On May 21, I wrote about the Brewer’s Blackbird collecting insects by the river. I returned to the gravel bar earlier this week and saw them again collecting insects. This time they were flying over to the nearby willows to feed a begging bird that I could hear but couldn’t quite see. I watched them for a while and the baby bird stayed hidden at the base of willows in the shadows. Eventually, the insatiably hungry juvenile came out to meet the parent foraging nearby, and I got a better look. I thought… Wait a minute, is that a juvenile cowbird? I remembered from my earlier post that Brewer’s Blackbirds will have a clutch size of 3 - 7 eggs. So, should feeding only one bird lead me to be suspicious of cowbird parasitism?
The cowbird egg usually hatches out earlier than the host species. Afterward, they may roll the other eggs out of the nest. Cowbirds will often choose to parasitize smaller birds, so they have the advantage of competing for food with the other baby birds of the host family. They hatched out first, and they are bigger. From what I have read, they can literally starve out the other birds or literally crowd them out of the nest. I looked up juvenile Cowbirds and they look similar to juvenile Brewer’s Blackbirds. Hmm… What do you think? It got me reading and thinking about it more anyways.
Be sure to click on photo to get a better look.
Check out this introductory paragraph from The Cornell Lab of Ornithology Birds of the World: “The Brown-headed Cowbird, North America's best known brood parasite, lays its eggs in the nests of many different species. Originally these ‘Buffalo Birds’ were limited to short-grass plains, where they followed herds of North American Bison (Bison bison) and fed on the insects stirred up by their movement. The Brown-headed Cowbird has since dispersed widely as European settlement in North America opened forests and homogenized the environment into the agricultural and suburban landscapes of today. The expansion of the Brown headed Cowbird has exposed new species and naive populations to brood parasitism, and the pressure on such host populations can be substantial. During the breeding season, female Brown-headed Cowbirds wander widely, overlap the home ranges of other females, and may lay 40 eggs per season.”
