Brown-headed Cowbird - Montana Field Guide

Brown-headed Cowbird - Molothrus ater

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Native Species

Global Rank: G5
State Rank: S5B

Agency Status
USFWS: MBTA
USFS:
BLM:
PIF:

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Length: 19 cm; heavy conical bill and a brown iris; walks with the squared tail uptilted. 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 paler above, heavily streaked below, with pale edgings on the feathers on the back; young males molting to adult plumage are patchy black, brown, and buff; mass of adult male is 39 to 57 g, female is smaller. 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 to 3.4 g (Walkinshaw 1983).

For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see Marks et al. 2016, Birds of Montana.

Montana Range Range Descriptions

All Ranges

Summer

Migratory

(Click legend blocks to view individual ranges)

Western Hemisphere Range

Relative Density

Recency

SUMMER (Feb 16 - Dec 14)

Direct Evidence of Breeding

Indirect Evidence of Breeding

No Evidence of Breeding

WINTER (Dec 15 - Feb 15)

Regularly Observed

Not Regularly Observed

(Observations spanning multiple months or years are excluded from time charts)

In the Bozeman area, normal migration periods are May 8 to 30 with no discernible movement in the fall.

Species prefers areas with low or scattered trees among grassland vegetation—woodland edges, brushy thickets, prairies, fields, pastures, orchards, or even residential areas.

Details on Creation and Suggested Uses and Limitations
How Associations Were Made
We associated the use and habitat quality (common or occasional) of each of the 82 ecological systems mapped in Montana for vertebrate animal species that regularly breed, overwinter, or migrate through the state by:
1. Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Foresman 2012, Adams 2003, and Werner et al. 2004);
2. Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
3. Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
4. Calculating the percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system to get a measure of "observations versus availability of habitat".
Species that breed in Montana were only evaluated for breeding habitat use, species that only overwinter in Montana were only evaluated for overwintering habitat use, and species that only migrate through Montana were only evaluated for migratory habitat use. In general, species were listed as associated with an ecological system if structural characteristics of used habitat documented in the literature were present in the ecological system or large numbers of point observations were associated with the ecological system. However, species were not listed as associated with an ecological system if there was no support in the literature for use of structural characteristics in an ecological system, even if point observations were associated with that system. Common versus occasional association with an ecological system was assigned based on the degree to which the structural characteristics of an ecological system matched the preferred structural habitat characteristics for each species as represented in scientific literature. The percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system was also used to guide assignment of common versus occasional association. If you have any questions or comments on species associations with ecological systems, please contact the Montana Natural Heritage Program's Senior Zoologist.

Suggested Uses and Limitations
Species associations with ecological systems should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning. These potential lists of species should not be used in place of documented occurrences of species (this information can be requested at: mtnhp.org/requests) or systematic surveys for species and evaluations of habitat at a local site level by trained biologists. Users of this information should be aware that the land cover data used to generate species associations is based on imagery from the late 1990s and early 2000s and was only intended to be used at broader landscape scales. Land cover mapping accuracy is particularly problematic when the systems occur as small patches or where the land cover types have been altered over the past decade. Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections). Finally, although a species may be associated with a particular ecological system within its known geographic range, portions of that ecological system may occur outside of the species' known geographic range.

Literature Cited
- Adams, R.A. 2003. Bats of the Rocky Mountain West; natural history, ecology, and conservation. Boulder, CO: University Press of Colorado. 289 p.
- Dobkin, D. S. 1992. Neotropical migrant land birds in the Northern Rockies and Great Plains. USDA Forest Service, Northern Region. Publication No. R1-93-34. Missoula, MT.
- Foresman, K.R. 2012. Mammals of Montana. Second edition. Mountain Press Publishing, Missoula, Montana. 429 pp.
- Hart, M.M., W.A. Williams, P.C. Thornton, K.P. McLaughlin, C.M. Tobalske, B.A. Maxell, D.P. Hendricks, C.R. Peterson, and R.L. Redmond. 1998. Montana atlas of terrestrial vertebrates. Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT. 1302 p.
- Hutto, R.L. and J.S. Young. 1999. Habitat relationships of landbirds in the Northern Region, USDA Forest Service, Rocky Mountain Research Station RMRS-GTR-32. 72 p.
- Maxell, B.A. 2000. Management of Montana's amphibians: a review of factors that may present a risk to population viability and accounts on the identification, distribution, taxonomy, habitat use, natural history, and the status and conservation of individual species. Report to U.S. Forest Service Region 1. Missoula, MT: Wildlife Biology Program, University of Montana. 161 p.
- Werner, J.K., B.A. Maxell, P. Hendricks, and D. Flath. 2004. Amphibians and reptiles of Montana. Missoula, MT: Mountain Press Publishing Company. 262 p.

Commonly Associated with these Ecological Systems
Forest and Woodland Systems
Grassland Systems
Human Land Use
Recently Disturbed or Modified
Shrubland, Steppe and Savanna Systems
Wetland and Riparian Systems
Occasionally Associated with these Ecological Systems
Forest and Woodland Systems
Sparse and Barren Systems
- Great Plains Badlands
Wetland and Riparian Systems
- Emergent Marsh
- Great Plains Open Freshwater Depression Wetland

Diet consists mainly of anthropods and seeds.

Species is a brood parasite; nests are chosen by females, but are that of another species. Care given to cowbird eggs and young is provided by the host and reflects characteristics of that species. Brood consists of four to five eggs; incubation time 10 to 13 days. Young able to fly 10 to 11 days after hatch.

Literature Cited AboveLegend: View Online Publication
- Bent, A.C. 1958. Life histories of North American blackbirds, orioles, tanagers, and their allies. U.S. Natl. Mus. Bull. 211. Washington, D.C.
- Friedmann, H. 1929. The cowbirds: a study in the biology of social parasitism. Charles C. Thomas, Publisher, Springfield, Illinois.
- Marks, J.S., P. Hendricks, and D. Casey. 2016. Birds of Montana. Arrington, VA. Buteo Books. 659 pages.
- Walkinshaw, L.H. 1983. Kirtland's warbler: the natural history of an endangered species. Cranbrook Institute of Science, Bloomfield Hills, Michigan. 207 pp.
Additional ReferencesLegend: View Online Publication
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