Last spring I received an interesting email from one of our customers, accompanied by a photograph. The photo showed a Dark-eyed Junco feeding what appeared to be a rather large baby while perched in a shrub.
Our customer, Katy, asked: “I am not sure if you answer wild bird questions”. (YES we DO, Katy–all the time!). “We have a window bird feeder and this morning I observed a junco feeding what I believe is a baby house finch. Both kinds of birds are common in our area–San Mateo, California, just South of San Francisco. I was able to get pictures.” Katy R.
It really helped that Katy told us where she lived and also included some photos. When I first looked at the picture, I thought, “What is that baby with the huge bill?” And it seemed too light and way too big to be a baby house finch. I sent the pictures on to Mindy in our Call Center for some collaboration. Mindy has tons of birds in her backyard and just like me, loves a challenge. Together we looked at lots of pictures on-line of baby juncos, baby house finches and other similar birds, but nothing seemed quite right. The baby juncos were way too dark, and the baby house finches were much smaller than the bird in Katy’s picture.
After looking up lots of pictures, just by chance, we Googled “Junco feeding baby bird”–and immediately came up with the image shown above! Not only did the baby look exactly like the one in Katy’s picture, but the picture was also taken in San Mateo–what a coincidence! As it turned out, Katy’s junco was feeding a baby Brown-headed Cowbird. And not only did we learn what the baby bird was in Katy’s photo, but a whole lot more!
We knew that cowbirds don’t build their own nests–cowbirds are parasitic, laying their eggs in other birds nests and depending upon other birds to feed their offspring. But we learned a few more interesting facts. This parasitic behavior on the part of the cowbird raises some interesting problems for both birds. If the adoptive parent realizes the new egg isn’t theirs, they may try to remove it, destroying the cowbird egg as well as their own eggs in the process. Or they may abandon the nest, along with the cowbird egg and their own eggs and start fresh somewhere else. Some birds even build new nests on top of old ones, until they finally get a nest with no cowbird eggs in it. If the cowbird egg is accepted and incubated by the new parents, it’s often the first egg to hatch. Then the baby cowbird pushes all the other eggs out of the nest, eliminating any competition for food.
It sounds like the cowbird is one nasty trickster, doesn’t it? But baby cowbirds are large and need lots of protein. If the adoptive parent is a small bird, it may become exhausted and not able to keep up with the food demands of this large baby. So even with the lack of competition from the adoptive parent’s own hatchlings, the baby cowbird’s future is not assured! And a mother cowbird actually lays about 40 eggs over the course of two years, with only 2 or 3 of those eggs actually hatching and reaching maturity. So, although it seems like the mother cowbird has no responsibilities at all (she doesn’t, does she?), and the babies are nothing but freeloaders, life is not as easy as it first may seem for cowbirds. Some birds have good strategies for dealing with cowbird eggs, while other birds don’t. And sometimes birds actually benefit by having a cowbird in their nest–albeit in a small way. For instance, bluebirds will frequently hatch a cowbird egg along with their own eggs. But when predators strike a bluebird nest, they’ll likely target the largest baby first–the unfortunate cowbird. Often the smaller bluebird hatchlings are left untouched.
So we want to thank you Katy, for asking us about your photo and giving us an opportunity to learn a little more about cowbirds in the process. There is no doubt that Nature has some very perplexing methods when it comes to the survival of birds–as well as that of other species. And although we may not understand the bigger picture, it’s always extremely interesting to observe birds involved in their complex and intriguing behaviors. And that’s why we find birds so fascinating!
— R. Brune