Walking through the neighborhood this afternoon, I had my attention drawn to a high-pitched burbling sound. Across the street from me were three Brown-headed Cowbirds. They were walking through a yard, foraging in the wet grass and occasionally picking up some food (a worm or larva or something- I couldn’t tell at that distance). The fascinating thing about watching this group of cowbirds moving along was that it was a group of two males and one female.
Now Brown-headed Cowbirds are brood parasites, which means that they lay their eggs in the nests of other birds and do not provide any parental care at all. As a result, this group was not a family group of birds. It couldn’t be. Once a female cowbird drops her eggs off in the nest of some other bird, she’s done. In fact, she may lay as many as 40 eggs in a single breeding season in nests that range over an area as large as 450 hectares (1100 acres or 1.7 square miles).
Well, if it’s not a family group, then who were those three birds? One of the clues came from watching the behavior of one of the males. He stayed between the female and the other male the whole time I watched him and he was the one making the high-pitched vocalizations. In fact, each time that he vocalized, he lowered his head, raised his tail, spread his wings and aimed himself directly at the other male. He was displaying and mate guarding. Even though cowbirds are not particularly good parents, they do appear to be devoted partners. In most populations, they do seem to be mostly monogamous.
So if these birds are monogamous and mate guard, what was that third male doing hanging around? In the time that I watched the group, he didn’t make any moves to get closer to the female, nor did he respond to the vocalizations of the calling male. He just kept plodding along, following the other two birds through the grass. Well, one idea that could explain this male’s behavior stems from the fact that these birds never get to interact with their parents in the nest (the whole brood parasite thing). If you never get to see what your parents do, then how can you learn how to court and woo a mate yourself? Apparently, the answer to this question is to find someone else to watch. Male cowbirds who hang around older, adult males and pairs have a higher reproductive success later in life than males who spend less time around other adults. So in order to be successful and to learn how to do his own mate guarding, he’s got to spend time watching someone else do it.
So in the end, that female had a quite vocal male declaring his ardor for her and trying to drive the other male away. It reminded me a lot of that old Michael Jackson/Paul McCartney song, which is probably a good model for what the young male had to learn. Sing like these two and you shouldn’t have any problems with courtship next season.