The Imposter

What's going on here? I've been seeing wild birds a lot in the chicken coop, warming up and eating the chicken feed. I don't mind. Whatever gets you through the winter. Then today, I was mindlessly collecting eggs (I'm a practitioner of mindlessness) when I noticed we have an unusually small egg: 

Is that a cowbird egg? (I think cowbirds lay eggs in other birds' nests and let the other birds raise their young.)

We don't have any miniature chickens, and all of our chickens are way too old to lay practice eggs.