sangamon (17167), Shelby (17173), St. Clair (17163), Union (17181), Vermilion (17183), Wayne (17191), Whiteside (17195), Will (17197)*, Williamson (17199)*
IN Allen (18003), Bartholomew (18005), Benton (18007), Boone (18011), Brown (18013), Daviess (18027), Dubois (18037), Elkhart (18039)*, Fountain (18045), Fulton (18049), Gibson (18051), Greene (18055), Hamilton (18057), Hancock (18059)*, Henry (18065), Huntington (18069), Jackson (18071)*, Jasper (18073)*, Jay (18075), Jennings (18079), Johnson (18081), Kosciusko (18085), La Porte (18091), Lagrange (18087), Lake (18089), Marion (18097), Marshall (18099), Monroe (18105), Montgomery (18107), Newton (18111), Noble (18113), Parke (18121), Pike (18125), Porter (18127), Posey (18129), Pulaski (18131), St. Joseph (18141), Starke (18149)*, Steuben (18151), Sullivan (18153), Tippecanoe (18157), Vermillion (18165), Vigo (18167), Warren (18171), Warrick (18173), Wayne (18177)*
KS Comanche (20033), Douglas (20045), Jefferson (20087), Linn (20107), Osage (20139), Stafford (20185)
KY Ballard (21007)*, Bath (21011), Bullitt (21029)*, Carroll (21041)*, Daviess (21059)*, Fulton (21075), Hardin (21093)*, Henderson (21101), Hopkins (21107)*, Jefferson (21111)*, Larue (21123)*, Marion (21155)*, Muhlenberg (21177), Nelson (21179)*, Rowan (21205), Spencer (21215)*, Union (21225), Washington (21229)*
MA Barnstable (25001), Berkshire (25003), Bristol (25005), Dukes (25007)*, Essex (25009), Franklin (25011), Hampden (25013), Hampshire (25015), Middlesex (25017), Norfolk (25021)*, Plymouth (25023), Worcester (25027)
MD Anne Arundel (24003)*, Baltimore County (24005), Cecil (24015), Charles (24017), Frederick (24021), Garrett (24023)*, Prince Georges (24033), Somerset (24039), Talbot (24041), Worcester (24047)
MI Arenac (26011), Bay (26017), Benzie (26019), Berrien (26021), Cass (26027), Chippewa (26033), Delta (26041), Grand Traverse (26055), Hillsdale (26059), Houghton (26061), Huron (26063), Jackson (26075), Manistee (26101), Mason (26105), Monroe (26115), Muskegon (26121), Roscommon (26143), Schoolcraft (26153)*, St. Clair (26147), Tuscola (26157), Washtenaw (26161), Wayne (26163)
MO Adair (29001), Andrew (29003), Boone (29019), Buchanan (29021), Cass (29037), Chariton (29041), Clark (29045), Clay (29047), Daviess (29061), Gentry (29075), Holt (29087), Jasper (29097), Lafayette (29107), Lincoln (29113), Linn (29115), Mercer (29129), Nodaway (29147), Pike (29163), Platte (29165), Ray (29177), Saline (29195), St. Charles (29183), Stoddard (29207), Sullivan (29211), Vernon (29217), Wayne (29223)
MS Bolivar (28011)*, Hancock (28045), Harrison (28047), Jackson (28059), Madison (28089), Noxubee (28103), Oktibbeha (28105), Rankin (28121), Tallahatchie (28135), Washington (28151)
NC Beaufort (37013), Brunswick (37019), Carteret (37031), Craven (37049)*, Currituck (37053), Dare (37055), Hyde (37095), Moore (37125)*, New Hanover (37129), Onslow (37133), Pamlico (37137), Pender (37141), Tyrrell (37177)*, Washington (37187), Wayne (37191)
NE Arthur (31005), Cherry (31031), Fillmore (31059), Knox (31107), Lancaster (31109), Platte (31141), Polk (31143)*, Saunders (31155), York (31185)*
NH Cheshire (33005), Merrimack (33013), Rockingham (33015), Strafford (33017)
NJ Atlantic (34001), Bergen (34003), Cape May (34009), Cumberland (34011), Essex (34013), Gloucester (34015), Hudson (34017), Hunterdon (34019), Middlesex (34023), Monmouth (34025), Morris (34027), Ocean (34029), Passaic (34031), Sussex (34037), Warren (34041)
NM Chaves (35005)*, Quay (35037)*, Socorro (35053)*
NV Clark (32003), Elko (32007), Nye (32023), White Pine (32033)
NY Albany (36001), Allegany (36003), Broome (36007), Cayuga (36011), Chautauqua (36013), Chemung (36015), Clinton (36019), Columbia (36021), Dutchess (36027), Erie (36029), Essex (36031), Franklin (36033), Genesee (36037), Greene (36039), Herkimer (36043), Jefferson (36045), Kings (36047), Lewis (36049), Livingston (36051), Monroe (36055), Nassau (36059), Niagara (36063), Oneida (36065), Onondaga (36067), Orange (36071), Orleans (36073), Oswego (36075), Putnam (36079), Queens (36081), Rensselaer (36083), Richmond (36085), Rockland (36087), Schuyler (36097), Seneca (36099), St. Lawrence (36089), Suffolk (36103), Sullivan (36105), Ulster (36111), Warren (36113), Washington (36115), Wayne (36117), Westchester (36119)
OH Butler (39017), Carroll (39019), Clark (39023), Erie (39043)*, Greene (39057), Huron (39077), Lake (39085), Licking (39089), Lucas (39095), Mahoning (39099)*, Marion (39101), Ottawa (39123), Putnam (39137), Sandusky (39143)*, Seneca (39147), Summit (39153), Trumbull (39155), Union (39159), Warren (39165), Wayne (39169)*
OK Wagoner (40145)
OR Harney (41025), Jackson (41029)*, Klamath (41035)
PA Bedford (42009), Berks (42011)*, Blair (42013), Butler (42019)*, Chester (42029)*, Crawford (42039), Delaware (42045), Erie (42049), Huntingdon (42061), Indiana (42063), Lawrence (42073), Mercer (42085), Northumberland (42097), Philadelphia (42101), Schuylkill (42107)*
RI Newport (44005), Providence (44007), Washington (44009)
SD Brown (46013), Charles Mix (46023)*, Day (46037)*, Deuel (46039)*, Hughes (46065), Kingsbury (46077)*, Lincoln (46083)*
TN Blount (47009)*, Bradley (47011), Davidson (47037)*, Franklin (47051)*, Gibson (47053)*, Grundy (47061), Hamilton (47065), Knox (47093)*, Lake (47095), Maury (47119), Meigs (47121), Obion (47131)*, Overton (47133)*, Putnam (47141)*, Warren (47177)*
UT Box Elder (49003)*, Carbon (49007)*, Davis (49011)*, Salt Lake (49035)*, Uintah (49047)*, Washington (49053)*
VT Addison (50001), Chittenden (50007), Franklin (50011), Grand Isle (50013), Orange (50017), Orleans (50019), Rutland (50021), Washington (50023)
WI Ashland (55003), Bayfield (55007), Buffalo (55011), Burnett (55013), Columbia (55021), Crawford (55023), Dane (55025), Dodge (55027), Door (55029), Douglas (55031), Fond Du Lac (55039), Grant (55043), Green Lake (55047), Iowa (55049), Jackson (55053), Jefferson (55055), Juneau (55057), Kenosha (55059), Kewaunee (55061)*, Manitowoc (55071), Marathon (55073), Marquette (55077), Monroe (55081), Oconto (55083), Outagamie (55087), Ozaukee (55089), Pepin (55091), Portage (55097), Racine (55101), Richland (55103), Sauk (55111), Shawano (55115), Sheboygan (55117), Walworth (55127), Washington (55131), Waukesha (55133), Waupaca (55135), Winnebago (55139), Wood (55141)
WV Mason (54053)
WY Lincoln (56023), Sublette (56035), Teton (56039), Uinta (56041)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
01 Piscataqua-Salmon Falls (01060003)+, Merrimack (01070002)+*, Contoocook (01070003)+, Nashua (01070004)+, Concord (01070005)+, Merrimack (01070006)+, Upper Connecticut-Mascoma (01080104)+, West (01080107)+, Middle Connecticut (01080201)+, Deerfield (01080203)+, Chicopee (01080204)+, Lower Connecticut (01080205)+, Charles (01090001)+, Cape Cod (01090002)+, Blackstone (01090003)+, Narragansett (01090004)+, Pawcatuck-Wood (01090005)+, Quinebaug (01100001)+, Thames (01100003)+*, Quinnipiac (01100004)+, Housatonic (01100005)+, Saugatuck (01100006)+
02 Upper Hudson (02020001)+, Hudson-Hoosic (02020003)+, Mohawk (02020004)+, Middle Hudson (02020006)+, Rondout (02020007)+, Hudson-Wappinger (02020008)+, Lower Hudson (02030101)+, Bronx (02030102)+, Hackensack-Passaic (02030103)+, Sandy Hook-Staten Island (02030104)+, Raritan (02030105)+, Southern Long Island (02030202)+, Middle Delaware-Mongaup-Brodhead (02040104)+, Middle Delaware-Musconetcong (02040105)+, Lower Delaware (02040202)+, Schuylkill (02040203)+*, Brandywine-Christina (02040205)+, Cohansey-Maurice (02040206)+, Broadkill-Smyrna (02040207)+, Mullica-Toms (02040301)+, Great Egg Harbor (02040302)+, Chincoteague (02040303)+, Upper Susquehanna (02050101)+, Chenango (02050102)+, Lower West Branch Susquehanna (02050206)+, Upper Juniata (02050302)+, Raystown (02050303)+, Lower Susquehanna-Swatara (02050305)+*, Chester-Sassafras (02060002)+, Gunpowder-Patapsco (02060003)+, Choptank (02060005)+, Patuxent (02060006)+*, Monocacy (02070009)+, Middle Potomac-Anacostia-Occoquan (02070010)+, Lower Potomac (02070011)+, Western Lower Delmarva (02080109)+, Eastern Lower Delmarva (02080110)+
03 Albemarle (03010205)+, Pamlico (03020104)+, Pamlico Sound (03020105)+, Middle Neuse (03020202)+, Lower Neuse (03020204)+, White Oak River (03020301)+, Upper Cape Fear (03030004)+*, Lower Cape Fear (03030005)+, Northeast Cape Fear (03030007)+, Coastal Carolina (03040208)+, Upper Savannah (03060103)+, Middle Savannah (03060106)+, Ogeechee Coastal (03060204)+, Upper Oconee (03070101)+, Oklawaha (03080102)+, Lower St. Johns (03080103)+*, Cape Canaveral (03080202)+*, Everglades (03090202)+, Florida Bay-Florida Keys (03090203)+, Peace (03100101)+, Myakka (03100102)+, Sarasota Bay (03100201)+, Hillsborough (03100205)+, Tampa Bay (03100206)+, Apalachicola (03130011)+, Apalachicola Bay (03130014)+, St. Andrew-St. Joseph Bays (03140101)+, Oostanaula (03150103)+, Noxubee (03160108)+, Pascagoula (03170006)+, Mississippi Coastal (03170009)+, Middle Pearl-Strong (03180002)+, Lower Pearl. Mississippi (03180004)+
04 St. Louis (04010201)+, Beartrap-Nemadji (04010301)+, Bad-Montreal (04010302)+, Sturgeon (04020104)+, Manitowoc-Sheboygan (04030101)+, Door-Kewaunee (04030102)+, Oconto (04030104)+, Tacoosh-Whitefish (04030111)+, Upper Fox (04030201)+, Wolf (04030202)+, Little Calumet-Galien (04040001)+, Pike-Root (04040002)+, Milwaukee (04040003)+, St. Joseph (04050001)+, Upper Grand (04050004)+, Pere Marquette-White (04060101)+, Muskegon (04060102)+, Betsie-Platte (04060104)+, Boardman-Charlevoix (04060105)+, Manistique (04060106)+*, St. Marys (04070001)+, Au Gres-Rifle (04080101)+, Kawkawlin-Pine (04080102)+, Pigeon-Wiscoggin (04080103)+, Lake Huron (04080300)+, Lake St. Clair (04090002)+, Detroit (04090004)+, Huron (04090005)+, Ottawa-Stony (04100001)+, Raisin (04100002)+, St. Joseph (04100003)+, Blanchard (04100008)+, Cedar-Portage (04100010)+, Sandusky (04100011)+*, Huron-Vermilion (04100012)+, Cuyahoga (04110002)+, Ashtabula-Chagrin (04110003)+, Grand (04110004)+, Chautauqua-Conneaut (04120101)+, Buffalo-Eighteenmile (04120103)+, Niagara (04120104)+, Oak Orchard-Twelvemile (04130001)+, Upper Genesee (04130002)+, Lower Genesee (04130003)+, Irondequoit-Ninemile (04140101)+, Salmon-Sandy (04140102)+, Seneca (04140201)+, Oneida (04140202)+, Chaumont-Perch (04150102)+, Upper St. Lawrence (04150301)+, Oswegatchie (04150302)+, Indian (04150303)+, Grass (04150304)+, Raquette (04150305)+, St. Regis (04150306)+, English-Salmon (04150307)+, Mettawee River (04150401)+, Otter Creek (04150402)+, Winooski River (04150403)+, Lamoille River (04150405)+, Missiquoi River (04150407)+, Lake Champlain (04150408)+, St. Francois River (04150500)+
05 Upper Allegheny (05010001)+, Conewango (05010002)+, Middle Allegheny-Tionesta (05010003)+*, French (05010004)+, Conemaugh (05010007)+, Youghiogheny (05020006)+*, Shenango (05030102)+, Mahoning (05030103)+*, Connoquenessing (05030105)+, Tuscarawas (05040001)+, Walhonding (05040003)+*, Licking (05040006)+, Upper Scioto (05060001)+, Upper Great Miami (05080001)+, Whitewater (05080003)+*, Raccoon-Symmes (05090101)+, Little Miami (05090202)+, Middle Ohio-Laughery (05090203)+, Licking (05100101)+, Middle Green (05110003)+, Upper Wabash (05120101)+, Salamonie (05120102)+, Tippecanoe (05120106)+, Middle Wabash-Little Vermilion (05120108)+, Vermilion (05120109)+, Sugar (05120110)+, Middle Wabash-Busseron (05120111)+, Embarras (05120112)+, Little Wabash (05120114)+, Skillet (05120115)+, Upper White (05120201)+, Lower White (05120202)+, Driftwood (05120204)+, Upper East Fork White (05120206)+*, Muscatatuck (05120207)+, Lower East Fork White (05120208)+, Patoka (05120209)+, Obey (05130105)+*, Collins (05130107)+*, Caney (05130108)+*, Lower Cumberland-Sycamore (05130202)+*, Silver-Little Kentucky (05140101)+*, Salt (05140102)+*, Rolling Fork (05140103)+*, Lower Ohio-Little Pigeon (05140201)+, Highland-Pigeon (05140202)+, Saline (05140204)+*, Tradewater (05140205)+*, Lower Ohio (05140206)+
06 Watts Bar Lake (06010201)+*, Middle Tennessee-Chickamauga (06020001)+, Hiwassee (06020002)+, Upper Elk (06030003)+, Lower Duck (06040003)+
07 Lac Qui Parle (07020003)+*, Upper St. Croix (07030001)+, Lower St. Croix (07030005)+, Buffalo-Whitewater (07040003)+, Black (07040007)+, Lower Chippewa (07050005)+, Apple-Plum (07060005)+, Lake Dubay (07070002)+, Castle Rock (07070003)+, Lower Wisconsin (07070005)+, Kickapoo (07070006)+, Copperas-Duck (07080101)+, Upper Rock (07090001)+, Kishwaukee (07090006)+, Green (07090007)+*, Bear-Wyaconda (07110001)+, The Sny (07110004)+, Salt (07110007)+, Peruque-Piasa (07110009)+, Kankakee (07120001)+, Iroquois (07120002)+, Chicago (07120003)+, Des Plaines (07120004)+, Upper Illinois (07120005)+*, Upper Fox (07120006)+, Lower Fox (07120007)+, Lower Illinois-Senachwine Lake (07130001)+, Lower Illinois-Lake Chautauqua (07130003)+, Lower Sangamon (07130008)+, Salt (07130009)+, Lower Illinois (07130011)+, Cahokia-Joachim (07140101)+, Upper Mississippi-Cape Girardeau (07140105)+, Big Muddy (07140106)+, Cache (07140108)+*, Upper Kaskaskia (07140201)+, Middle Kaskaskia (07140202)+*, Lower Kaskaskia (07140204)+*
08 Obion (08010202)+, North Fork Forked Deer (08010204)+*, Lower Mississippi-Helena (08020100)+*, Lower St. Francis (08020203)+, Cache (08020302)+*, Lower Arkansas (08020401)+, Bayou Meto (08020402)+*, Tallahatchie (08030202)+, Deer-Steele (08030209)+
10 Fort Randall Reservoir (10140101)+, Middle Niobrara (10150004)+, Upper James (10160003)+, Lewis and Clark Lake (10170101)+, South Big Sioux Coteau (10170103)+*, Middle Big Sioux Coteau (10170201)+*, Lower Big Sioux (10170203)+*, Lower Platte-Shell (10200201)+, Salt (10200203)+, Dismal (10210002)+, Tarkio-Wolf (10240005)+, Nodaway (10240010)+, Independence-Sugar (10240011)+, Delaware (10270103)+, Lower Kansas (10270104)+, Upper Big Blue (10270201)+*, West Fork Big Blue (10270203)+*, Turkey (10270204)+, Upper Little Blue (10270206)+, Upper Grand (10280101)+, Thompson (10280102)+, Lower Grand (10280103)+, Upper Chariton (10280201)+, Lower Chariton (10280202)+, Upper Marais Des Cygnes (10290101)+, Lower Marais Des Cygnes (10290102)+, Little Osage (10290103)+, Harry S. Missouri (10290105)+, South Grand (10290108)+, Lower Missouri-Crooked (10300101)+, Lower Missouri-Moreau (10300102)+
11 Rattlesnake (11030009)+, Upper Cimarron-Bluff (11040008)+, Lower Verdigris (11070105)+, Spring (11070207)+, Upper Canadian-Ute Reservoir (11080006)+*, Lower Arkansas-Maumelle (11110207)+, Lower Little (11140109)+
13 Rio Grande-Albuquerque (13020203)+*, Upper Pecos-Long Arroyo (13060007)+*
14 Upper Green (14040101)+, Blacks Fork (14040107)+, Lower White (14050007)+*, Lower Green-Diamond (14060001)+*, Duchesne (14060003)+*, Price (14060007)+*
15 Lake Mead (15010005)+, Upper Virgin (15010008)+*, Fort Pierce Wash (15010009)+*, Lower Virgin (15010010)+*, White (15010011)+, Muddy (15010012)+, Las Vegas Wash (15010015)+, Havasu-Mohave Lakes (15030101)+, Imperial Reservoir (15030104)+, Lower Colorado (15030107)+, Middle Gila (15050100)+, Lower Salt (15060106)+, Lower Gila-Painted Rock Reservoir (15070101)+, Agua Fria (15070102)+, Centennial Wash (15070104)+, Lower Gila (15070201)+
16 Upper Bear (16010101)+, Lower Bear-Malad (16010204)+*, Jordan (16020204)+*, Long-Ruby Valleys (16060007)+
17 Snake headwaters (17040101)+, Beaver-Camas (17040214)+, Upper Rogue (17100307)+*, Middle Rogue (17100308)+*, Harney-Malheur Lakes (17120001)+*, Silvies (17120002)+*, Donner Und Blitzen (17120003)+
18 Upper Klamath Lake (18010203)+, San Luis Rey-Escondido (18070303)+, San Diego (18070304)+, Owens Lake (18090103)+, Salton Sea (18100204)+
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History
Basic Description: A marsh bird (bittern).
General Description: In comparison to other herons, bitterns are more solitary and rely more heavily on auditory than visual communication. Bitterns also have a laterally compressed trunk, short legs, short outer toes, and long, curved toenails that enable them to travel through and grasp the dense, emergent vegetation typical of nesting habitats (Kushlan 1978, Hancock and Kushlan 1984). The smallest member of the heron family, the least bittern ranges between 28-36 cm in length, average 33 cm, and has a wingspan of 43 cm (Cramp 1977, NGS 1983). It has a straight, slender, pointed bill and a large buffy patch on the inner half of the upper side of each wing. The head is slightly crested. Whitish, highly visible lines border the scapular feathers. Sexes are similar in size, but plumage is dimorphic. The crown and back of the female is purple-chestnut, whereas those of the male are black, and the neck of the female is darkly streaked (Palmer 1962). Juvenal plumage is similar to that of the adult female, but the crown of the juvenile is paler and more brown, and the breast and throat are browner with heavier streaking. The only seasonal difference in plumage is a high gloss in spring (Weller 1961). A rare, darker morph occurs, known as "Cory's bittern," in which paler areas of typical plumage appear chestnut-colored (Palmer 1962, Hancock and Kushlan 1984). Young are covered with long, soft, ochre-colored down above and whiter down beneath (Palmer 1962, McVaugh 1975). Detailed descriptions and illustrations of the plumage development of young are provided by McVaugh (1975).

NEST: Usually built over shallow water (0.1-1.0 m deep) (Palmer 1962, Kushlan 1973, Aniskowicz 1981) and tend to be near, less than ten meters from open water (Weller 1961). A nesting platform with a canopy is made by pulling down and crimping surrounding emergent vegetation, such as cattail or bulrush (Weller 1961).

EGGS: Elliptical, pale blue or pale green, smooth and non-glossy, averaging 31 by 24 mm (Bent 1926, Harrison 1978).

VOCALIZATIONS: Quite vocal with a varied repertoire of calls. The male's advertisement call most frequently heard in spring is a dove-like cooing characterized as "uh-uh-uh-uh-uh-oo-oo-oooo-oo-ooah" (Palmer 1962). Females may respond with "ticking" calls (Hancock and Kushlan 1984). When alarmed, three calls may be uttered: a loud, shrieking "quoh," a hissing "hah," or cackling "tut-tut-tut" (Palmer 1962, Hancock and Kushlan 1984).

Diagnostic Characteristics: Small size, yellow color, and a dark crown are characteristics that distinguish least bitterns from all other bitterns and herons (Hancock and Kushlan 1984). Diagnostic field characteristics include a vivid, greenish-black crown, back, and tail; brownish and white neck, sides, and underparts; and chestnut-colored wings with conspicuous, contrasting, pale-colored wing patches. No other small heron has large buffy patches on the upper side of the otherwise dark wings.
Reproduction Comments: Nesting densities can be high (e.g. 15 nests in 0.8 ha, Wood 1951), but are often on the order of 1/hectare (Palmer 1962, Kent 1951).

In the north-central U.S., nests in June-July (Brewer et al. 1991). In southern Florida, most nesting was initiated after 18 May, with large numbers of fledglings appearing after 15 June (Frederick et al. 1990). Peak nesting (including first and second broods) occurs in the northeastern U.S. from late-May to early-July (Palmer 1962, Swift et al. 1988). Copulation occurs on the nest before and during incubation (Weller 1961). Females lay one egg daily to complete clutches of two to seven eggs, usually four to five eggs (Trautman 1940, Kent 1951, Weller 1961, Hansen 1984). Clutch-size seems to increase with latitude in North America (Palmer 1962). Early clutches, laid before mid-June, may be smaller than later clutches (Weller 1961). Incubation by both sexes begins upon deposition of the first or second egg, and lasts 17-20 days (Weller 1961, Aniskowicz 1981). The semi-altricial young hatch asynchronously over a three day period (Weller 1961, Palmer 1962). Young, brooded for several days, leave the nest after five to 15 (usually 10) days but return occasionally to be fed regurgitated food by their parents, mostly by the male (Gabrielson 1914, Weller 1961, Cogswell 1977). Hansen (1984) and Nero (1950) noted that the youngest hatchling among broods grew the slowest because it received less food. Can probably rear second broods (Kent 1951, Weller 1961). Although few data are available on territoriality and mating systems, most nest solitarily and probably form seasonal, monogamous pair-bonds (Weller 1961, Palmer 1962). Occasionally breeding colonies form near abundant food sources (Audubon 1840, Weller 1961, Palmer 1962, Kushlan 1973). Descriptions of courtship and territorial behavior are given by Weller (1961). Age of first breeding is unknown (Palmer 1962).

NESTING SUCCESS: Success of 38 nests was estimated by Weller (1961) to be 84% (nests in which one or no eggs hatched), with an average of three young hatched per nest. Kent (1951) observed that eggs in 70% of 20 nests hatched, and listed abandonment, predation, cannibalism, and disease as causes of nest loss. In Kansas, four of five nests studied by Hansen (1984) were depredated, at least one by a raccoon (PROCYON LOTOR). High water also may destroy nests (McVaugh 1975), marsh wrens (CISTOTHORUS PALUSTRIS) may puncture eggs (Bent 1926), and mink (MUSTELA VISON) may take eggs and nestlings (Hancock and Kushlan 1984).

Ecology Comments: Usually solitary, secretive. In six tidal marshes along the Hudson River in New York, Swift (1987) observed an average of up to 0.5 individuals per ha during playback surveys using tape-recorded vocalizations in 1986 and 1987. Other reports of nesting densities in North America vary widely: 15 nests per ha (n = 15 nests: Palmer 1962), one nest per ha (n = 26 nests: Palmer 1962), and one nest per ha (n = 19 nests: Kent 1951). Approximately equal numbers of males and females were observed by Frederick et al. (1990) during the breeding season in the Florida Everglades.
Non-Migrant: Y
Locally Migrant: Y
Long Distance Migrant: Y
Mobility and Migration Comments: Arrives at nesting areas in the northeastern U.S. in early to mid-April or early May (Bent 1926); arrival may be delayed until late April in more northern states, including Maine (Palmer 1949) and New York (Swift 1987). Generally leaves northern breeding areas by September-October (Palmer 1962). Migrates apparently at night. Breeding populations south of the U.S. are mostly sedentary. North American birds migrate as far south as Colombia. Seasonal movements in Costa Rica are related to water levels (Stiles and Skutch 1989). Young may linger at nesting areas until October before beginning their southward migration (Palmer 1962).
Estuarine Habitat(s): Herbaceous wetland, Scrub-shrub wetland
Palustrine Habitat(s): Bog/fen, HERBACEOUS WETLAND, Riparian, SCRUB-SHRUB WETLAND
Habitat Comments: BREEDING: Tall emergent vegetation in marshes, primarily freshwater, less commonly in coastal brackish marshes and mangrove swamps. Prefers marshes with scattered bushes or other woody growth. In the northeastern U.S., breeds mainly in wetlands along lakes, rivers, and estuaries on the coastal plain; generally absent from the Appalachian highlands and mountainous parts of New York and northern New England (Gibbs and Melvin 1992). Readily uses artificial wetlands.

Habitats vary throughout North America, but nesting usually occurs among dense, tall growths of emergent vegetation (particularly cattail (TYPHA spp.), sedge (CAREX spp.), bulrush (SCIRPUS spp.), or common reed (PHRAGMITES AUSTRALIS)), interspersed with some woody vegetation and open, fresh water (Weller 1961, Palmer 1962, Kushlan 1973, Swift 1987, Frederick et al. 1990). Both fresh and brackish marshes are used (Palmer 1962, Swift 1987, Andrle and Carroll 1988). Occurrences have been associated particularly with cattail, vegetated edges along deep, open waters (Weller 1961), and nutrient-rich microhabitats (Kushlan 1973). Nests typically 0.15-0.75 m above water near open water (Terres 1980, Harrison 1979, Brewer et al. 1991), in water typically 10-50 cm deep. Spend nearly all the diurnal period in dense, grass-like vegetation; open habitats such as mats of emergent vegetation are rarely used (Frederick et al. 1990).

Weller and Spatcher (1965) found more nests at two Iowa marshes during years when ratios of emergent vegetative cover to open water were equal (the "hemi-marsh" condition). More densely or sparsely vegetated wetlands contained fewer nests, and interspersion of water and cover may thus be an important characteristic of breeding habitats. In Wisconsin, were restricted to deep-water and shallow-water cattail habitats, apparently avoiding areas of dry cattail, river bulrush (SCIRPUS FLUVIATILIS), and sedge (Manci and Rusch 1988). Variably-sized wetlands were used in Maine, with dense, tall stands of cattail, which were often associated with relatively stable water regimes at managed impoundments and coves on lakes (Gibbs and Melvin 1990). On moist-soil impoundments in Missouri, associated with waters up to 50 cm deep and rank, dense vegetative cover bordering open water (Fredrickson and Reid 1986). Were not associated with open, sparse, or short vegetative cover or muddy openings (Fredrickson and Reid 1986).

Among six tidal marshes along the Hudson River in New York, presence was related to the extent of tall bulrush-cattail cover and site elevation (i.e., depth of tidal flooding) (Swift 1987). Favored sites had tall, dense vegetation and low-lying, "wetter" sites (peak water depths averaged 70 cm), perhaps because nests are usually placed over water or near open water (Weller 1961, Palmer 1962, Kushlan 1973, Aniskowicz 1981). In tidewater areas of Chesapeake Bay, Stewart and Robbins (1958) reported abundance in narrow-leaved cattail (TYPHA ANGUSTIFOLIA) marshes, common in other coarse marsh types and weak-stemmed brackish marsh types, but scarcity in salt marshes.

In the Florida Everglades, observed from airboats at densities (birds/km) of 0.13 in canals, 0.04 in open grasslands, and 0.37 along airboat trails (Frederick et al. 1990). Most individuals were seen in mixed sawgrass (CLADIUM JAMAICENSE) and cattail (29% of birds), homogeneous sawgrass (23%), and homogeneous cattail (9%), and, within these vegetative associations, were seen twice as frequently in dense than sparse stands (Frederick et al. 1990). Open sloughs, rush prairies, mats of emergent vegetation, and burn areas were used infrequently (Frederick et al. 1990, but see Kushlan 1973). The strong association with cattail in northern regions (e.g., Weller 1961, Swift 1987, Manci and Rusch 1988, Gibbs and Melvin 1990) may occur because cattail is the most common tall plant growing in dense stands above deep water in most northern areas (Frederick et al. 1990). In South Carolina, nests frequently were associated with boat-tailed grackle colonies (Post and Seals 1993).

Sites where bitterns were flushed during spring migration in Missouri (n = 61) had a mean water depth of 23 cm, vegetation height of 64 cm, and stem density of 287 stems/meter squared (Reid 1989), and were dominated by burreed (SPARGANIUM EURYCARPUM), water smartweed (POLYGONUM COCCINEUM), and cattail. Reid (1989) observed a shift in habitat use to taller (121 cm), sparser (165 stems/meter squared) stands in fall (n = 15 flush sites). Feeding platforms (n = 52) occurred over water averaging 29 cm in depth and among stands of emergent vegetation averaging 199 stems/meter squared.

NON-BREEDING: Overwintering birds occur in brackish and saline swamps and marshes (Palmer 1962, Hancock and Kushlan 1984), but little is known about wintering habitats.

Adult Food Habits: Carnivore, Invertivore, Piscivore
Immature Food Habits: Carnivore, Invertivore, Piscivore
Food Comments: Eats small fishes, amphibians, leeches, slugs, snails, crustaceans, insects, and occasionally small mammals (Palmer 1962); possibly the eggs and young of marsh-nesting blackbirds. Feeds young by regurgitation. Forages in shallow water or along banks. Heavy growths of cattail, bulrush, wild rice, burreed, water smartweed, and reeds are favored feeding sites (Brewer et al. 1991). The repertoire of feeding behaviors is relatively small (Kushlan 1978), and among 28 recognized behaviors used by herons, only four are employed: standing in place; walking slowly; neck swaying (to overcome glare, to increase camouflage, or to have muscles in movement when strike begins); and wing-flicking, which involves quick, repeated extension and retraction of wings that may startle prey from hiding (Sutton 1936). The small size and highly compressed trunk enable it to move easily through dense vegetation. It forages by stalking along branches and reeds, and often clings to clumps of vegetation above water-level, aided by its short, outer toes and long, curved claws. It may build foraging platforms of bent reeds at productive feeding sites (Weller 1961, Reid 1989), and frequent these platforms during late-incubation and brood-rearing periods (Reid 1989). Clinging to emergent vegetation and constructing platforms enables it to forage at water depths as great as those of the largest North American herons (25-60 cm depth).

The small, thin bill is probably an adaptation for securing small, fast-moving prey (Kushlan 1978), an observation supported by dietary analyses. Contents of 93 stomachs from Florida included 40% fish (top minnows, mud minnows, yellow perch, and sunfish), 21% dragonflies, 12% other aquatic insects, and 10% crustaceans (mainly crayfish (Howell 1932)). Six stomachs from Pennsylvania contained small mammals, beetles, fish, a snake, and vegetable matter (Warren 1890). Nesting birds consumed damselflies, grasshoppers, other insects, and tadpoles in Iowa (Weller 1961). Also may prey on the eggs and young of yellow-headed blackbirds (XANTHOCEPHALUS XANTHOCEPHALUS) (Roberts 1936, cited in Hancock and Kushlan 1984). Other foods include frogs, lizards, mollusks, and leeches (Bent 1926).

Length: 33 centimeters
Weight: 86 grams
Economic Attributes Not yet assessed
Management Summary
Stewardship Overview: The Least Bittern (Ixobrychus exilis) breeds in marshes in the western United States and throughout much of the eastern United States, the Caribbean, and parts of Mexico and northern Central America (Gibbs et al. 1992). Most of the birds in the United States migrate to the neotropics for winter, but some remain in southern parts of the United States, including southern California (especially in the Salton Sink and the lower Colorado River valley), southern and coastal Texas, and the coastal plain of Maryland south to Florida (Rosenberg et al. 1991, Gibbs et al. 1992, Patten et al. 2003). Nests in freshwater wetlands with dense, tall growths of emergent vegetation interspersed with open water, and feeds predominantly on insects, crustaceans, and fish.  Because the Least Bitten is a secretive marsh bird, its population status and trends are poorly understood (Gibbs et al. 1992, Sterling 2008), but population declines have likely been extensive given the widespread loss of marsh habitat across its range (Gibbs et al. 1992, Sterling 2008). The primary threat to this species is the loss and degradation of its freshwater marsh habitat (Gibbs et al. 1992).

Management and Research needs across their range include: 1) Preserve, protect, and restore shallow marsh habitat (larger than 10 ha) with dense emergent veg­etation; 2) Identify and protect existing marsh habitat used for breeding, wintering, and dispersing; 3) Manage summer wetlands to increase the availability of suitable Least Bittern habitat; 4) Minimize disturbance impacts dur­ing nesting season; 5) Research breeding biology across range, and identify occurrences that support core populations and key factors that limit nest success; 6) Sample eggs at key sites to measure contaminants and effects of toxins on Least Bitterns; 7) Monitor effects of selenium, other toxins, and salts where water quality in breeding habitat has been compromised; 8) Determine the mini­mum patch size of suitable breeding habi­tat; 9) Study social structure and nest success of ?loose colonies? compared to non-colonial populations; 10) Implement rangewide monitoring protocol to determine population status and monitor trends (Gibbs et al. 1992, Sterling 2008). Census techniques for wetland birds include using tape-recorded vocalizations to elicit responses from breeding birds. Standardized techniques should be used throughout their range to monitor populations of this secretive species. Management of impoundments on federal and state wildlife areas for dense, emergent vegetation and deep water will help create suitable habitat and encourage the expansion of populations (Gibbs and Melvin 1992).

Restoration Potential: Readily use wetlands created by artificial impoundments (Andrle and Carroll 1988, Gibbs and Melvin 1990), and seem adaptable to a wide range of wetland habitats, e.g., brackish, coastal marshes, artificial impoundments, and natural freshwater, palustrine, lacustrine, and riverine emergent wetlands. These traits could facilitate restoration or expansion of populations in regions where marshland losses have been high (Connecticut, Rhode Island, central New York, New Jersey, and Maryland) or where marshlands were scarce originally (central Pennsylvania and West Virginia). Information on reproduction and population structure is too scarce, however, to state anything definitive about the potential for management to stabilize or increase populations (Gibbs and Melvin 1992).
Preserve Selection & Design Considerations: Wetland area is a consideration for preserve design because frequency of occurrence is greater on larger than on smaller wetlands (Brown and Dinsmore 1986). A minimum area of five or more hectares may be sufficient to support nesting activity (Brown and Dinsmore 1986, Gibbs and Melvin 1990). Cattail stands of at least 15 acres interspersed with open water are most suitable for breeding (Evers 1992). Gibbs and Melvin (1990), however, observed territorial birds on wetlands as small as 0.4 ha in Maine. Vegetative features of wetland preserves should include dense (> 100 stems per square meter) and tall (greater than one meter) stands of emergent vegetation (e.g., cattail or bulrush) in deep-water (10-50 cm) well-interspersed with patches of open water (Weller and Spatcher 1965, Fredrickson and Reid 1986, Reid 1989). Flooded vegetation is likely required for nesting (Weller 1961). Stable water levels will prevent nest flooding, may reduce predation, and can promote dense growth of emergents. If a dynamic wetland system is stabilized over a long period, however, productivity of a site can decline. Retaining riparian vegetation can buffer the wetland ecosystem against upland runoff that may contain silt and contaminants (Gibbs and Melvin 1992).
Management Requirements: Preservation, protection, and improvement of wetland habitats, particularly large (greater than five ha), shallow wetlands with dense growths of robust, emergent vegetation, is the most urgent management need. Wetlands used for breeding also need to be protected from chemical contamination, siltation, eutrophication, and other forms of pollution that directly harm bitterns or their food supplies. Equal ratios of cover to open water are preferred, so wetland managers may need to periodically reverse vegetative succession while maintaining suitable habitats nearby to serve as alternate nesting areas during wetland manipulations (e.g., at other wetlands in a complex).

Because bitterns occur in many states at wetlands managed by state and federal agencies for waterfowl, there is ample opportunity for making minor alterations to existing management schemes to improve nesting habitat. For example, dense stands of cattail and bulrush, often eliminated with cutting, burning, or flooding treatments to improve waterfowl habitat, can be partially retained as habitat. Maintaining stands of deep-water (10-30 cm) cattail is important because water levels at or below the base of emergent vegetation may reduce nesting activity by least bitterns (Weller 1961), which prefer foraging over deep water (10-50 cm). Where littoral vegetation is scarce, moist-soil plant management (Fredrickson and Taylor 1982) provides a cost-effective method involving water level manipulation to reestablish and promote growth of dense stands of emergent vegetation. Complete drawdowns, sometimes undertaken for waterfowl management, should be avoided so that populations of small fish and dragonfly larvae, which make up the majority of the diet, are conserved for the following season. Liming and fertilizing dikes and adjacent fields can increase the productivity and raise the pH of many nutrient-poor, acidic wetlands in the northeastern region. Infestations of purple loosestrife, which are detrimental, can be controlled with herbicides, physical removal, and burning (Gibbs and Melvin 1992).

Monitoring Requirements: Special surveys need to be developed to better determine population distribution and assess trends and responses to habitat features (Gibbs and Melvin 1992). Detected too infrequently on North American Breeding Bird Survey (BBS) routes during the period 1966-89 to assess population trends in any state or province in North America except Florida. Breed in localized habitats that usually occur away from roadsides, and their seasonal peak of vocal activity (May) usually occurs when BBS routes are not run. Extremely low sighting frequencies on Christmas Bird Counts prompted Butcher (1989) to state that a special monitoring program should be initiated immediately.

Broadcast of tape-recorded calls is useful for eliciting responses from these rarely seen birds and surveying their populations (Manci and Rusch 1988, Swift et al. 1988, Gibbs and Melvin 1990, Kibbe 1989). Such surveys can be coordinated with surveys of other marshbirds (Swift 1987, Manci and Rusch 1988, Gibbs and Melvin 1990), many of which are also of management concern (e.g., pied-billed grebe (PODILYMBUS PODICEPS), American bittern, and black tern (CHLIDONIAS NIGER). If so designed, surveys can be used to evaluate population responses to habitat features in addition to assessing population trends. Surveys should initially be conducted annually (for two to three years) at many sites to provide baseline data on population distributions and abundance in a given region. Repeated surveys (e.g., once every 3-5 years) can then be used to determine population trends. Surveys should be conducted during the peak of nesting activity, which may be later than for other marsh birds, e.g., in early June (Swift et al. 1988, Frederick et al. 1990). Local populations should be exposed to no more than three surveys per year to avoid undue harassment of nesting birds. Standardization of survey methodology and coordination of state and regional surveys is necessary for results to be comparable among years and areas. Coordination of surveys might best be accomplished by having a single regional or national agency responsible for administering survey efforts (Gibbs and Melvin 1992).

Management Research Needs: The following are suggested by Gibbs and Melvin (1992):

Initiate standardized monitoring programs to determine abundance and distribution and to evaluate trends in populations.

Conduct studies of wetland vegetation, water levels, water quality, and minimum wetland area during nesting, migration, and over-wintering seasons.

Conduct studies on breeding biology to examine movements and patterns of habitat use, causes and rates of juvenile and adult mortality, sources of nest failure, ability to renest, juvenile dispersal patterns, mating systems and philopatry, and diet. This information could be obtained partly through radio-telemetry and banding studies. Telemetry studies are contingent upon development of appropriate capture techniques.

Identify major stop-over sites for overwintering and migrating.

Determine the effects of diseases, parasites, contaminants, and weather during breeding and overwintering.

Monitor contaminant levels in birds and their eggs in agricultural and industrialized regions.

Evaluate the effects of open-water management and mosquito-ditching programs at brackish and salt marshes.

Evaluate the effects of marshland invasion by phragmites and purple loosestrife.

Biological Research Needs: 1) Research breeding biology across range, and identify occurrences that support core populations and key factors that limit nest success; 2) Sample eggs at key sites to measure contaminants and effects of toxins on Least Bitterns; 3) Monitor effects of selenium, other toxins, and salts where water quality in breeding habitat has been compromised; 4) Determine the mini­mum patch size of suitable breeding habi­tat; 5) Study social structure and nest success of ?loose colonies? compared to non-colonial populations (Sterling 2008).
Population/Occurrence Delineation
Use Class: Breeding
Minimum Criteria for an Occurrence: Evidence of breeding (including historical); and potential recurring breeding at a given location, minimally a reliable observation of one or more breeding pairs in appropriate habitat. Be cautious about creating EOs for observations that may represent single breeding events outside the normal breeding distribution.
Separation Barriers: None.
Separation Distance for Unsuitable Habitat: 5 km
Separation Distance for Suitable Habitat: 5 km
Separation Justification: Separation distance arbitrary and does not attempt to delineate populations or metapopulations. No available information on dispersal characteristics. Presumably a significant number of birds end up nesting in sites at least several kilometers from their natal area.
Date: 23Jul2004
Author: Hammerson, G.
Population/Occurrence Viability
U.S. Invasive Species Impact Rank (I-Rank) Not yet assessed
NatureServe Conservation Status Factors Edition Date: 29Dec2017
NatureServe Conservation Status Factors Author: Rev. Davidson, A.D. (2017); Hammerson, G. (2011)
Management Information Edition Date: 12Jan2018
Management Information Edition Author: Rev. Davidson, A.D. (2018); GIBBS, J.P., AND S.M. MELVIN; REVISIONS BY G. HAMMERSON AND D.W. MEHLMAN
Management Information Acknowledgments: Parts of this abstract were originally published by the U.S. Fish and Wildlife Service in Schneider and Pence (1992). Funding for the preparation of the original document was made possible by the U.S. Fish and Wildlife Service, Newton Corner, MA. The authors are grateful to the library staff of the Smithsonian Institution, the Patuxent Wildlife Research Center (U.S. Fish and Wildlife Service), and the Peabody Museum at Yale University for bibliographic assistance. J. Longcore and D. McAuley facilitated a bibliographic computer search and provided other logistical support. The Maine Department of Inland Fisheries and Wildlife provided office space and computer access during the preparation of this report. Critical review of an earlier draft of the report was provided by P. Frederick, F. Reid, and B. Swift. K. E. and H. C. Gibbs provided much indirect support to J.P.G.
Element Ecology & Life History Edition Date: 27Dec1994
Element Ecology & Life History Author(s): HAMMERSON, G.

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