|The egg in the middle of the top row is a cowbird egg. ©Janet Allen|
We had sent a photo in to Cornell since they are researching variability in eggs, and the house sparrows are a perfect candidate for this research. I was surprised that there was so much variability and now I know why.
Cornell was going to use my photo in the next issue of BirdScope, but they just emailed me that they won't be able to use it. On closer inspection, they realized that the egg in the middle of the top row isn't a house sparrow egg, it's a cowbird egg! It's a little bigger and a little splotchier.
I always hate to see the cowbirds around. Even though they're native, they didn't evolve in our area and so our songbirds have no defenses against them. As with the house sparrows, it's human intervention that has caused the problem. In the case of cowbirds, the herds of bison they used to travel with no longer roam the prairies. When they moved east, they didn't live in unlogged forested areas ... but now there are so many roads in forests that cowbirds easily find songbird nests to parasitize.
Birds that evolved with cowbirds have strategies for dealing with them. Some toss them out, some build on a new nest layer on top of them, and so on. Songbirds that didn't evolve with cowbirds don't have these defenses and end up raising the cowbird babies instead of their own. Some experts say this might lead some songbirds, already challenged, to extinction.