Male and female brown-headed cowbirds look different from each other. Males are a little bit larger than females, with body lengths from 19 to 22 cm, an average wingspan of 36 cm and body masses of 42 to 50 g. Males have a glossy, black body and a brown head, with a pointed gray beak. Females have body lengths from 16 to 20 cm, wingspans from 32 to 38 cm and body masses of 38 to 45 g. Females are dull brown and have lightly colored streaks on their breast, with a pointed gray beak. ("Brown-headed Cowbird", 2013; Ortega, 1998; Tacutu, et al., 2013; Tekiela, 2000)
Brown-headed cowbirds (Molothrus ater) are found throughout the United States, northern Mexico and most of Canada. They used to be found mostly in the Great Plains of North America but their range expanded in all directions during the 19th and 20th centuries. The largest populations of brown-headed cowbirds are still found in the central parts of North America, especially from southern Canada to Oklahoma. Most brown-headed cowbirds migrate, but some travel further than others and most travel alone. Brown-headed cowbirds usually travel to the same breeding areas each year, but may go to different areas in the winter. (Ortega, 1998; Peterjohn, et al., 2000; Rothstein and Robinson, 2000)
Brown-headed cowbirds are found near forest edges, riparian zones, thickets, prairies, fields, cattail marshes, pastures, orchards and suburban areas. In the morning, brown-headed cowbirds are found along the edges of closed, forested habitats. In the afternoon, they are often found foraging in open habitats, such as prairies. (Lowther, 2003; Ortega, 1998; Rothstein, et al., 1986)
Among brown-headed cowbirds, females choose their mates. Males try to attract females through displays and perched songs. Males give song-spread displays in which they fluff their feathers, spread their wings and bow to the female. Males are chosen based on their song spreads and the number of flight whistles they can perform. These birds are mostly monogamous, but their mating systems can vary in different populations. Most populations have more males than females, so females can be choosy. Males are usually monogamous throughout the breeding season and guard their female from other males. Females, on the other hand, tend to mate outside of their pair. Because males do not give their mates food, nesting resources, protection from predators or help raise the young, there is no advantage for females to breed with only one male. Males that are not in a pair-bond may mate with unguarded females, often when the female's mate is foraging. Females with larger home ranges than males are more likely to mate outside of their pair. (Darley, 1982; Ortega, 1998)
Brown-headed cowbirds are brood parasites, meaning that they lay their eggs in the nest of another bird, a 'host species'. These birds use the nests of several different species, which means that their breeding season varies with the breeding season of their hosts. The earliest breeding seasons begin in early to mid-April and the latest seasons end in early August, but egg laying normally occurs from May to June. The number of eggs per breeding season varies, but brown-headed cowbirds may lay up to 77 eggs in one mating season. Because they are brood parasites, the number of available host nests influences the number of eggs laid per breeding season. Female brown-headed cowbirds lay eggs very quickly to avoid being caught by the nest's host. On average, they lay eggs in 41 seconds, compared to 20.7 to 103 minutes in other passerines. Their eggs hatch in 10 to 11.6 days. In many cases, this short incubation time allows them to hatch before host eggs and get food and begin growing before host hatchlings. The hatchlings of brown-headed cowbirds show exaggerated begging, which causes the host parents to feed cowbirds more than their own hatchlings. Cowbirds out-compete host hatchlings, resulting in the death of some, or all of the host's hatchlings. Fledgling brown-headed cowbirds normally leave the nest 10 to 11 days after hatching and gain independence from their foster parents at 25 to 39 days old. At this point, they find and join a flock of other brown-headed cowbirds. (Ortega, 1998; Rothstein, 2004; Tacutu, et al., 2013)
Brown-headed cowbirds likely became brood parasites because early cowbirds followed bison herds and could not stay in one place long enough to care for their young. Brown-headed cowbirds are often thought of as lazy or neglectful parents because they do not provide care for their young, however, they are careful about choosing a good nest for their eggs and continue to check on the nest after laying their eggs. If a host rejects a cowbirds' egg, female cowbirds may destroy the host's eggs or sometimes their hatchlings, this is called “mafia behavior”. It is uncommon, but some brown-headed cowbirds feed their hatchling in the parasitized nest, feeding only their own offspring and pecking the host's nestlings on the head when they beg for food. (Hoover and Robinson, 2007; Ortega, 1998)
The longest known lifespan for a wild brown-headed cowbird is 16.9 years. (Tacutu, et al., 2013)
Brown-headed cowbirds are social. During the mornings they tend to be more solitary but they may also mate or look for host nests during this time. In the afternoon, brown-headed cowbirds form flocks to forage. During the winter months, they form very large flocks with other members of family Icteridae and European starlings. While roosting, they may form groups of up to 38 million individuals. When they are in groups, they form hierarchies of dominance based on the number of displacements, song spreads and flight whistles a male performs. To determine their placement on the hierarchy, brown-headed cowbirds have "triangle and quadrangle ceremonies". In these activities, male cowbirds stand in a circle and give song spreads to one another to determine their social status. In flocks, most singing is performed by the dominant male, inferior males that sing may be attacked. (Darley, 1982; Ortega, 1998)
Brown-headed cowbirds have large home ranges because they may travel between nesting and feeding sites each day. Their home range size depends on their specific habitat. In Ontario females use 4.5 ha and males use 6.6 ha, although they have a much larger home range in the Sierra Nevada Mountains. Some brown-headed cowbirds travel 7 km from nesting to feeding sites each day, while others stay within the same area to nest and forage. Because they do not need to care for their young they are free to travel to areas with more food sources. Instead of guarding territory, males usually guard their mates. Females, on the other hand, tend to be more territorial, especially in areas with many host nests. Because there are more nests, cowbirds can keep a smaller home range, making it easier to defend. (Orians, 1961; Ortega, 1998; Rothstein, et al., 1984)
Brown-headed cowbirds make a variety of vocalizations, including flight whistles, single-syllable calls, perched songs, keks or chucks and chatter. They use their vocalizations for courtship, species and individual identification, aggression and alerts to threats. Brown-headed cowbirds know their songs without learning them. Although they are raised in the nests of other species, they do not learn the songs of those species. Their ability to know their songs without being taught is unique; the songs of most other songbirds must be learned. There is little or no difference between the vocalizations made in different populations, however, there are differences between individuals. (Ortega, 1998; Rothstein, et al., 2000; West, et al., 1979)
Flight whistles are a form of long distance communication given exclusively by males and consist of pure tones, between 3 and 9 kHz. Flight whistles vary and may include trills, they are often given before or during flight and within 5 seconds of copulation, they also function as an alarm call. Single-syllable calls are given by males and consist of a single pure tone, with a fundamental frequency between 2 and 8 kHz. Males generally have 1 or 2 in their repertoire, their functions are similar to flight whistles, but they are given more often when conspecifics are nearby. Perched songs are used exclusively by males and have a frequency range of 0.5 to 12 kHz, which is the widest frequency range of any bird song. Males have a repertoire of 1 to 8 different perched songs. When used in courtship, these songs are accompanied by a song-spread display. They also function in male to male aggression, identification and establishment of social hierarchies. Keks or chucks are short notes given by males and females. They are not detectable beyond 5 m of the bird, so relatively little is known about this vocalization. Chatter is given primarily by females and may be used in response to other vocalizations. It consists of several elements with frequencies from 2 to 6 kHz. There is little or no variation between subspecies or different populations, however, there are differences between individuals, this may imply that sounds are used in identification. (Ortega, 1998; Rothstein, et al., 2000; West, et al., 1979)
Brown-headed cowbirds mainly forage for food on the ground in open habitats such as grasslands. They often forage near herds of animals, such as cows, that stir up insects from the grass as they walk. Seventy-five percent of their diet is made up of plant matter including fruits and seeds. They also eat spiders and arthropods, such as grasshoppers, leafhoppers and beetles. During the breeding season, females eat mollusk shells to increase their calcium level, which helps them produce eggs. Although both males and females eat eggs, females may do so to get more calcium. During the winter, they mostly eat grains. ("Brown-headed Cowbird", 2013; Ortega, 1998; Reilly, Jr (ed.) and Pettingill, Jr (ed.), 1968)
Blue Jays are common egg predators, but red squirrels, northern flying squirrels and yellow-bellied sapsuckers also attack brown-headed cowbirds during incubation. Broad-winged hawks and barred owls are common predators of nestlings, as well as northern goshawks, Cooper's hawks, sharp-shinned hawks, northern saw-whet owls, blue jays and northern flying squirrels. (Cox, et al., 2012; Hannon, et al., 2009)
Due to their brood parasite habits, their breeding behavior impacts many other species. They use 226 different host species of all different sizes, from warblers that weigh 8 to 15 g, to blackbirds that weigh over 100 g. However, they mostly parasitize only 132 species, most commonly yellow warblers, red-eyed vireos, song sparrows, wood thrushes and common yellow throats. The population size and the number of offspring of the host species may be impacted by brown-headed cowbirds because they remove and destroy eggs and out-compete host hatchlings, in addition, hosts often abandon parasitized nests. Brown-headed cowbirds pose a threat to some endangered species, so programs have been designed to control their population. These programs kill thousands of cowbirds annually in an attempt to increase host populations. Control programs have been used to protect Kirtland's warblers, least bell's vireos, black-capped vireos and southwestern willow flycatchers. While cowbirds have some part in reducing the populations of endangered species, their actual role is probably not as large as it was originally thought. (Ortega, 1998; Rothstein, 2004; Smith, et al., 2000)
Programs that help to protect endangered bird species from brown-headed cowbirds are expensive, costing about one million dollars each year. (Rothstein, 2004)
Brown-headed cowbirds help control insect populations that may be pests to humans.
Brown-headed cowbirds have a large population throughout North America. Their population has expanded due to human activities like farming, deforestation and urbanization. (Ortega, 1998)
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