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Brown-headed Cowbirds
May 06, 2021 by Haya Aldoori

Springtime breathes new life into High Park as many of the residents are having babies! One High Park resident with a particularly interesting approach to raising its young is the Brown-headed Cowbird.

Cowbirds are “brood parasites” which means they don’t make a nest, but instead lay their eggs in the nests of other bird species (hosts), who then raise the young for them.

Photo by Andrew Yee

During the breeding period, a female Cowbird quietly searches for female birds of other species that are actively laying eggs. Once a suitable host is found, the Cowbird will sneak onto the resident bird’s nest when it is away. It will usually damage or remove one egg (or more) before replacing that egg with one (or more) of her own.

The host species will either reject or accept the Cowbird’s egg. Rejecters recognize and remove Cowbird eggs from their nests immediately after they are laid. These are mostly large birds with beaks that are big enough to easily get rid of those unwanted eggs.

Accepters, which are usually smaller birds, will rarely eject the Cowbird eggs. Instead, they end up incubating the eggs and then feeding the relatively large and noisy young Cowbirds as if they were their own. Since the young Cowbirds have a lot of food requirements, more often than not the nesting success of the host species becomes negatively impacted. It is still a bit unclear why certain bird species accept this form of parasitism. It could be because that the host just does not notice that there is a Cowbird egg in the nest, or the host does not want to risk breaking or losing her own eggs by attempting to remove the Cowbird egg, or raising the Cowbird is a better option than the risk of not nesting at all.

On your next spring visit to High Park, keep an eye out for Brown-headed Cowbirds!

FUN FACTS:

Brown-headed Cowbirds can lay eggs in the nests of more than 220 species of birds

Brown-headed Cowbirds can lay more eggs than any other wild bird. A single female is capable of laying nearly one egg per day at the peak of the breeding season and produces a total of 30-40 eggs over the 2-3 month breeding period (May-July). Because female cowbirds usually lay only one egg in a host nest, this means that anywhere between 30-40 nests can be parasitized per female in one season.

To learn more about birds with the High Park Nature Centre, check out our “Birds of the City” Online Naturalist Course for adults!

Resources:

https://www.allaboutbirds.org/guide/Brown-headed_Cowbird/overview https://nationalzoo.si.edu/migratory-birds/news/brown-headed-cowbirds-buffalo-birds-modern-scourge

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