Rapid temporal change in frequency of infanticide in a passerine bird associated with change in population density and body condition

Sexually selected infanticide, whereby unmated males obtain a mate by killing the dependent offspring of an already mated female, is a common alternative reproductive strategy in many animals. I estimated the frequency of infanticide in a population of barn swallows, Hirundo rustica, during the period 1977--2002. Population size decreased by more than a factor 10 during this period, and this decrease was associated with an increase in mortality, selecting for adults with better body condition. Density-dependent effects acted on infanticide through the relationship between the relative number of unmated males and population size. Because males in prime condition are better able to defend their nests against infanticidal males, the frequency of infanticide decreased as mean tail length and mean body condition of males increased during the study period. Therefore, a rapid decrease in population size, a decrease in the abundance of unmated males, and a concomitant increase in body condition have changed the importance of infanticide from being a major cause of mortality accounting for more than 25% of all nestling mortality to being almost completely absent during a period of 25 years. Copyright 2004.