Brown-headed Cowbird

Brown-headed Cowbirds are easily recognized as members of the blackbird family (Icterids). Gregarious birds, they are often found in mixed flocks with other blackbirds and starlings. These flocks can number in the hundreds of thousands in the fall and winter after the breeding season. 
It is a year round resident of the eastern, southern US and coastal California. During breeding season their range is widespread across North America. It winters in the Southeastern US and Mexico.

Adult Male (spring/summer)

Rich brown head and black body with green glossy sheen. The bill is stout: almost finch like at first glance. Wings are long and pointed and tail is relatively short.

Adult Female (spring/summer)

Gray-brown overall, with a paler head and underparts. The breast has fine, faint streaks.

Juvenile

Scaly back: all feathers fringed pale. Underparts are paler with distinct fine streaks on the breast.

General:

Stocky blackbird with stout bill. Length: 19-22cm. Wing: 36cm. Weight: 42-50grams.

Behaviour:

Roost colonially; tend to congregate in large mixed flocks in open fields. They eat grain and insects, foraging on the ground. Flight is direct with constant wing beats. They are the most well known of North American practitioners of brood parasitism: eggs are laid in another species’ nest to be hatched and raised there.

Habitat:

Brown-headed Cowbirds tend to congregate in open fields, often near livestock, but will occupy forest edges during breeding season to utilize host nests. Habitat can vary with time of day, with breeding behavior taking place in the morning (near woodland nests) and foraging in open fields occurring later in the day.

Information:

Unable to build their own nests, Brown-headed Cowbirds rely on other birds to hatch and rear their young by laying eggs in other species’ nests. When placing her egg in a host nest, a female cowbird will often peck or dislodge a host egg. A female can lay as many as 40 eggs in a season. While the young cowbirds may not oust host eggs and young from the nest per se, cowbird eggs do hatch sooner, grow faster and put a lot of pressure on the host young and parent by competing for food. If the host species is larger than the cowbirds, the host species is usually able to bear the burden, but in smaller species the young are usually cannot compete and starve. For this reason cowbirds are considered to be a contributing factor to the decline of some songbird species. They lay their eggs in the nests of over 220 species, with the most common being the Yellow Warbler, Song and Chipping sparrows, Red-eyed Vireo, Eastern and Spotted towhees, and Red-winged Blackbird.

Similar species:

Conservation Status:

Originally associated with American Bison, the Brown-headed Cowbird’s range has increased with the clearing of fields and the introduction of livestock. It is a species listed as Least Concern, having thrived with human influence beginning in the 1800s as forests were cleared and the spread of agricultural lands increased its feeding habitat.

As a brood parasite, Brown-headed Cowbird capture rate peaks in mid-summer during the breeding season. Cowbirds are short distance migrants, moving south for the winter as seen by our zero capture rate between September – March.

Molt Summary:

PF: HY incomplete-complete (Jul-Oct), PB: AHY complete (Jul-Oct); PA absent-limited.
Preformative molt is rarely complete with a varying number of underwing covs possibly retained.

Birds in full juvenal plumage like this hatch year (HY) bird in May can easily be separated from adults. In addition to other characteristics of birds in juvenal plumage, the feathers of the underwing develop later than other feathers, hence, this area is often devoid of feathers for a period after the bird has fledged as shown in the photo below.

The following two photos show the underwing coverts of second year (SY) males, the red arrows pointing to the darkly pigmented and glossy black replaced coverts.
Although replaced underwing coverts are easy to see on males as in these photos, the contrast between retained and replaced feathers on females is much more difficult to detect and due to this some are impossible to age.

This hatch year (HY) male in July had already completed its incomplete preformative molt. The dorsal view of this wing shows very fresh replaced feathers – the inner primaries and primary tips showing the brownish wash contrasting with the blacker replaced secondaries producing an obvious molt limit (red arrow).

Cowbirds are members of the Icteridae family and can have complete preformative and adult prebasic molts although the preformative molt is rarely complete with a number of underwing coverts normally retained.

Therefore, the greater underwing coverts of adult birds show little or no contrast between adjacent feathers – males being uniformly blackish and females like this ASY in May below grayish brown.