The Influence of Habitats on Mating Systems of North American Passerine Birds
1966
Regardless of sex ratio, a polygynous mating is expected to be adaptive for the females as well as for the male. Two possible selective bases for the evolution of polygyny are considered: 1) One male may make it advantageous to several females to mate with him by appropriating a large share of a limited number of nest sites. 2) When a large share of the food for the young is obtained from the territory, local variations in food availability could influence the mating system. Given a sufficient minimum level of food supply, the difference in food availability between two males' territories may be great enough to permit a female to rear more young on the better territory, unaided by her mate, than she could on the poorer one even with full assistance from her mate. In this case, selection would favor a female pairing with the male on the better territory, even if this meant establishing a polygynous association. Fourteen of 291 species of North American passerine birds have been reported to be regularly polygynous or promiscuous. Thirteen of the 14 breed in marshes, prairies, or savannah—like habitats, where productivity resulting from solar energy is concentrated into a narrow vertical belt; that in a forest is spread unevenly over a broad vertical belt. Thus, the density of productivity–and of avian food sources–is potentially much greater in marshes and prairies than in forests. Hence, marshes and prairies are more likely than forests to present the minimum requisite food supply and sufficiently great differences in available food between territories for selection to favor polygyny.
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