Brown-headed Cowbird - Molothrus ater

[OldGreenVulture]

Brown-headed Cowbird - Molothrus ater



Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Passeriformes
Family: Icteridae
Genus: Molothrus
Species: Molothrus ater

The brown-headed cowbird (Molothrus ater) is a small obligate brood parasitic icterid of temperate to subtropical North America. They are permanent residents in the southern parts of their range; northern birds migrate to the southern United States and Mexico in winter, returning to their summer habitat around March or April.

Description
The brown-headed cowbird is typical for an icterid in general shape, but is distinguished by a finch-like head and beak and is smaller than most icterids. The adult male is iridescent black in color with a brown head. The adult female is slightly smaller and is dull grey with a pale throat and very fine streaking on the underparts. The total length is 16–22 cm (6.3–8.7 in) and the average wingspan is 36 cm (14 in). Body mass can range from 30–60 g (1.1–2.1 oz), with females averaging 38.8 g (1.37 oz) against the males' average of 49 g (1.7 oz).



Ecology
The species lives in open or semi-open country and often travels in flocks, sometimes mixed with red-winged blackbirds (particularly in spring) and bobolinks (particularly in fall), as well as common grackles or European starlings. These birds forage on the ground, often following grazing animals such as horses and cows to catch insects stirred up by the larger animals. They mainly eat seeds and insects.

Before European settlement, the brown-headed cowbird followed bison herds across the prairies. Their parasitic nesting behaviour complemented this nomadic lifestyle. Their numbers expanded with the clearing of forested areas and the introduction of new grazing animals by settlers across North America. Brown-headed cowbirds are now commonly seen at suburban birdfeeders.



Reproduction
The brown-headed cowbird is an obligate brood parasite: it lays its eggs in the nests of other small passerines (perching birds), particularly those that build cup-like nests. The brown-headed cowbird eggs have been documented in nests of at least 220 host species, including hummingbirds and raptors. The young cowbird is fed by the host parents at the expense of their own young. Brown-headed cowbird females can lay 36 eggs in a season. More than 140 different species of birds are known to have raised young cowbirds.

Unlike the Common Cuckoo, the brown-headed cowbird is not divided into gentes whose eggs imitate those of a particular host.

Some host species, such as the house finch, feed their young a vegetarian diet. This is unsuitable for young brown-headed cowbirds, meaning almost none survive to fledge.

Male behavior and reproductive success
Social behaviors of cowbird males include aggressive, competitive singing bouts with other males and pair-bonding and monogamy with females. By manipulating demographics so juveniles only had access to females, juvenile males developed atypical social behavior; they did not engage in the typical social singing bouts with other males, did not pair bond with females, and were promiscuous. This demonstrates that there is great flexibility in the behavior of cowbirds, and that the social environment is extremely important in structuring their behavior. Adult males housed with juvenile males were shown to have greater reproductive success compared to adult males housed with other adult males. Being housed with juvenile males honed the reproductive skills of the adult males by providing them with a more complex social environment. This finding was further studied by comparing the behaviors and reproductive success of males exposed to a dynamic flock, consisting of changing individuals, with males exposed to a static group of individuals. The individuals that stayed with the same group had a stable, predictable relationship between social behavior and reproductive success; the males that sang high amounts to females experienced the greatest reproductive success. The adult males that were exposed to a rotating roster of new individuals had an unpredictable relationship between social variables and reproductive success; these males were able to copulate using a much greater variety of social strategies. The males who lived in static flocks had high levels of consistency in their behaviors and reproductive success across multiple years. The males in dynamic flocks experienced varying levels of dominance with other males, differing levels of singing to females, and differing levels of reproductive success.



Brood parasitism
Behavior
Brown-headed cowbirds do not raise their own young, instead laying their eggs in the nests of other bird species. Because of this, cowbirds are not exposed to species-typical visual and auditory information like other birds. Despite this, cowbirds are able to develop species-typical singing, social, and breeding behaviors.

Host response
The acceptance of a cowbird egg and rearing of a cowbird can be costly to a host species. In the American redstart, nests parasitized by cowbirds were found to have a higher rate of predation, likely due in part to the loud begging calls by the cowbird nestling, but also partly explained by the fact that nests likely to be parasitized are also more likely to be predated.

Host birds sometimes notice the cowbird egg, to which different host species react in different ways. Rejection manifests in three forms: nest desertion (e.g., Blue-gray Gnatcatcher), burying of the egg under nest material (e.g., Yellow Warbler), and physical ejection of the egg from the nest (e.g., Brown Thrasher). Brown-headed cowbird nestlings are also sometimes expelled from the nest. The gray catbird rejects cowbird eggs over 95% of the time; experimentation has suggested that in this species, the cost of accepting an egg is much higher (0.79 catbird fledglings) than the cost of rejecting an egg (0.0022 catbird fledglings).

Nestlings of host species can also alter their behavior in response to the presence of a cowbird nestling. Song sparrow nestlings in parasitized nests alter their vocalizations in frequency and amplitude so that they resemble the cowbird nestling, and these nestlings tend to be fed equally often as nestlings in unparasitized nests.



Parasite response
It seems that brown-headed cowbirds periodically check on their eggs and young after they have deposited them. Removal of the parasitic egg may trigger a retaliatory reaction termed "mafia behavior". According to a study by the Florida Museum of Natural History published in 1983, the cowbird returned to ransack the nests of a range of host species 56% of the time when their egg was removed. In addition, the cowbird also destroyed nests in a type of "farming behavior" to force the hosts to build new ones. The cowbirds then laid their eggs in the new nests 85% of the time.

Human intervention
Humans sometimes engage in cowbird control programs, with the intention of protecting species negatively impacted by the cowbirds' brood parasitism. A study of nests of Bell's vireo highlighted a potential limitation of these control programs, demonstrating that removal of cowbirds from a site may create an unintended consequence of increasing cowbird productivity on that site, because with fewer cowbirds, fewer parasitized nests are deserted, resulting in greater nest success for cowbirds.

From Carnivora.

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[OldGreenVulture]

Juvenile cowbirds sneak out at night

Date: November 2, 2015
Source: University of Illinois at Urbana-Champaign
Summary:
A new study explores how a young cowbird, left as an egg in the nest of a different species, grows up to know it's a cowbird and not a warbler, thrush or sparrow.


An advanced tracking system revealed the secret habits of young cowbirds, like this one with a radio transmitter on its back.

A new study explores how a young cowbird, left as an egg in the nest of a different species, grows up to know it's a cowbird and not a warbler, thrush or sparrow.

The study, published in Animal Behaviour, reveals that cowbird juveniles leave the host parents at dusk and spend their nights in nearby fields, returning just after daybreak. This behavior likely plays a role in the cowbirds' ability to avoid imprinting on their host parents.

"If I took a chickadee and I put it in a titmouse nest, the chickadee would start learning the song of the titmouse and it would actually learn the titmouse behaviors," said Matthew Louder, who conducted the study as a Ph.D. student with Illinois Natural History Survey avian ecologist Jeff Hoover and INHS biological surveys coordinator Wendy Schelsky. "And then, when it was old enough, the chickadee would prefer to mate with the titmouse, which would be an evolutionary dead end," he said.

Louder is now a postdoctoral researcher with East Carolina University in North Carolina and Hunter College in New York.

The imprinting process is widespread among birds and other animals, but brood parasites like the cowbird appear to be resistant to imprinting. They will imprint on a different species if confined with that species for an extended period of time in a cage, but the birds don't appear to do so in the wild.

Cowbird hosts, such as the prothonotary warblers in this study, have their own habits and habitats, and seldom choose to live where the cowbirds live or eat what they eat. Prothonotary warblers, for example, live in forests and dine on insects and caterpillars. Cowbirds spend most of their adult lives in open fields and prairies, and while they do eat insects, about three-quarters of their diet consists of seeds.

"Among other things, cowbirds have got to learn to eat like cowbirds or they're not going to survive very long," Hoover said.

The researchers wanted to test the hypothesis that cowbird moms are the ones that lead their offspring out of the forest. There was some support for this idea. A recent study from the same team found that cowbird females don't simply abandon their eggs in another species' nest. They pay attention to whether the young birds survive, sometimes wrecking the nests of birds that kick the cowbird eggs out of their nests.

The cowbird females also return to nests where young cowbirds survived to fledging age. Cowbird females are often spotted in the vicinity of cowbird nestlings, Schelsky said, and sometimes respond (with vocalizations, not food) to the nestlings' begging calls.

To track the birds in the forest and prairie, the researchers put radio telemetry transmitters on the cowbird nestlings and on adult female cowbirds in the forest where the host parents made their nests. The team took blood from the birds and conducted genetic analyses to match the juveniles (and their radio signals) to their biological mothers.

But tracking the birds, even with the radio transmitters, was next to impossible, Louder said. He tried for a year, but was unable to get meaningful data. Then study co-author Michael Ward, a professor of natural resources and environmental sciences at the University of Illinois, came up with a new approach.

"He helped construct an automated telemetry system," Louder said. "We put up three radio towers, each with six antennas on it, so you have 360-degree directional coverage. All three towers track one individual cowbird at a time and then move to the next individual."

With this system, Louder could track the location of each study bird every one-to-two minutes.

"We were able to watch the juveniles and see if they left the forest at the same time as a female and, if so, whether that female was their mom," he said.

"Strangely enough, the juveniles did not follow the females out of the forest," Louder said. Instead, they left on their own, after dark, returning only the following morning, he said.

"I started seeing this in the data and I thought it was wrong," Louder said. So he went to the forest and followed a single juvenile cowbird for one night. The bird left the forest in the evening, moving to a rosebush on the adjacent prairie. It was out there all night, alone.

"As soon as the sun came up, the juvenile flew back into the forest and to the warbler's territory," Louder said. "Without the automated radio telemetry, I would have assumed that it had stayed in the forest all night."

The discovery doesn't explain how cowbirds find their way into a cowbird flock, where they learn most of their social and survival skills and eventually find a mate. But it does offer some insight into the processes that allow young cowbirds to avoid imprinting on their hosts, the researchers said.

"Clearly, there's a lot more to these birds than people would have thought," Hoover said. "We still have more layers to peel away from this onion that is the cowbird."

Story Source: University of Illinois at Urbana-Champaign. "Juvenile cowbirds sneak out at night." ScienceDaily. www.sciencedaily.com/releases/2015/11/151102152607.htm (accessed November 3, 2015).




Journal Reference:
Matthew I.M. Louder, Michael P. Ward, Wendy M. Schelsky, Mark E. Hauber, Jeffrey P. Hoover. Out on their own: a test of adult-assisted dispersal in fledgling brood parasites reveals solitary departures from hosts. Animal Behaviour, 2015; 110: 29 DOI: 10.1016/j.anbehav.2015.09.009

Highlights
• Few studies have addressed the mechanism of species recognition in brood parasites.
• We examined the movements of fledgling parasitic brown-headed cowbirds.
• We tested whether adult female cowbirds guide juveniles from their hosts.
• Fledgling cowbirds departed from the host solitarily and roosted alone.
• Brood parasites may avoid (mis)imprinting on the host by spatial segregation.
Brood-parasitic offspring sexually (mis)imprinting on the foster parents is considered one of the greatest constraints to the evolution of interspecific avian brood parasitism. While most nonparasitic juvenile birds learn the behaviours and mate choice preferences from their own parents, social parasites must avoid misimprinting on their host species' phenotype in order to accurately recognize conspecifics. One possible mechanism to assure accurate species recognition by juvenile parasites is to begin to associate with adult parasitic conspecifics, known as the ‘first contact’ scenario, whereby adult female parasites facilitate the dispersal of their offspring away from hosts, thus providing accurate referents for conspecific recognition. Using an automated radiotelemetry system, we determined the presence or absence (every 1–2 min during three breeding seasons; 516 315 search occasions) of radiotagged parasitic adult female brown-headed cowbirds, Molothrus ater, and compared their departures from a forest study site with those of genetic offspring or experimentally transplanted (nonrelated) juvenile cowbirds within the female's egg-laying range. Contrary to our predictions, we found no support for the facilitation of juvenile cowbird dispersal by adult female cowbirds. Juvenile cowbirds typically were not located within their mother's home range when departing the forest and, likewise, departure events for natal and experimentally transplanted juveniles (

www.sciencedirect.com/science/article/pii/S0003347215003401

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