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BROOD PARASITE
Brown-headed Cowbird (Molothrus ater)
Distribution

Breeding: Occupies most of our continent south of the Arctic. Range has expanded east and west from Great Plains with clearing of forests and introduction of cattle, with which it now associates.

Winter: Winters throughout much of U.S. and south to southern Mexico.

Highly gregarious at all seasons; postbreeding flocks in the fall generally number 50-200. Feeds and roosts in enormous flocks with other blackbirds and starlings, especially in winter.

Larry McQueen illustration of Brown-headed Cowbird pair

Brown-headed Cowbird range mapBreeding habitat

Woodland, forest (especially deciduous), forest edge, grassland, farmlands, suburban gardens, shade trees.

Description

The smallest of our blackbirds (6–8 inches or 15–20 centimeters); with a stubby, conical bill and relatively short tail. Dark eye in all plumages. Western birds are slightly smaller than eastern birds.male Brown-headed Cowbird by Bob Schmitz CLO slide #626.4

Male: Brown head contrasting with glossy black-green body.

Female: Uniformly drab gray-brown.

Juvenile: Same size as adult cowbird; paler than adult female and more heavily streaked below; pale feather edgings give a scaled appearance to the back.

Vocalizations

Song: Male’s song is a squeaky gurgling, usually ending with a few high, squeaky whistles. The female’s chatter is often given in unison with the male’s song during courtship.

Calls: Calls include a harsh rattle or chatter, usually given by the female, a short chuck note given by both sexes when alarmed, and squeaky whistles given by the male, especially in flight. The loud begging calls of fledgling cowbirds are distinctive.

Behavior and displays

Male

If you observe any of the following displays, mark "males displaying" in the "Cowbird" section of the Visit Form.

• Bill-Tilt: Bird lifts head and points bill skyward. Feathers may be sleeked. Context: Given when competing for dominance with another bird. Sometimes followed by topple-over display.

• Topple-Over: This display has been described by writers as everything from gallant bowing to motions suggestive of extreme nausea. The bird fluffs body feathers, arches neck, spreads tail and wings, and seems to fall on his face. Display often ends with a brief bill-wiping. Song often accompanies display. Context: Given on perches or on the ground. Given by males to females or to other males; may function as courtship to females and as a competitive display to other males.

• Head-Forward: Bird fluffs body feathers, raises wings, and thrusts head forward. Context: This display occurs during territory defense and during feeding when many birds are present. May be given toward other species as well.

Female

Female Brown-headed Cowbirds perform some of the same displays as males of the species. Females tend to be "sneaky" when searching for host nests, skulking quietly through the undergrowth or canopy leaves. A female Brown-headed Cowbird often locates a potential host nest during its construction phase. She then regularly visits the nest prior to laying while the host species are absent. The female cowbird lays eggs at dawn; she removes (and occasionally eats) an egg of the host the day before or the day after the parasitic egg is laid. If only one host egg is present, she does not remove it (otherwise the host might abandon its now eggless nest).

• Bill-Tilt is performed when competing for dominance with another female cowbird.

• Head-Forward may occur during territory defense.

Nesting

Height: Brown-headed Cowbirds are known to lay their eggs in the nests of treetop species as well as ground nesters.

Eggs: White to grayish-white, evenly marked with browns, often heavier markings at large end. Host may desert nest, build floor over cowbird egg, throw the egg out, or accept it. Approximately one-third of all parasitized nests hold more than one cowbird egg. Brown-headed Cowbird eggs in a Blue-winged Warbler's nest CLO slide#626.3
Two Brown-headed Cowbird eggs in the nest of a Blue-winged Warbler

A female Brown-headed Cowbird has a long reproductive period with an extraordinarily short interval between clutches. In fact, this cowbird is the only wild passerine ever reported not to show regression of ovaries and oviducts following clutch completion. Indeed, the physiological demarcation between clutches sometimes is not at all clear, leading ornithologists to characterize female cowbirds as "passerine chickens." Each female’s laying cycle appears adapted to take advantage of a continuous supply of host nests for about a two-month period. An average female lays about 80 eggs, 40 per year for two years. About 3% of those 80 eggs reach maturity—an average of 2.4 adults per female. Clearly, such numbers more than compensate for the excessive loss of eggs and young in the nests of inappropriate hosts. Each pair of cowbirds replaces itself with an average of 1.2 pairs—which will double a cowbird population in eight years.

Incubation period: By host; 10–13 days.

Nestling period: 10–11 days.

A major adaptation for parasitism seen in nestling and fledgling cowbirds is their rapid development. Cowbird eggs usually hatch one day ahead of the host’s eggs. In addition, cowbird nestlings usually are larger and grow faster than the host’s young, which enable them to garner more than their fair share of the food brought to the nest. Cowbird fledglings do not recognize their foster parents as individuals, but respond positively to all adults of their foster parents’ species. Fledglings receive more food than would the equivalent weight of host young, probably because their loud and persistent calling causes them to be fed more often.

Even though 97% of cowbird eggs and nestlings fail to reach adulthood, cowbird parasitism reduces production of young by the parasitized species. Abandonment of a nest by a parasitized host may preclude renesting and result in zero production for that pair during that breeding season. The reproductive success of birds that suffer the presence of a cowbird chick in their nest will therefore be significantly lower than that of unparasitized conspecifics in the same population.

 

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