7-17-2023 4:08am

July Bird of the Month : Brown-headed Cowbird

Lesley Misko Editor

by Michael Strzelecki*

The brown-headed cowbird is a common and non-descript bird seen in the Boyertown area, often in fields and meadows. They are deep black in color and the males have brownish heads. They are seen occasionally on feeders but more likely walking around beneath the feeder scavenging for seeds that other birds dropped. Here is an image of a brown-headed cowbird from the National Audubon Society’s website

The reason the brown-headed cowbird warrants a bird-of-the-month column is because of its peculiar breeding methods. It is what is known as a “brood parasite.” Instead of building their own nests, the female cowbirds lay their eggs in the nests of other birds and hope that those birds will hatch the eggs and raise the young. Essentially, adopt their young. They do not take part in the parenting of their young.

It is not uncommon to see a backyard nest with four or five eggs in it where all the eggs are of the same type, except for one usually larger egg. This is the cowbird egg.

So how do other birds receive the concept of having to adopt a baby from a different bird species? Most reject the eggs. Larger birds will often just toss the cowbird egg out of the nest and continue on with their own. No harm, no foul to their own clutch.

Smaller birds that do not have the wherewithal to pick up the usually larger eggs of the cowbird will often start their breeding process anew. This means abandoning the eggs they have already laid. What they do is keep the same nest, but build a new floor in the nest over the already-laid eggs. They then will lay another clutch of eggs. Cowbirds often return and lay an egg with the new clutch, and this breeding dance continues. A warbler nest was once found with six layers of eggs caused by cowbirds.

In some cases, the nesting bird will actually accept the cowbird egg as her own and hatch and feed it. The problem is that the baby cowbird hatches more quickly than most other eggs in the nest and becomes dominant. It is typically larger than the other hatchlings and eats more. It has been documented where cowbird hatchlings that mature more quickly will actually toss the other hatchlings from the nest so as to be an only child.

While hiking at Susquehanna State Park, in Maryland, I happened upon a yellow warbler feeding a much-larger baby cowbird that it chose to adopt. As seen from these pictures, the actual process of a yellow warbler adopting and feeding a young cowbird must be exhausting, as the cowbird baby is so much larger than her own young and requires so much more food than she is used to gathering.

While it is easy to dismiss the cowbird as a lazy parent, there is actually a biological disposition as to why this brood parasite behavior occurs. Cowbirds historically have followed herds of bison and cattle across large swaths of prairie, feeding off the bugs that the animals kick up from the grass as they move and migrate. They were always on the go. If a female cowbird would have taken the time to build a nest, incubate eggs, and raise the young, she would have been separated from, not only her own flock, but the herd of bison or cattle that was responsible for her food. Brood parasite parenting allows propagation of the species.

Watch backyard feeders for cowbirds. The males will be identifiable due to its deep brown head. Females are non-descript and look similar to the females of other species as well as certain immature birds. As seen in the picture below, male cowbirds perform elaborate mating displays to attract the females. They will often dance in front of the female and cause lots of commotion. Sometimes the most unremarkable-looking birds are the most interesting.

* Mike Strzelecki is a freelance travel and outdoor writer, and 1981 graduate of Boyertown Area Senior High School. He writes from his house in Baltimore, Maryland. In his spare time, he joins his wife on adventures around the country observing and photographing birds.

Photos by Mike Strzelecki.