Bird Polygamy and Polyandry Mating

Polygamy occurs in two forms in bird societies: polygyny (a male with several female mates) and polyandry (a female with several male mates). Polygyny is much more common than polyandry. Among backyard birds, three masters of polygyny are Red-winged Blackbirds, House Wrens, and Eastern Meadowlarks. Males of each of these species may have several female mates during the same breeding season.

The male Red-winged Blackbird achieves its polygyny by impressing first one female, then another. At any one moment it may have several females nesting on its territory, all in different stages of breeding. One may be just beginning, the second well into incubation of eggs, and the third busy feeding the young. This scheduling of the females allows the male to devote different levels of effort to the various females on his territory, thereby not spreading himself too thinly.

Why should a female Red-winged Blackbird select a male that already has a mate? The answer is still unknown. With a bachelor, a female is likely to receive undivided attention during the hard weeks when food has to be brought to the chicks every few minutes. By choosing a male who already has at least one female on his territory, a female is certain to receive only partial help. The female is not unaware of the situation; it interacts with other females on a male's territory. The most plausible explanation is that a female's priority is a male with the best territory. The males without mates may have chosen such bad sites that a female would be worse off with them than it would be sharing a mate with another female.

Polyandry is rare among birds, especially in North America. Of all the species regularly found in the lower 48 states, only the Spotted Sandpiper and Wilson's Phalarope are polyandrous. Several more can be found in Alaska, including the Red Phalarope, a species that carries polyandry to its extreme. The female Red Phalarope is bigger and more colorful than the male and plays no role in incubating or caring for the young. The female spends a week or so with one male, courting it and laying eggs in its nest. Once the nest is full, the female begins searching for another mate. Long before the eggs have been hatched by any of the dedicated males, he female Red Phalarope has headed south in migration.