The Invasive Brown-Headed Cowbird

 

Species Physical Description: 

​

Brown-headed cowbirds are a type of blackbird, with certain distinct features that make them different. A major difference between brown-headed cowbirds and other blackbirds are their beaks; brown-headed cowbirds have shorter and thicker beaks. Other differences include that brown-headed cowbirds have shorter tails and thicker heads. Brown-headed cowbirds are sexually dimorphic, meaning that males and females, apart from different organs, have different physical characteristics such as body size and color pattern. The male brown-headed cowbirds primarily have black bodies and their heads are light to dark brown, while the female brown-headed cowbirds are brown all over their body and heads. Males have an average body mass of up to 49 grams, a wingspan of 14.2 in and length of 7-8 in, while females have an average body mass of 38 grams, a wingspan of 12 in, and length of 6-7 in. The eggs of brown-headed cowbirds are unique from usual bird eggs as they are either white or grey and have multiple brown spots all over. You can easily spot the eggs of brown-headed cowbirds as there are usually one or two eggs that look different from the rest of the eggs in the nest.

​

 

 

 

 

 

​

 

​

Range and Habitat:
 

Brown-headed cowbirds are found throughout the United States, northern Mexico and most of Canada. The largest populations of brown-headed cowbirds are found in their native habitat of central North America, specifically from southern Canada to Oklahoma. Brown-headed cowbirds are found in grasslands with low and scattered trees as well as woodland edges, brushy thickets, prairies, fields, pastures, orchards, and residential areas that provide sufficient resources. They prefer these open habitats because they provide ample forage and host nests, and they usually avoid unbroken forest. Brown-headed cowbirds lay their eggs in host nests found in habitat similar or identical to the habitat in which they live or in forest edges. Most brown-headed cowbirds migrate, but some travel further than others and most travel alone. The population size is extremely large; therefore it does not approach the threshold for vulnerable.

 

Additional information on the specific ranges and habitats can be viewed in the chart displayed below.

​

​

Life History:
 

The brown-headed cowbird is North America's most common parasitizing species, exclusively laying their eggs in the nests of other birds: over 200 different species of birds have been recorded. Female cowbirds prefer to parasitize nests with host eggs already present, and nests in which the host bird adds eggs daily.  The cowbird eggs hatch faster than other species, which allows the cowbird to get more food and attention from the parents. A typical cowbird nesting consists of 1-7 eggs, with an incubation period of 10-12 days and a nestling period of 8-13 days. The host species will continue to feed the cowbird fledgling for approximately 16 to 28 days after leaving the nest. The cowbird reaches sexual maturity at about 1 year of age and has a lifespan of about 16 years. Cowbirds may be monogamous, polygynous, or polygamous. The variation in mating relationships may be due to sex-ratios or cowbird densities. In North America, brown-headed cowbirds migrate about 330 miles to areas for spring breeding season (often April to July) in large flocks composed of other species of blackbirds for protection. The time of migration varies for cowbirds living in different locations.

​

​

Diet and Nutritional Requirements:

​

The brown-headed cowbird diet consists of invertebrates and seeds, which are found in short grasses and the bare ground. The diet of the cowbird varies from region to region, but in California in spring and summer, over half of their diet consists of insects, spiders and snails. Approximately one-quarter of the cowbird diet is made up of grasshoppers and beetles. Since cowbirds lay large amounts of eggs within a single season they require high levels of calcium and they maintain these high levels by eating snail shells and bird eggs that they steal from nests. Cowbirds often feed in pastures and feedlots because the presence of the grazing animals provides them with easy access to invertebrates, as well as waste grain. During the fall and winter, cowbirds tend to rely more on waste grain as a food source than insects, as it becomes much more available. They are often found feeding in stables, feedlots, campgrounds, picnic areas, bird feeders, and garbage dumps in California. 

 

Additional information on the diet of the cowbird can be viewed in the chart displayed below.

​

​

​

​

​