Molothrus ater - (Boddaert, 1783)
Brown-headed Cowbird
Taxonomic Status: Accepted
Related ITIS Name(s): Molothrus ater (Boddaert, 1783) (TSN 179112)
French Common Names: vacher à tête brune
Spanish Common Names: Tordo Cabeza Café
Unique Identifier: ELEMENT_GLOBAL.2.102638
Element Code: ABPBXB7030
Informal Taxonomy: Animals, Vertebrates - Birds - Perching Birds
Image 21635

© Dennis Donohue

 
Kingdom Phylum Class Order Family Genus
Animalia Craniata Aves Passeriformes Icteridae Molothrus
Genus Size: C - Small genus (6-20 species)
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Concept Reference
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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: Molothrus ater
Taxonomic Comments: Range-wide mtDNA data reveal no evidence of long-standing population separations (Ball and Avise 1992). See Fleischer et al. (1991) for information on gene flow between subspecies obsecurus and artemisiae in the Sierra Nevada, California.
Conservation Status
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NatureServe Status

Global Status: G5
Global Status Last Reviewed: 09Apr2016
Global Status Last Changed: 04Dec1996
Ranking Methodology Used: Ranked by inspection
Rounded Global Status: G5 - Secure
Nation: United States
National Status: N5 (19Mar1997)
Nation: Canada
National Status: N5B,NUN,N5M (08Jan2018)

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 (S5), Alaska (S3B), Arizona (S5), Arkansas (S5), California (SNRB,SNRN), Colorado (S5), Connecticut (S5B), Delaware (S5), District of Columbia (S4), Florida (SNR), Georgia (S5), Idaho (S5B), Illinois (S5), Indiana (S4S5), Iowa (S5B,S5N), Kansas (S5B,S3N), Kentucky (S5B,S4N), Louisiana (S5), Maine (S4N,S4S5B), Maryland (S5), Massachusetts (S5), Michigan (S5), Minnesota (SNRB), Mississippi (S5B,S5N), Missouri (SNR), Montana (S5B), Navajo Nation (S5B), Nebraska (S5), Nevada (S5B), New Hampshire (S5B), New Jersey (S5B,S5N), New Mexico (S5B,S5N), New York (S5B), North Carolina (S5B,S5N), North Dakota (SNRB), Ohio (S5), Oklahoma (S5), Oregon (S5), Pennsylvania (S5B,S5N), Rhode Island (S5B), South Carolina (SNR), South Dakota (S5B), Tennessee (S5), Texas (S5B), Utah (S5B,S3N), Vermont (S5B), Virginia (S5), Washington (S4N,S5B), West Virginia (S4N,S5B), Wisconsin (S5B), Wyoming (S5B,S5N)
Canada Alberta (S5B), British Columbia (S5), Manitoba (S5B), New Brunswick (S3B,S3M), Newfoundland Island (SNA), Northwest Territories (S4B), Nova Scotia (S2B), Ontario (S4B), Prince Edward Island (S1S2B), Quebec (S4?), Saskatchewan (S5B), Yukon Territory (S3B)

Other Statuses

IUCN Red List Category: LC - Least concern

NatureServe Global Conservation Status Factors

Range Extent Comments: Breeding range extends from southeast Alaska, northern British Columbia, and southern Mackenzie to Newfoundland, and south to central Mexico, southern Texas, Gulf Coast, and southern Florida (AOU 1983, Lowther 1993). Winter range extends from northern California, southern New Mexico, Kansas, Great Lakes region, New England, and Nova Scotia south to southern Baja California, Oaxaca, central Veracruz, Gulf Coast, and southern Florida (AOU 1983, Lowther 1993). This species historically occurred in the Great Plains west of the Mississippi River and often was associated with bison that stirred up insects and uncovered seeds. In the 1700s and 1800s, European settlers fragmented the once unbroken expanse of eastern deciduous forest, leading to cowbird expansion throughout the eastern United States and Canada (Mayfield 1965, 1977). Fragmentation of forests and cowbird expansion occurred more recently in the western United States (Verner and Ritter 1983).

Short-term Trend: Decline of 10-30%
Short-term Trend Comments: Increased range as forested areas were cleared during human settlement; increases were related to more abundant forest edge habitat and to increased food supplies (e.g., in pastures, feedlots). Overall, population trend is downward; breeding populations throughout the west coast ranges, northern Rockies, and much of the eastrn U.S. (except coastal areas from New England to the Deep South) are declining; populations within interior basins, ranges, deserts, and most of the northern prairie are increasing; perhaps declines are related to reduced availability of farm grain (data analysis by S. Droege, North American Breeding Bird Survey 1966-1993, Christmas Bird Count 1959-1988).

Other NatureServe Conservation Status Information

Distribution
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Global Range: Breeding range extends from southeast Alaska, northern British Columbia, and southern Mackenzie to Newfoundland, and south to central Mexico, southern Texas, Gulf Coast, and southern Florida (AOU 1983, Lowther 1993). Winter range extends from northern California, southern New Mexico, Kansas, Great Lakes region, New England, and Nova Scotia south to southern Baja California, Oaxaca, central Veracruz, Gulf Coast, and southern Florida (AOU 1983, Lowther 1993). This species historically occurred in the Great Plains west of the Mississippi River and often was associated with bison that stirred up insects and uncovered seeds. In the 1700s and 1800s, European settlers fragmented the once unbroken expanse of eastern deciduous forest, leading to cowbird expansion throughout the eastern United States and Canada (Mayfield 1965, 1977). Fragmentation of forests and cowbird expansion occurred more recently in the western United States (Verner and Ritter 1983).

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 AK, AL, AR, AZ, CA, CO, CT, DC, DE, FL, GA, IA, ID, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MS, MT, NC, ND, NE, NH, NJ, NM, NN, NV, NY, OH, OK, OR, PA, RI, SC, SD, TN, TX, UT, VA, VT, WA, WI, WV, WY
Canada AB, BC, MB, NB, NF, NS, NT, ON, PE, QC, SK, YT

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; NatureServe, 2005; WILDSPACETM 2002

Ecology & Life History
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Basic Description: A small bird (blackbird, cowbird).
General Description: This species has a heavy conical bill and a brown iris. Adult male has a plain brown head, neck, and upper chest, with the rest of the plumage glossy greenish black. Adult female is gray-brown above, paler below. Juvenile is gray-brown, usually with a finely streaked breast, with pale edgings on the feathers on the back; young males molting to adult plumage are patchy black, brown, and buff. Length is about 7.5 inches (19 cm).

EGGS: vary from pure white to bluish-white; entire egg is covered with chocolate brown or yellowish-tan specks or blotches, which are often heaviest around the larger end (Friedmann 1929). Eggs average 21.5 by 16.4 mm (Bent 1958), with a mass of 3.2-3.4 g (Walkinshaw 1983).

Reproduction Comments: This is an obligate brood parasite; females always deposit their eggs in the nests of other bird species,usually resulting in the death of some or all of the host species eggs or nestlings. Females never build a nest, incubate eggs, or tend young. Eggs have been found in the nests of more than 200 species, and young cowbirds are known to have been raised by at least 139 host species (Eastzer et al. 1980).

Females usually locate nests to parasitize by watching nest-building activities from a perch (Thompson and Gottfried 1976, 1981), especially in semi-open or open habitats. Females have also been observed walking on the ground in dense woods, quietly searching for activity or nests and noisily flapping through dense shrubbery, supposedly to flush incubating females (Norman and Robertson 1975).

Egg laying, in the nests of other bird species, generally extends from April to July, with most eggs laid in May-June. Individual females lay up to a few dozen eggs each season, but usually much fewer (Jackson and Roby 1992, Holford and Roby 1993). Females may lay eggs in different nests or lay multiple eggs in a single nest. They often, but not always, remove and eat one of the host eggs so that the clutch size is the same when the host female returns. The incubation period (11-12 days, sometimes 10) is often shorter than that of the host species. The host birds feed and tend the nestling and recently fledged cowbirds. A nestling cowbird may weigh up to 10 g when the host nestlings, weighing 1-3 g, hatch and the smaller nestlings are often crushed, crowded out of the nest, or starved (Friedmann 1929, 1963; Mayfield 1965). Cowbird nestlings stay in the nest 10-12 days, begin feeding themselves around day 20-22, and become independent when 25-39 days old (Woodward 1983). Young cowbirds just out of the nest are fed more by the host parent than an equivalent mass of host young. During this period, cowbird young perch at the same height and have the same home ranges as their host (Woodward 1983). Hatching success is not necessarily higher than that of their host species. In Pennsylvania, 57.3% of the eggs of several passerine hosts in unparasitized nests hatched but only 42.6% of the cowbird eggs hatched (Friedmann 1963). Approximately 15% of all cowbird eggs laid in Kansas grasslands resulted in a cowbird young leaving the nest (Zimmerman 1983). Cowbird survival in the nest is similar to host nestling survival but, in Vancouver, cowbird survival was much lower after leaving the nest, possibly because the young cowbird's loud begging calls may attract predators (Smith 1981).

Ecology Comments: Females hold and defend territories, whereas males have home ranges (0.4-2.5 ha in Ontario; Darley 1982) but do not defend a territory (Ankney and Scott 1982). Breeding and feeding areas of breeding females may be up to several kilometers apart: mean of 1.2 km in Illinois-Missouri; mean of 4.0 km in one area in California (Thompson 1994), and up to 6.7 km in the Sierra Nevada (Rothstein et al. 1980, 1984). NON-BREEDING: Roosts and forages in mixed flocks with red-wing blackbirds (AGELAIUS PHOENICEUS) and grackles (QUISCALUS QUISCULA).
Non-Migrant: Y
Locally Migrant: Y
Long Distance Migrant: Y
Mobility and Migration Comments: Northern breeding populations are long-distance migrants. Most migrant cowbirds arrive in northern breeding areas in late March-April. Adult males and females arrive first, followed two weeks later by yearling males, who are followed one week later by yearling females (Darley 1982). Southward migration begins in August, peaks in September, and extends into October. Specific timing varies with latitude.
Palustrine Habitat(s): FORESTED WETLAND, Riparian
Terrestrial Habitat(s): Cropland/hedgerow, Forest - Conifer, Forest - Hardwood, Forest - Mixed, Grassland/herbaceous, Old field, Savanna, Shrubland/chaparral, Suburban/orchard, Woodland - Conifer, Woodland - Hardwood, Woodland - Mixed
Habitat Comments: Breeding habitat includes woodland, forest (primarily deciduous), forest edge, city parks, suburban gardens, farms, and ranches. Cowbirds often are associated with forest-field edge habitat and clearings in forests. Feedlots, pastures, and fields with livestock also attract cowbirds, especially in predominately forested areas. In the Sierra Nevada of California, cowbirds feed in horse corrals, meadows with herds of cattle, and at bird feeders in towns, and few cowbirds are found more than 10 km from these food sources (Verner and Ritter 1983). In this same area, the number of cowbirds in meadows decreases as the distance of the meadow from pack stations and horse corrals increases. In migration and winter, cowbirds often occur in open situations, cultivated lands, fields, pastures, and scrub.

Before European settlement of the eastern U.S., Brown-headed Cowbirds were apparently limited to the open grasslands of the United States, mostly west of the Mississippi River. However, because the cowbirds need suitable perches for searching for nests to parasitize, the species was probably limited in the breeding season to prairie riparian corridors (Mayfield 1965).

The main reason for cowbird expansion outside of the Great Plains appears to be the fragmentation of once unbroken forest tracts into small pieces of forest interspersed with fields, pastures, roads, and towns (Mayfield 1977b, Kerlinger and Doremus 1981).

Adult Food Habits: Frugivore, Granivore, Invertivore
Immature Food Habits: Frugivore, Granivore, Invertivore
Food Comments: Feeds almost entirely in open habitats such as pastures, old fields, and prairies (Whitcomb et al. 1981). Diet includes insects (wasps, ants, beetles, grasshoppers and caterpillars), waste grain (corn, wheat, oats, sunflowers, rice), and seeds from noncommercial plants such as panic grass, ragweed (Ambrosia artemisifolia), barnyard grass (Echinochloa crusgalla) and yellow foxtail (Choetochloa glauca) (Friedmann 1929, Bent 1958). Females eat some of the eggs removed from nests (Condor 94:579-584). Nestlings are fed the typical insect diet of their host (Friedmann 1929).
Adult Phenology: Diurnal
Immature Phenology: Diurnal
Length: 19 centimeters
Weight: 49 grams
Economic Attributes Not yet assessed
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Management Summary
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Stewardship Overview: In areas where cowbirds are detrimentally impacting the reproduction of native birds, the following actions may be appropriate:

Reduce or eliminate the amount of forest-field edge habitat by removing fencerows and other perches in grasslands and designing preserves with proportionally larger amounts of "interior" habitat and less forest-field edge habitat.

Reduce or eliminate livestock herds and supplementary feeding areas, such as agricultural fields, feedlots, and bird feeders.

Trap and remove cowbirds.

Species Impacts: Cowbird parasitism apparently is contributing to the decline of some songbird populations by reducing the reproductive success of the host species. In the eastern U.S., birds have only recently been exposed to brood parasitism (Mayfield 1977, Brittingham and Temple 1983) and many species lack appropriate responses to minimize the impact of cowbird parasitism. Cowbird parasitism has contributed to the decline in numbers of endangered species such as Kirtland's warbler (DENDROICA KIRTLANDII), least Bell's vireo (VIREO BELLI PUSILLUS), southwestern willow flycatcher (EMPIDONAX TRAILLII EXTIMUS), and black-capped vireo (VIREO ATRICAPILLUS) (Robinson et al. 1995). Because cowbirds do not rely on just one host, parasitism pressure usually does not relax even when a single host's numbers decrease.

Friedmann (1963) summarized the reactions of hosts to cowbird eggs. Female hosts may 1) continue laying and incubating eggs without apparently noticing the cowbird eggs, 2) throw out the strange eggs, 3) bury the eggs by building a new floor to the nest, or 4) desert the nest. Most of the lower reproductive success experienced by hosts is from egg mortality caused by nest desertion or removal of host eggs by the female cowbird (Gates and Gysel 1978, Smith 1981). In Kirtland's warbler, unparasitized clutches averaged 4.4 eggs but parasitized nests contained an average of only 2.2 warbler eggs (Walkinshaw 1983). Nonparasitized dickcissel (SPIZA AMERICANA) nests averaged 4.0 eggs while parasitized nests contained an average of 2.4 dickcissel eggs (Zimmerman 1983). In most species, fewer host young fledge in parasitized nests than in unparasitized nests (Friedmann 1963). For instance, Smith (1981) estimated that cowbird parasitism reduced the average number of song sparrows (MELOSPIZA MELODIA) alive at age six days by 0.38 young.

Parasitism rates vary according to host species (Friedmann 1929, 1963) and habitat (Gates and Gysel 1978, Brittingham and Temple 1983) and vary within a species' range (Mayfield 1965, Zimmerman 1983). On a community-wide basis, some reported parasitism rates include 30.8% of the nests in central Pennsylvania, 13.2% in eastern Washington, 22.4% in Michigan and 1% and 21% in the Sierra Nevada (Ricklefs 1969, Verner and Ritter 1983). The birds most prone to parasitism are open-nesting species that are smaller than cowbirds, have a longer incubation period, feed insects to their young, and do not desert or destroy cowbird eggs. Most of the host species are flycatchers, finches, vireos, and warblers (Friedmann 1929). Within a species, the rate of parasitism can vary greatly. For instance, in a study in Ontario, there was no parasitism of red-eyed vireo (VIREO OLIVACEUS) nests but 72.2% of nests in Michigan were parasitized (Friedmann 1963). In the center of the dickcissel's range, parasitism rates vary from 31% to 95% (Zimmerman 1983).

Timing of breeding can also determine how seriously a host is affected by cowbird parasitism. Cowbirds generally breed from early May through early July; any single-brooded species that breeds outside of this period, and double-brooded species, are less affected by cowbird parasitism (Friedmann 1963, Zimmerman 1983). In a Michigan study, only 3% of the nests failed because of parasitism in May, July, and August, while 11% of the June nests were parasitized (Gates and Gysel 1978). Cowbird parasitism rates can differ among habitat types. The nesting substrate of the host may be important. In riparian habitats of central Iowa, cowbirds did not parasitize nests in deciduous or evergreen trees but did parasitize nests (at a rate of 6-8%) in deciduous saplings, shrubs, forbs, and grass (Best and Stauffer 1980). In a study on the effect of fire on field sparrows (SPIZELLA PUSILLA) in Iowa, Best (1979) found that two years before a burn 11% of the nests were parasitized by cowbirds. The first season after the late April burn, in which almost all of the herbaceous vegetation and litter was completely burned, no parasitized sparrow nests were found.

In Michigan, Gates and Gysel (1978) found that as distance from the forest-field edge increased, the rate of parasitism decreased. Johnson (pers. comm.) obtained similar results in Minnesota prairies. Nests less than 45 m from the prairie-forest edge had a higher probability of being parasitized than did prairie nests farther than 45 m from an edge (Johnson pers. comm.). One possible reason for the increased rate of parasitism near forest-field edges is because there is a higher density of host nests in edge habitats (Gates and Gysel 1978) and this may attract more cowbirds. In addition, edge provides observation perches nest-searching females. Brittingham and Temple (1983) studied the incidence of cowbird parasitism in Wisconsin deciduous forests in areas with openings such as campgrounds, fields, and logged areas. Forest nests within 100 m of a forest opening >0.2 ha had a parasitism rate of 65% while only 18% of the nests more than 300 m from an opening were parasitized. Cowbirds fed in open areas and parasitized nests that were the shortest distances from feeding areas.

Preserve Selection & Design Considerations: The main reason for the recent spread of the cowbird's breeding distribution appears to be a dramatic increase in the amount of field-forest edge habitat and increased availability of feeding areas. Therefore, the best long-term control of cowbirds probably is to eliminate as much field-forest edge habitat as possible and to reduce the number of nearby food sources. With endangered populations with very limited distributions, removing cowbirds is probably the optimal method for immediately reducing the incidence of cowbird parasitism.
Management Requirements: TRAPPING: An effective but labor-intensive method of rapidly decreasing cowbird parasitism in a small region is to live-trap the birds in decoy traps baited with grain or seeds (Shake and Mattsson 1975). The cowbirds are then removed and destroyed. In the Kirtland's warbler project in Michigan, blackbird traps operating between May 1 and July 15 successfully removed cowbirds within a 1 km radius area of the trap and during 1972-1981, 33,536 cowbirds were removed (Mayfield 1977). The parasitism rates of warbler nests dropped from 74.6% during 1957-1971 to 6.1% during 1972-1977 (Walkinshaw 1983). Cowbird removal substantially increased warbler reproductive success. During 1966-1971, an average of 0.8 warblers survived to leave the nest, and this number increased to 2.7 warblers per nest after cowbird removal (Walkinshaw 1983).

ELIMINATION OF EDGE: Because parasitism rates are highest near forest-field edges (Gates and Gysel 1978, Johnson pers. comm.), one method of cowbird control may be to eliminate as much edge habitat as possible. In grasslands, nest parasitism by cowbirds can be reduced by removing any structures that provide an observation perch, such as fences, posts, telephone lines, or fencerows. In all habitats, preserves can be designed with a high ratio of non-edge, "interior" habitat to edge habitat. Efforts should be made to avoid fragmenting large tracts of forest or grasslands into smaller pieces with more field-forest edge. Finally, the nearby presence of livestock or supplemental food (e.g., pastures, feedlots, grain fields) may attract cowbirds and should be reduced or eliminated, if possible.

See Glahn et al. (1991) for information on the impact of ground-based surfactant roost control treatments on local urban and agricultural blackbird/starling problems.

Population/Occurrence Delineation
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Group Name: Passerines

Use Class: Breeding
Subtype(s): Foraging Area, Nest Site, Nesting Colony
Minimum Criteria for an Occurrence: Evidence of historical breeding, or current and likely 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.

Mapping Guidance: Breeding occurrences include nesting areas as well as foraging areas.

For swallows and other species that have separate nesting and foraging areas, separations are based on nest sites or nesting areas, not to locations of foraging individuals. For example, nesting areas separated by a gap larger than the separation distance are different occurrences, regardless of the foraging locations of individuals from those nesting areas. This separation procedure is appropriate because nesting areas are the critical aspect of swallow breeding occurrences, tend to be relatively stable or at least somwhat predictable in general location, and so are the basis for effective conservation; foraging areas are much more flexible and not necessarily static.

Separation Distance for Unsuitable Habitat: 5 km
Separation Distance for Suitable Habitat: 5 km
Separation Justification: Significant dispersal and associated high potential for gene flow among populations of birds separated by tens of kilometers (e.g., Moore and Dolbeer 1989), and increasing evidence that individuals leave their usual home range to engage in extrapair copulations, as well as long foraging excursions of some species, make it difficult to circumscribe occurrences on the basis of meaningful population units without occurrences becoming too large. Hence, a moderate, standardized separation distance has been adopted for songbirds and flycatchers; it should yield occurrences that are not too spatially expansive while also accounting for the likelihood of gene flow among populations within a few kilometers of each other.

Be careful not to separate a population's nesting areas and foraging areas as different occurrences; include them in the same occurrence even if they are more than 5 km apart. Mean foraging radius (from nesting area) of Brown-headed Cowbird females was 4.0 kilometers in California, 1.2 kilometers in Illinois-Missouri (Thompson 1994). Yellow-headed Blackbirds, Brewer's Blackbirds, and probably Red-winged Blackbirds all forage up to 1.6 kilometers away from breeding colony (Willson 1966, Horn 1968). In one study, Brewer's Blackbirds were found as far as 10 kilometers from nesting area (Williams 1952), but this may be unusual.

For swallows and other parrerines with similar behavioral ecology, separation distance pertains to nest sites or nesting colonies, not to locations of foraging individuals. For example, nesting areas separated by a gap of more than 5 km are different occurrences, regardless of the foraging locations of individuals from those nesting areas. This separation procedure is appropriate because nesting areas are the critical aspect of swallow breeding occurrences, tend to be relatively stable or at least somwhat predictable in general location, and so are the basis for effective conservation; foraging areas are much more flexible and not necessarily static.

Be cautious about creating EOs for observations that may represent single breeding events outside the normal breeding distribution.

Unsuitable habitat: Habitat not normally used for breeding/feeding by a particular species. For example, unsuitable habitat for grassland and shrubland birds includes forest/woodland, urban/suburban, and aquatic habitats. Most habitats would be suitable for birds with versatile foraging habits (e.g., most corvids).

Date: 10Sep2004
Author: Hammerson, G.

Use Class: Migratory stopover
Subtype(s): Foraging Area, Roost Site
Minimum Criteria for an Occurrence: For most passerines: Evidence of recurring presence of migrating individuals (including historical) and potential recurring presence at a given location; minimally a reliable observation of 25 birds in appropriate habitat.

For swallows: Evidence of recurring presence of migrating flocks (including historical) and potential recurring presence at a given location; minimally a reliable observation of 100 birds in appropriate habitat (e.g., traditional roost sites).

Occurrences should be locations where the species is resident for some time during the appropriate season; it is preferable to have observations documenting presence over at least 7 days annually.

EOs should not be described for species that are nomadic during nonbreeding season: e.g., Lark Bunting.

Be cautious about creating EOs for observations that may represent single events.

Separation Distance for Unsuitable Habitat: 5 km
Separation Distance for Suitable Habitat: 5 km
Separation Justification: Separation distance somewhat arbitrary but intended to define occurrences of managable size for conservation purposes. Occurrences defined primarily on the basis of areas supporting concentrations of birds, rather than on the basis of distinct populations.

For swallows and other species with similar behavioral ecology, the separation distance pertains to communal roost sites rather than to foraging areas; the former tend to be more stable and specific over time than the latter.

Date: 03Sep2004
Author: Hammerson, G., and S. Cannings

Use Class: Nonbreeding
Subtype(s): Foraging Area, Roost Site
Minimum Criteria for an Occurrence: Any area used traditionally in the nonbreeding season (used for populations that are not resident in a location year-round). Minimally, reliable observations of 10 or more individuals in appropriate habitat for 20 or more days at a time. For G1-G3 species, observations of fewer individuals could constitute an occurrence of conservation value. Sites used during migration should be documented under the 'migratory stopover' location use class.

Separation Distance for Unsuitable Habitat: 5 km
Separation Distance for Suitable Habitat: 5 km
Separation Justification: Separation distance is necessarily arbitrary but attempts to balance the high mobility of birds with the need for occurrences of reasonable spatial scope. Note that a population's roost sites and foraging areas are parts of the same occurrence, even if they are more than 5 km apart.

For swallows and other species with similar behavioral ecology, the separation distance pertains to communal roost sites rather than to foraging areas; the former tend to be more stable and specific over time than the latter.

Date: 03Sep2004
Author: Hammerson, G.

Use Class: Nonmigratory
Minimum Criteria for an Occurrence: Occurrences are based on evidence of historical presence, or current and likely recurring presence, at a particular location. Such evidence minimally includes collection or reliable observation and documentation of one or more individuals in or near appropriate habitat.

These occurrence specifications are used for nonmigratory populations of passerine birds.

Separation Barriers: None.
Separation Distance for Unsuitable Habitat: 5 km
Separation Distance for Suitable Habitat: 5 km
Separation Justification: Significant dispersal and associated high potential for gene flow among populations of birds separated by tens of kilometers (e.g., Moore and Dolbeer 1989), and increasing evidence that individuals leave their usual home range to engage in extrapair copulations, as well as long foraging excursions of some species, make it difficult to circumscribe occurrences on the basis of meaningful population units without occurrences becoming too large. Hence, a moderate, standardized separation distance has been adopted for songbirds and flycatchers; it should yield occurrences that are not too spatially expansive while also accounting for the likelihood of gene flow among populations within a few kilometers of each other.

Be careful not to separate a population's nesting areas and breeding-season foraging areas as different occurrences; include them in the same occurrence even if they are more than 5 km apart. Blue jays have small summer home ranges but fly up to 4 kilometers to harvest mast (Tarvin and Woolfenden 1999). Flocks of pinyon jays range over 21-29 square kilometers (Ligon 1971, Balda and Bateman 1971); nesting and foraging areas may be widely separated. Tricolored blackbirds forage in flocks that range widely to more than 15 kilometers from the nesting colony (Beedy and Hamilton 1999).

Unsuitable habitat: Habitat not normally used for breeding/feeding by a particular species. For example, unsuitable habitat for grassland and shrubland birds includes forest/woodland, urban/suburban, and aquatic habitats. Most habitats would be suitable for birds with versatile foraging habits (e.g., most corvids).

Date: 10Sep2004
Author: Hammerson, G.
Notes: These specs pertain to nonmigratory species.
Population/Occurrence Viability
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U.S. Invasive Species Impact Rank (I-Rank) Not yet assessed
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Authors/Contributors
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NatureServe Conservation Status Factors Edition Date: 01Feb2010
NatureServe Conservation Status Factors Author: Hammerson, G.
Management Information Edition Date: 26Jun1985
Management Information Edition Author: DRILLING, N.; REVISIONS BY D.W. MEHLMAN AND G. HAMMERSON
Element Ecology & Life History Edition Date: 01Feb2010
Element Ecology & Life History Author(s): Hammerson, G.

Zoological data developed by NatureServe and its network of natural heritage programs (see Local Programs) and other contributors and cooperators (see Sources).

References
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  • Alabama Breeding Bird Atlas 2000-2006 Homepage. 2009. T.M. Haggerty (editor), Alabama Ornithological Society. Available at http://www.una.edu/faculty/thaggerty/BBA%20website/Index.htm.

  • Alabama Ornithological Society. 2006. Field checklist of Alabama birds. Alabama Ornithological Society, Dauphin Island, Alabama. [Available online at http://www.aosbirds.org/documents/AOSChecklist_april2006.pdf ]

  • Allen, C. R., S. Demarais, and R. S. Lutz. 1994. Red imported fire ant impact on wildlife: an overview. The Texas Journal of Science 46(1):51-59.

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