Experimental confirmation of the polygyny threshold model for red-winged blackbirds

The polygyny threshold model assumes that polygynous mating is costly to fe males and proposes that females pay the cost of polygyny only when compensa ted by obtaining a superior territory or male. We present, to the authors' knowledge, the first experimental field test to demonstrate that females tr ade mating status against territory quality as proposed by this hypothesis. Previous work has shown that female red-winged blackbirds (Agelaius phoeni ceus) in Ontario prefer settling with unmated males and that this preferenc e is adaptive because polygynous mating status lowers female reproductive s uccess. Other evidence suggests that nesting over water increases the repro ductive success of female red-winged blackbirds. Here we describe an experi ment in which females were given choices between two adjacent territories, one owned by an unmated male without any over-water nesting sites and the o ther by an already-mated male with over-water sites. Females overwhelmingly preferred the already-mated males, demonstrating that superior territory q uality can reverse preferences based on mating status and supporting the po lygyny threshold model as the explanation for polygyny in this population.