The Brown-headed Cowbirds (Molothrus ater) is a small icterid with a stocky build. It stands out for both its appearance and its parenting style. The males have a shiny black body with a chocolate-brown, rich head. Females, on the other hand, are dressed in a soft, grey-brown with a paler throat. The juveniles are usually streaked with brown to make them more camouflaged.
These birds can be found in large flocks foraging in grassy fields, open woodlands and farmlands. They are easy to spot in winter when they flock with other blackbirds or visit bird feeders. The most interesting thing about them is that they are Brood Parasites, which means they lay their eggs inside the nests and leave parenting to other birds!
Description
Adults have a short finch-like bill and dark eyes. The adult male can be identified by his iridescent black plumage with a brown head. The adult female is mostly grey with a pale throat and fine streaking on the plumage below.
Males and females have different sizes and colouration. The males are slightly bigger, measuring between 19 and 22 cm, with a wingspan of around 36 cm. The combination of black glossy feathers with a brownish head is easy to recognise, especially when the sun is bright. The birds have a small tail, a pointed, stout beak and a compact body.
The females are smaller and measure 16-20 cm, with wingspans ranging from 32-38 cm. The duller brown tones are more subdued, with some faint streaking along the chest. They also have the same pointed grey beak. The main differences between subspecies are the beak shape and wing size. These subtle differences help researchers understand the bird’s geographical range and behaviour.
Behaviour
Brown-headed Cowbirds are highly social and mobile. They are often seen in flocks of small cowbirds during breeding season, and large roosts in the cooler months. They are usually found perched on fences or shrubs, or tree branches, scanning for insects and potential nest hosts.
The Brown-headed Cowbird’s brood parasitism is one of its most interesting features. Females do not build their nests; instead, they lay eggs in those of other species of birds–usually smaller woodpeckers. One female can lay as many as 30 eggs per season. The cowbird’s eggs hatch sooner than the host species, and they grow faster.
Cowbirds do not just lay their eggs at random. They carefully observe potential hosts, sneaking into the nest at dawn and laying an egg. Some host birds raise the chicks as their own, while others reject the egg or abandon the nest. Cowbirds are parasitising more than 60 native bird species in regions such as Illinois. This has led to concerns over their impact on populations that are vulnerable.
Diet / Feeding
The Brown-headed Cowbird is a Ground-Foraging Omnivore. This means that they feed mainly by walking along the ground, eating both plant and animal matter. Around 75 per cent of their diet is plant material. This includes seeds and fruits during colder months, when insects are scarcer.
The grazing of large animals such as cattle can cause insects to be stirred up. They eat beetles and grasshoppers as well as spiders, moths, and other arthropods. During breeding season, females consume calcium-rich materials such as mollusc shells to aid in egg development. Both male and female cowbirds are known to consume eggs, possibly as an extra source of nutrients such as calcium. According to Bug Guide, this species is well documented.
Song / Calls
Brown-headed Cowbirds’ males sing an evocative bubbling song, which sounds like “glug glug gleeeeee”, usually from perches to attract mates or claim territories. The Brown-headed Cowbirds also use sharp calls or soft chattering to keep in touch when foraging, flying in groups or claiming territory. According to IUCN Red List, this species is well documented.
Distribution / Range
Brown-headed Cowbirds are permanent residents in the southern parts of their range, while the northern birds breed in open or semi-open country across most of North America and migrate to the southern United States and Mexico for the winter.
They often migrate in flocks, sometimes mixed with Red-winged Blackbirds or European Starlings.
Nesting / Breeding
Brown-headed Cowbird females can produce 36 eggs in a season.
This brood parasite will deposit its eggs in the nests of other small passerines (perching birds), particularly those that build cup-like nests, such as the Yellow Warbler.
The young cowbirds are then raised by the host parents. Over 140 different species of birds are known to have raised cowbirds. Host parents may notice the cowbird egg, and some species may destroy the eggs (such as the Grey Catbirds), while others may simply build a new layer over the bottom of the original nest.
Brown-headed cowbird nestlings are sometimes expelled from the nest.
Conservation Status
Currently, the IUCN Red List classifies the brown-headed cowbird as being of “Least Concern”. The Brown-headed Cowbird’s population is stable and widespread, largely due to the human-altered landscapes such as farms and suburbs that suit its breeding and feeding habits.
Its success, however, comes at the expense of other birds. Cowbirds are known to harm native songbirds. This is especially true for those birds with small populations or in decline. The species is still abundant in North America, even though its numbers have decreased slightly over the past few decades. This decline was primarily due to habitat changes, conservation efforts and other factors. It is important to monitor their population and the impact they have on other bird species, especially when there are endangered hosts.
