Cowbird pair  ©Janet Allen
A pair probably ready to parasitize our song sparrow's nest

Cowbirds, even though they're native to our continent, may actually be functioning as an invasive species. (See sidebar.)

The problem is that as forests in the east were cleared, cowbirds traveled east. Birds native to their original range in the prairies had evolved defenses against their nest predation, but our native birds of the east had not.

Today, we have so many roads going through forests (in addition to many other changes in our landscape) that there's a lot of the edge habitat cowbirds like.

Song sparrow raising a cowbird  ©Janet Allen
Song sparrow raising a cowbird

Year after year, we've watched the poor female song sparrow raise cowbird babies instead of her own babies.

The baby cowbirds quickly grow larger than the song sparrow, and she runs herself ragged trying to feed them.

Heartbreaking to watch.

Song sparrow babies ©Janet AllenSong sparrow babies

We were thrilled to see these baby song sparrows one year. This is the only time we've ever seen the song sparrow raise her own babies.

Cowbird egg  ©Janet Allen
A cowbird egg (on the left) among house sparrow eggs

Not so heartbreaking is finding cowbird eggs in the house sparrow nests. This has happened multiple times a season.

Reducing the house sparrow population is a good thing, but it does indicate that cowbirds are active in our yard. We have no way of knowing which other species' nests they've parasitized.