Some males mate with more than one female in a breeding season. This is a population definition. If some are mating with more than one female, others will be monogamous, mating with only one female, and still others will be forced to be celibate, assuming that females are mating with only one male. Polygyny is so common because male reproductive success is often a simple function of the number of females inseminated, which generates intense competition among males Kinds of polygyny 1. Female defense polygyny When females are in a herd or group, males can easily defend them resulting in female defense polygyny If females are clumped they can be monopolized economically by males.
Scattered females are less likely to be monopolized.
Females are distributed according to their ecological costs and benefits, and then males take advantage of their distribution, and may further clump them, but the males are not the cause of the female distribution. Yellow-bellied marmots form social units, females remaining near birth site, so clusters of mothers, daughters and aunts arise, probably for protection against coyotes and eagles. Males search for and defend meadows with these female clusters in them. If female gets parental care from male, she is less likely to join a male that already has a female on his territory. The resources would have to be good enough even for the second female to make it worth while Territory resources are great in red-winged blackbirds and indigo buntings where females that were later joiners got just as much reproductive success as females that were monogamous.