Fledging success peaks later than insect prey biomass in two aerial insectivores: Evaluating potential phenological mismatch and fitness consequences

Abstract Understanding potential impacts of climate change on phenology is limited for birds breeding in northern North America. We investigated phenology and fledging success of 2 species of aerial insectivorous birds—Contopus cooperi (Olive-sided Flycatcher) and Contopus sordidulus (Western Wood-Pewee)—experiencing population declines over the last decades, and their potential insect prey in southern Yukon, Canada, between 2013 and 2017. Our objective was to evaluate whether suboptimal timing between predator and prey resulting from climate change (the phenological mismatch hypothesis) could be contributing to reduced fledging success in these 2 species and to compare findings between the 2 species. Insect biomass revealed a broad peak across each breeding season and interannual variation. Population-wide breeding activity (nesting score) was 11.46 days later than insect biomass (calculated using an average daily insect weight from hanging Malaise traps) and 8.93 days later for transect counts of large insects for C. cooperi and 10.44 (Malaise) and 11.23 days (transects) later for C. sordidulus. Daily nest survival for entire nesting periods was low (0.23, 62 nests, and 0.29, 60 nests, for each species, respectively) and dependent on year and breeding attempt. Daily nest survival rate was negatively influenced by nest age, and positively by attempt (first or second), date of nest initiation in C. cooperi and by offset from insect biomass peak in C. sordidulus. Contrary to predictions, daily nest survival was higher for later nests. Insect biomass did not significantly influence daily nesting scores. Our results suggest that although breeding activity was mismatched with prey biomass peaks for the northern species we studied, it was not necessarily mis-timed, in that later nests were more successful. We discuss these unexpected results in the context of potential constraints on early breeding of northern insectivorous birds due to latitude or altitude.