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The Brown-headed Cowbird: An Abundant Brood Parasite

The Brown-headed Cowbird: An Abundant Brood Parasite

The most common brood parasite in North America, the Brown-headed Cowbird, is a relatively small, stocky passerine belonging to the Icteridae family – the blackbird family. This species is sexually dimorphic, but both sexes have a short, thick bill and a black eye.

The male is a bird with a brown head and black body that often has a glossy appearance. The females are brown overall with a lighter-coloured head and underside that is finely streaked. The juveniles are brown with a well-streaked underside and a scaly patterned back.

The Brown-headed Cowbird
  • Scientific nameMolothrus ater
  • Lifespan – 17 years (maximum recorded)
  • Size – 6.3 to 8.7 in (16 to 22 cm)
  • Weight – 1.3 to 1.8 oz (38 to 50 g)
  • Wingspan – 12.6 to 15 in (32 to 38 cm)
  • Status – Least concern

Vocalisations

The Brown-headed Cowbird produces a range of songs and calls. The typically heard song is a series of one-second-long, liquid-like, low-pitched gurgling notes followed by whistles made by the male. In addition to the male’s song, both sexes produce a range of chatters, clicks and whistling calls.

The flight call is a series of whistling calls intertwined with trills and buzzes. Females make a rolling chatter call that attracts males. Males also make a chatter call but less frequently than females. When feeding, males and females also produce a cluck note.

Distribution

The Brown-headed Cowbird is native to North America, where it is distributed throughout most of the United States, from the west coast to the east coast, north into southern Canada and south into northern Mexico.

It is particularly abundant in the central United States and the Canadian Prairies. The highest density of breeding birds occurs in North Dakota.

The populations in the north are short-distance migrants that may migrate as far as 530 miles to their wintering range in Mexico and the southern United States. Populations in parts of the central and southern United States do not migrate and remain year-round. This species becomes more localised outside the breeding season and widespread during the breeding season.

Brown-headed Cowbirds were once restricted to the grasslands of central North America but have now expanded their range dramatically because of human development and forest clearing. In the process, their numbers have also increased exponentially.

The Brown-headed Cowbird Map Distribution
The Brown-headed Cowbird Map Distribution

Habitat

Brown-headed Cowbirds occur in grasslands containing scattered low trees, pastures, fields, lawns, forest edges, woodland edges, prairies, thickets, orchards, meadows and suburban areas.

Food

The Brown-headed Cowbird mainly feeds on crop grains and seeds from grasses and weeds, making up half of their diet during summer. During winter, the percentage of grain and seeds in their diet increases to 90%. They also feed on insects such as caterpillars, beetles and grasshoppers, which comprise approximately one-quarter of their diet.

Female blackbirds eat other food sources because of the amount of energy they expend during reproduction. They require more significant amounts of calcium than the males because of the egg formation, so they also eat egg shells (usually from the host’s eggs) and snail shells to increase their calcium intake.

Feeding Behaviour

Brown-headed Cowbirds are named not only because of their brown heads but because of their relationship with cows and other livestock. Back in the 1800s, these birds would follow Bison over the Great Plains and catch insects in the air that were disturbed by the Bison’s movement and took flight.

Nowadays, the same symbiosis is seen with cows, in particular, who flush out insects that the birds eat.

The Brown-headed Cowbird

This species forages on the ground when not following larger animals around. They usually congregate and feed in huge mixed-species flocks containing other blackbirds, starlings and grackles during winter and while migrating. When they are not feeding, they perch on branches in trees.

Courtship

During the breeding season, male Brown-headed Cowbirds put on a delightful display to whoo females and attract a mate. The males often gather in groups to display to females in open areas such as lawns, where they walk with swagger to impress females.

The display put on by males also includes singing while lifting their wings, fanning their tails, puffing up their back and chest feathers and bowing. During the display, the female selects a mate based on the quality of their singing and flight performances.

Breeding

Brown-headed Cowbirds have a complex mating system which ranges from monogamy to a mix of polygony and monogamy to complete promiscuity. The mating type depends on the distribution of host nests and the ratio of males to females in a given location.

Monogamous and polygonous relationships exist when there is a high density of host nests – meaning the females have small home ranges and the males can guard their mates. In areas with widely distributed host nests, females range further and mate with many males, resulting in promiscuous relationships.

Nesting

Brood Parasitism

Brown-headed Cowbirds have a fascinating strategy for producing young and raising them. That is because they are brood parasites, which means they depend on other species or individuals of the same species to raise their young.

To be successful, the brood parasite manipulates the host using strategies such as egg mimicry to make them raise the young as if it were their own.

As a result of their reproduction strategy, Brown-headed Cowbirds never make their own nests or even raise their young. The energy that is saved by not building a nest is used in significant amounts to produce eggs.

Females may lay an average of almost one egg per day for the entire breeding season. The result is an average total of around 36 eggs laid in a season, and in exceptional circumstances, over 70 eggs may be laid by a single female.

Nest Habitats

Brown-headed Cowbirds lay their eggs in various nest types and habitats that vary depending on how each host species builds the nest and where they place it.

Some common nest types are those found in marshes, dome-shaped nests on the forest floor, cup-shaped nests inside shrubs, treetop nests and tree cavity nests. One of the critical factors in determining the host nest is that the host’s eggs need to be smaller than the Brown-headed Cowbird’s eggs.

Egg Laying

The female Brown-headed Cowbirds locate host nests by watching for the nest-building activity of host species or by scaring host birds off their nests. Before laying her eggs, the female Brown-headed Cowbird visits the host nest multiple times when the owner is away from the nest.

On the day of egg laying or one day before, the female Brown-headed Cowbird removes or eats at least one of the host’s eggs. She then lays her own egg or eggs in place of the removed egg. As a result, the clutch size can vary from only one egg to up to seven eggs in a single nest. Once the eggs have been laid, the females leave the eggs to be raised by foster parents.

Host Species

Brown-headed Cowbirds have been documented to lay their eggs in and parasitise over 220 bird species’ nests. Recent genetic studies suggest that each female has a preferred host species that they specialise in targeting. Some of the more commonly used host species are the following:

  • Red-winged Blackbird (Agelaius phoeniceus)
  • Ovenbird (Seiurus aurocapilla)
  • Golden-crowned Kinglet (Regulus satrapa)
  • Red-eyed Vireo (Vireo olivaceus)
  • Spotted Towhee (Pipilo maculatus)
  • Eastern Towhee (Pipilo erythrophthalmus)
  • Yellow Warbler (Setophaga petechia)
  • Song Sparrow (Melospiza melodia)
  • Chirping Sparrow (Spizella passerina)

While 140 hosts have been documented to raise young Brown-headed Cowbirds successfully, many hosts are unsuccessful, including the following host species:

  • Ferruginous Hawk (Buteo regalis)
  • Blue-winged Teal (Spatula discors)
  • Virginia Rail (Rallus limicola)
  • Spotted Sandpiper (Actitis macularius)
  • Killdeer (Charadrius vociferus)
  • Upland Sandpiper (Bartramia longicauda)
  • Wilson’s Phalarope (Phalaropus tricolor)
  • Common Tern (Sterna hirundo)
  • California Gull (Larus californicus)
  • Red-headed Woodpecker (Melanerpes erythrocephalus)
  • Ruby-throated Hummingbird (Archilochus colubris)

Many host species can’t distinguish Brown-headed Cowbird eggs from their own, but some victims of this parasitism recognise the Brown-headed Cowbird eggs. The problem for small species like the Yellow Warbler is that they are too small to remove the Brown-headed Cowbird eggs from the nest. Instead, they build a new nest on top of the old parasitised nest. Larger birds that recognise the eggs don’t have such an issue. They throw the eggs out of the nest, eat them or puncture them to prevent development within the egg.

Nestling Growth

The Brown-headed Cowbird eggs are a whitish or greyish colour with grey or brown spots. The incubation period is short – only 10 to 12 days. That gives Brown-headed Cowbirds a competitive advantage over their hosts since the Brown-headed Cowbird nestlings usually have a shorter incubation period and hatch earlier than the host’s nestlings. That allows the Brown-headed Cowbird chicks to get more food from the foster parents and outgrow their actual chicks.

The development of Brown-headed Cowbird chicks is also usually far quicker than the other chicks in the nest – taking only 8 to 13 days to grow and learn to fly. As a result, Brown-headed Cowbird nestlings often kill the smaller host species nestlings or throw the eggs out of the nest. After 25 to 29 days, the chicks become entirely independent of their hosts.

Conservation

Brown-headed Cowbirds were once limited to the Great Plains of the central United States in the 1800s. Since then, forest clearing and fragmentation because of development opened up a passage for this species to expand their range and become more abundant, particularly in the eastern United States.

The Brown-headed Cowbird Close Up

They are now a prevalent and widespread species with a breeding population estimated to comprise 130 million individuals. However, the population size has decreased slightly over the past five decades.

Brown-headed Cowbirds are captured for the pet trade in some areas, but their biggest threat is birds of prey and mammals. The following species predate them:

  • Barred Owl (Strix varia)
  • Northern Goshawk (Accipiter gentilis)
  • Broad-winged Hawk (Buteo platypterus)
  • Sharp-shinned Hawks (Accipiter striatus)
  • Cooper’s Hawk (Accipiter cooperii)
  • Blue Jay (Cyanocitta cristata)
  • Northern Saw-whet Owl (Aegolius acadicus)
  • Northern Flying Squirrel (Glaucomys sabrinus)

The massive abundance of this species and their brood parasitism strategy poses a threat to many species because it decreases host reproduction success. Unfortunately, some species that the Brown-headed Cowbird targets as a host are endangered. Species such as the Kirtland’s Warbler (Setophaga kirtlandii) and Black-capped Vireo (Vireo atricapilla) have small populations and are severely threatened by this parasitism.

Conclusion

The Brown-headed Cowbird is a fascinating, very successful, widespread species with an interesting reproduction strategy in the form of brood parasitism. They have benefitted from human development, which allowed them to expand their range in the east.

Brown-headed Cowbirds may be attracted to your yard if you have open lawns and feeders with plentiful seeds and grain. If a wintering mixed flock of Brown-headed Cowbirds and other blackbird species were to visit your yard, you’d be sure to hear them, and the spectacle would be great to witness.

Brown-headed Cowbirds make a large range of calls and songs. When looking for this species, they are best found by listening out for the male’s gurgling song and the female’s chattering call. In winter, they are often seen amongst mixed flocks when they roost communally in flocks of upwards of 100,000 individuals.

To many people, the Brown-headed Cowbird is considered a nuisance because they kill the eggs and juveniles of many more beautiful songbirds, including the endangered Black-capped Vireo and Kirtland’s Warbler. However, the behaviour is natural to this species, which is why they have become such a successful species.

Ultimately, the Brown-headed Cowbird extended its range, became so abundant and is now negatively impacting so many species because humans cleared forests and caused fragmentation, which opened the door for the Brown-headed Cowbird to have a range expansion. Humans disrupted the natural processes, and the forest barrier that prevented the Brown-headed Cowbird from expanding its range was broken down.

Overall, the Brown-headed Cowbirds have their place in the wilderness and should be appreciated for their clever breeding strategy, grand gatherings during winter and beautiful songs that fill the air during courtship displays.