Demography, environmental uncertainty, and the evolution of mate desertion in the snail kite.

Abstract

Rostrhamus sociabilis, an endangered hawk, has a unique mating system in Florida; when food is abundant, males or females desert their mates at nearly equal frequency (ambisexual mate desertion) in the midst of a nesting cycle. Kites have 1) a very high nest failure rate (mean 68%) due most often to unstable nest sites and predation; 2) a variable nesting season (5-10mo/yr); 3) an early age of first reproduction for a bird this size (10mo); 4) a high degree of iteroparity (double and potentially triple clutching within a season); and 5) unstable populations. Both nesting success and population size were directly related to Everglades water levels and resultant snail densities. Kites responded to large annual changes in food abundance, not by adjusting clutch size but by deserting their mates and presumably attempting to renest. Kite demographic traits appear to be adaptations to or results of an uncertain environment. Mate desertion behavior probably evolved in response to a smaller average clutch size; this would allow females to be highly iteroparous and avoid the costs of overinvestment, and should be strongly favored in a highly uncertain environment.-from Author