Black-throated blue warblers are small, territorial, insect-eating songbirds that breed in eastern North America and migrate to the Caribbean for the winter. Most males are "socially monogamous," meaning they are mated to a single female. About 10% of males are bigamous, with two females nesting on their territories. However, all is not as it appears: up to 50% of Black-throated Blue Warbler nests have young sired by a male that is not the territory holder. Dr. Sara Kaiser, a recently graduated Ph.D. student at the Cornell Lab of Ornithology and the Smithsonian Migratory Bird Center, has been studying the mating system of these warblers in the hardwood forests of New Hampshire.
The number of young that Black-throated Blue Warblers can produce in a 2-3 month breeding season is limited by the amount of food in the forest. Males can increase this number by surreptitiously mating with females that are already paired with other males. Searching for "extra-pair" mates requires time and energy. Males must decide when and where to invest their energy into this behavior because pursuing additional matings conflicts with their ability to guard their social mates from other males, or to feed their chicks.
Kaiser analyzed warbler behavior and genetic data collected from 2009 to 2012. She discovered that males mostly mated with extra-pair females while their "social" mate was incubating her eggs, and that males on food-rich territories sired fewer extra-pair young compared to males on food-poor territories. Why? Because females were more faithful to social mates that had territories with lots of food. She also found that males on food-rich territories mostly mated with females on neighboring territories, while males on food-limited territories mated with females several territories away.
Kaiser's results suggests that males on high quality territories spend more time "close to home," defending their territories from intruders. In contrast, males and females on poor territories need to travel further to forage. This could affect the encounter rates between males and females. Kaiser's research, to be published in 2014, helps explain how habitat quality affects where and how often seemingly monogamous songbirds mate outside their pair-bond.