An Avian Parasite
The Brown-headed Cowbird is one of my least favorite birds (on a par with the European Starling), but it is a very interesting bird nonetheless. It is North America’s most common “brood parasite.” A female cowbird makes no nest of her own, but instead lays her eggs in the nests of other bird species, which then raise the young cowbirds. Researchers believe that parasitism by cowbirds may be a significant factor contributing to the declining numbers of many songbirds in North America.
Cowbirds were historically open-country birds, associating with buffalo herds and later adapting to domestic cattle. The prairies and plains were the cowbirds’ homeland, but they have now expanded their ranges. Currently the Brown-headed Cowbird’s range includes all of the 48 contiguous states and southern Canada.
An average female lays about 80 eggs, 40 per year for two years. Only about 3% of those 80 eggs reach maturity—an average of 2.4 adults per female. 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.
Facts About the Brownheaded Cowbird
- Brown-headed Cowbird lay eggs in the nests of more than 220 species of birds. Recent genetic analyses have shown that most individual females specialize on one particular host species.
- Social relationships are difficult to figure out in birds that do not build nests, but male and female Brown-headed Cowbirds are not monogamous. Genetic analyses show that males and females have several different mates within a single season.
- Some birds, such as the Yellow Warbler, can recognize cowbird eggs but are too small to get the eggs out of their nests. Instead, they build a new nest over the top of the old one and hope cowbirds don’t come back. Some larger species puncture or grab cowbird eggs and throw them out of the nest. But the majority of hosts don’t recognize cowbird eggs at all.
- Cowbird eggs hatch faster than other species eggs, giving cowbird nestlings a head start in getting food from the parents. Young cowbirds also develop at a faster pace than their nest mates, and they sometimes toss out eggs and young nestlings or smother them in the bottom of the nest.
- In winter, Brown-headed Cowbirds may join huge roosts with several blackbird species. One such mixed roost in Kentucky contained more than five million birds.
- The oldest recorded Brown-headed Cowbird was 16 years 10 months old.
The above facts thanks to allaboutbirds.org