Odd One Out: Whose Young is it Anyways?

This time of year, birds are busy making nests, incubating their eggs and then raising their young. However, not all birds make nests. Certain species are considered brood parasites, instead of making their own nest and raising their young, they lay their eggs in the nests of other species, which are then subsequently raised by those parents. In Ontario, we see this behaviour most commonly with Brown-Headed Cowbirds.

Brown-Headed Cowbird (male). Photo by Heather Kerrison

During nesting season, female Brown-Headed Cowbirds search for nests of other species where the female is actively laying eggs. They then sneak into the nest and remove one or more of that birds eggs and replace them with one or more of their own. The "foster parents" will then unknowingly raise the cowbird, often at the expense of their own young.

Not only do cowbirds have a shorter incubation period than other species, and hatch first, they also grow quickly and end up being larger than the host species young very quickly. This works to their advantage because they are able to out-compete for food that the parents are bringing, often resulting in the reduced success of the host species offspring. What we observe is typically a large fledgling being fed by a species much smaller than it. You can watch a video of a Cowbird laying an egg in a Northern Cardinal's nest here. 

Chipping Sparrow with Cowbird

Above is an example of a Chipping Sparrow with a fledgling Cowbird that is already larger than it. Now, I know what you are thinking. How terrible! How horrible! They must be stopped. Cowbirds are often vilified due to this behaviour and people that come across a nest with a cowbird egg in it may want to remove it. Vigilanties for the bird community. Although it feels injust, it is important that we as humans, do not place our own values onto the natural world. Further, Cowbirds are protected under the Migratory Birds Convention Act (MBCA) and it is illegal to tamper with their eggs. More importantly, most birds do not have the ability to recognize the difference in eggs, only the overall egg mass in their nest. Therefore, if an egg is removed from their nest they may abandon it entirely. Even more shockingly, some studies have been able to prove that cowbirds will seek retribution if their eggs are removed from the nest, some researchers deeming this "mafia behaviour", where if their egg is removed they may return to the nest and destroy other eggs. You can read that study here. 

Fledgling Cowbird. Photo by Heather Kerrison. 

Sarah Winnicki, an avian ecologist at Kansas State University, has an interesting way to of shifting perspectives on this behaviour: 

"I try to convince people about how amazing [cowbirds’] evolutionary story is,” she says. “How did they learn to find nests, to lay eggs, and to re-find them later? How do they learn to time their reproduction to their hosts? As an ornithologist, this is incredible to me."

In a way, this behaviour does seem sneaky, unjust or unfair. In another sense, it is absolutely amazing that cowbirds have been able to evolve to not have to expend resource on making nests and raising their own young. After all, the natural world is all about survival and furthering your genes. In that sense, cowbirds have developed a great system. Look around this nesting season and see if you notice any of this behaviour! 


Warmly, 


Heather