Blackbirds that flock together breed better.

Blackbirds that flock together breed better

In the red-winged blackbird mating game, the female calls the shots. And when shopping for a mate, she seems to care more about real estate than looks, basing her choice on the territory the male bird controls. Now researchers say she also bases her choice on whether she knows his neighbors.

Gordon H. Orians and Les D. Beletsky of the University of Washington in Seattle found that male red-winged blackbirds with familiar neighbors attract significantly larger harems each year and thus have more offspring. They say their "familiar neighbor" hypothesis represents a new twist on what zoologists call the "dear enemy" relationship. Territorial birds and other creatures that battle each other over space also band together to defend nests from predators.

In the October PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol.86, No.20), Orians and Beletsky say they deduced their "familiar neighbor" hyphothesis from a decade of watching red-winged blackbirds, Agelaius phoeniceus, in Washington's Columbia National Wildlife Refuge. Beletsky says they found the strongest evidence of the "familiar neighbor" effect in pocket marshes, where males with familiar neighbors averaged 10.3 fledglings per year compared with 6.4 for birds that nested near strangers. In strip marshes, where fewer blackbirds nest, knowing the neighbors didn't significantly influence fledgling numbers.

Orians and Beletsky say other factors exert a stronger influence on breeding success than familiarity with neighbors. But they think the phenomenon may directly help males by reducing the time they spend defending their territory. Females may prefer nesting near familiar males because of the females' mating habits, which Orians says show "less than the highest Christian morals." Females "sneak copulations with other males," and those males may be more likely to help feed her young or defend her nest.

Ken Yasukawa, a behavioral ecologist at Beloit (Wis.) College, calls the finding "pretty exciting." While Yasukawa says he suspects the phenomenon holds true for other red-winged blackbird populations, he notes that species in different areas often behave differently. For example, while males in the Washington refuge rarely feed their nestlings, three-quarters of their Wisconsin counterparts do, Yasukawa says.