While taking a closer look at the empty bird’s nest found on a fallen branch of our crabapple tree, we found a second mystery. The nest had several eggshell fragments inside.
As you can see, some of the fragments are pale blue. Others, though, are whiter, with noticeable brown blotches. It may be hard to see in the photograph, but the fragments suggest the eggs were not the same size, either. The white-and-brown egg looks to have been larger.
What bird or birds laid these eggs? Are there two types of eggs in this one tiny nest?
Flashback to the Past
When I was a teenager, we watched a phoebe build a nest and lay eggs in it, over the door of a cottage. The first two eggs were small and white. Then one morning a much larger egg appeared in the nest. With my parents help, we investigated. A brown headed cowbird had somehow sneaked one of her eggs in our phoebe’s nest! The memory of this outrageous cheating is still vivid in my mind. So I had a suspicion about what I might be seeing in this newer crabapple tree nest.
Cowbirds are cunning. Or just lazy. The females lay their eggs in other birds’ nests then fly away and never look back. They don’t raise their own young.
What makes it seem especially unjust to those of us who would prefer the world to follow some rules, is that the cowbird chicks usually survive, and the foster parents’ chicks usually don’t. Cowbird eggs are large, and tend to hatch faster than other birds’ eggs. The chicks also tend to grow a bit faster and are aggressive. It’s been observed that cowbird chicks will deliberately push other nestlings out of the nest, kill them, and break other eggs open.
Some songbirds will stubbornly keep raising the cowbird chick, not apparently aware that it has replaced their own offspring. Other birds will abandon a nest with a cowbird egg, or push the egg out of the nest before it hatches. According to Cornell University’s website, goldfinches win by a stranger twist of fate: the cowbird chicks cannot apparently survive on the seeds-only diet provided by the goldfinch parents to their nestlings.
The Tale Told by Two Eggshell Fragments
I may be mis-reading it, but this is the tale I read from the eggshell bits in the nest in our crabapple tree. I think the nest was a chipping sparrow nest (see my article: 3 Steps to Identifying an Empty Bird’s Nest from a Crabapple Tree) The blue eggshell bits are from a chipping sparrow egg. The mottled brown and white bits are from a larger cowbird egg.
Did a cowbird win in this case? I don’t think so. I think both chicks were lost while still in their eggs. I think the reason the shells are still in the nest is that a predator, probably a squirrel, ate the eggs.
I hope the sparrows had better luck with a new nest in another location. And that no cowbirds came to their new home.
Has a cowbird ever dropped an egg in a nest you’ve been watching? Did you remove the egg? If not, did the parents raise all of the chicks successfully? Share your experiences with a comment.