Severe recent decrease of adult body mass in a declining insectivorous bird population

Migratory bird species that feed on air-borne insects are experiencing widespread regional declines. Agricultural intensification in the breeding range is often regarded as one of the main drivers of these declines. Here, we tested the hypothesis that body mass in breeding individuals should reflect habitat quality in an aerial insectivore, the tree swallow (Tachycineta bicolor), along a gradient of agricultural intensity. Our dataset was collected over 7 years (2005–2011) and included 2918 swallow captures and 1483 broods. Analyses revealed a substantial decline of the population over the course of the study (−19% occupancy rate), mirrored by decreasing body mass. This trend was especially severe in females, representing a total loss of 8% of their mass. Reproductive success was negatively influenced by intensive agriculture.
Sébastien Rioux Paquette, Fanie Pelletier, Dany Garant, Marc Bélisle. Proc Royal Society B Published 21 May 2014. DOI: 10.1098/rspb.2014.0649