A couple weeks ago a customer was very excited to send me a photo of an adult female Bullock’s oriole feeding “its” baby. Upon close inspection I could see that the baby was bigger than the adult that was feeding it . It also had none of the coloration of an oriole. I had to report to him that the Bullock’s Oriole was actually feeding a Brown-headed Cowbird baby.
He began watching his feeders more closely and realized cowbirds, male and female, were all over his feeders…and none of them were feeding their own babies.
Cowbirds are considered a brood parasite. That means they lay their eggs in another bird’s nest with the expectation that the host species will incubate the egg, feed the nestling once hatched, and continue to feed it after it has fledged until it is ready to forage on its own. This adaptation has been very successful for hundreds, if not thousands of years. Unfortunately it has been to the detriment of many other bird species. Over 200 species of birds are parasitized by the cowbird, including vireos, sparrows, blackbirds (orioles and cowbirds being in the blackbird family), warblers, flycatchers, and finches. It has even been documented that cowbirds have laid their eggs in hummingbird nests. Cowbirds have been called the chicken of the passerine world as they can lay up 40 eggs in a breeding season.
Why would a cowbird be so irresponsible? Well, the Brown-headed Cowbird evolved hanging around migrating buffalo herds on the short-grass prairie region west of the Mississippi. The cowbird’s food source happens to be the insects feeding on the dung left by the migrating herds. Since their food source was always on the move they didn’t have the luxury of taking the time to build a nest, lay eggs, incubate the eggs and fledge their own young. So they evolved the strategy of dumping their egg in another species nest resulting in the host species caring for the egg. Unfortunately many species do not recognize the cowbird egg as not their own. Many bird eggs are off-white with brown specks or splotches. Cowbird eggs are also off-white with brown splotches so they are difficult to tell apart. In contrast, a Robin’s blue egg is brightly colored blue so Robins do not make a good host for cowbirds.
If a species does recognize the egg as not their own there are several strategies they can use to rid their nest of the egg. They can simply peck it, pick it up and dump it over the side. They can build another layer of nest material above the cowbird egg and continue to lay their own clutch in the next layer. (One yellow warbler nest was found to have six layers in the nest with a cowbird egg in each layer). Or they can simply abandon the nest and start all over, securing a nest site, building a new nest and laying another clutch of eggs. While doing bird research on cowbird parasitism my husband and I witnessed a poor Bell’s Vireo couple that started in May building their first nest and laid a clutch of eggs, but soon came the cowbirds taking out a Bell’s Vireo egg and depositing one of their own. The Bell’s Vireos eventually abandoned the nest and started all over. As soon as they got the next nest built and started laying eggs, here came the cowbirds. This continued all summer long, the tired Vireos building one nest after another and each time the cowbirds laid their eggs in the nest. Five nests later, toward the end of the summer, the Vireos still had not successfully fledged any of their own species. This type of energy output actually shortens the lifespan of these birds, and most critically diminishes the population of many of our favorite songbirds.
Humans have intervened by trapping and killing cowbirds in areas where a host species is on the brink of extinction. However, North America is home to not just the Brown-headed cowbird, but also the Bronzed cowbird. The Shiny Cowbird has made its way from the Caribbean up to Florida and is moving northward. The Screaming and Giant Cowbirds are currently in South America and Mexico and also expanding their range northward. We’re probably not going to rid the world of cowbirds even with active trapping programs. More importantly, it’s important to put our energies toward preserving habitat, especially river corridors with lush riparian forests that provide nesting sites that our songbirds use for breeding.
Cowbirds are a fascinating bird in that they don’t imprint on their host parents as most birds do. For instance, cowbirds don’t end up singing like warblers or vireos after being raised by them. Instead at the end of the breeding season cowbirds fly south for the winter as migratory birds do. They end up all together in hundreds and even thousands of large flocks of cowbirds congregating around horse pens and stock yards. Here they spend much of their time eating the insects and flies that pester these animals. Hence they provide a valuable service.
Even though the cowbird may be the bird that bird lovers love to hate, there is still much to learn about the behavior of cowbirds. Watch your feeders carefully – if you see a bird feeding a baby bigger than itself, it might just be feeding a brown-headed cowbird.
Dena is a seasoned naturalist, birding guide and educator. As manager of Jay’s Bird Barn in West Sedona she is available to answer questions about wild birds and enhancing your backyard habitat. You may contact her at (928) 203-5700 or firstname.lastname@example.org.