Spring migration is winding down. The arrival of Blackpoll Warblers, Black-billed Cuckoos, Saltmarsh Sparrow and Nelson’s Sparrow signifies the end of the spring spectacle.
Having returned to Maine from the south, male birds are setting up territories and trying to attract a mate with songs and displays. We have a tendency to project an idyllic image of a happy bird couple raising a family. That image is often inaccurate.
For starters, not every bird will be able to find a mate. Some of the best evidence for this statement comes from an experiment that was done in Maine more than 50 years ago. The methodology of the study will be reprehensible to some. Nevertheless we learned much from this experiment.
The researchers mapped out a 40-acre forest plot. In early June they determined that 154 territorial birds (males) were present. Then the removals began. The researchers shot as many of the singing males as they could. Within two weeks the density of male birds was reduced to 21 percent and kept at that level until July 11. By July 11, 528 adult birds had been killed. That’s 3.5 times the original density of birds.
This removal experiment tells us there are lots of unmated males that are lurking around, hoping for a chance to acquire a territory and a mate. These non-territorial birds are called satellite males in the ornithological literature. The experiment shows that there must be many satellite males waiting for an opportunity.
As in mammals, birds show a 50-50 proportion of females-males. With so many satellite males, there must be unmated females present as well who never come into the vicinity of an unmated male. The failure of some birds to find a mate challenges our notion of wedded bliss in birds.
An even greater challenge to that notion lies in the fact that avian social life has soap-opera aspects. Cheating on a mate occurs frequently. Thanks to the development of DNA fingerprinting techniques, we can determine the paternity of nestlings. Although 90 percent of bird species are classified as monogamous, 30 percent of nestlings are sired by a male other than the female’s mate.
It’s a two-way street. Females seek multiple partners to fertilize their eggs and males seek as many female partners outside of the pair-bond as they can find. These dalliances are referred to as extra-pair copulations and their importance in the field is indicated by the fact that every ornithologist knows the initialized version, EPC, of this behavior.
The male incentive for EPCs is clear: to father as many baby birds as possible. What’s the advantage for a female who can produce only a few eggs? The best argument is that the female is seeking to increase the variability of her nestlings. The environment is always changing and having greater variation in her offspring increases the chances that one or two of those offspring will better fit the demands of the environment in the future.
The reproductive life of a territorial male is pretty good. He fathers at least some of the eggs laid by his mate and perhaps has some EPCs with females on neighboring territories. But what about those unmated satellite males? Do they get to reproduce at all? Satellite males do not have a territory because they are outcompeted by the stronger males who can defend a territory. Weaker males are usually younger as well.
Some males engage in a type of trickery called delayed plumage maturation. In the second year of their life, their plumage resembles that of a female. This disguise allows them to slip into the territory of a male (he’s got cheating on his mind) and sneak a quick mating with the resident female. Check your field guide to see the second-year plumage of American Redstarts, Baltimore Orioles and Red-winged Blackbirds.
Herb Wilson teaches ornithology and other biology courses at Colby College and perches in South China. He welcomes reader comments and questions at: [email protected]