Caterpillar phenology predicts differences in timing of mountain chickadee breeding in urban and rural habitats

To ensure the survival of their offspring, birds need to precisely time their reproduction: when offspring have the highest demand for food, food resources should be most abundant. In temperate environments, caterpillars are often a key food source for nestlings, so many insectivorous bird species time their reproduction to correspond to the peak abundance of caterpillars in their habitat. Mountain chickadees (Poecile gambeli) are small songbirds that naturally inhabit coniferous forests, but are also found in urban areas. Reproductive timing of these birds may be altered by urbanization, as mountain chickadees in the city have been shown to breed earlier than those in natural habitat. This study aimed to determine if caterpillar abundance drives reproductive timing of mountain chickadees and if urbanization alters the timing of caterpillar abundance. Birds in both urban and rural habitats were monitored throughout the breeding season. Caterpillar abundance was estimated at each nest location by collecting samples of caterpillar excrement (frass). We found that in both urban and rural habitat, frass mass changed throughout the breeding season, but the date of maximum frass mass occurred about one week earlier in urban habitat. However, in both habitats maximum frass mass occurred when offspring were approximately 11 days old. Our results suggest that mountain chickadees time their reproduction to correspond to caterpillar abundance, and birds in urban environments may be reproducing earlier to correspond with earlier peak caterpillar abundance in the city.