Geographic and ecological variation in clutch size of tree swallows

In many birds, populations breeding at higher latitudes have larger clutch sizes. It has been hypothesized that this relationship results from the gre ater availability of food at higher latitudes. We examined geographic varia tion in clutch size of Tree Swallows (Tachycineta bicolor) in relation to l atitude, longitude, laying date, breeding density, elevation, and summer ac tual evapotranspiration (AE). AE is a measure of terrestrial primary produc tivity, and we assumed that it was correlated with resource abundance. Clut ch size data were compiled from nest records throughout the breeding range in the United States and Canada (n = 7,459). Summer AE estimates were made from long-term climatological records, and breeding density was estimated f rom Breeding Bird Surveys. All variables were averaged for each 1 degrees b lock of latitude and longitude (n = 85 latilong blocks). We found a strong positive relationship between clutch size and latitude. Absolute levels of resource abundance (summer AE) were not related to clutch size but were rel ated inversely to latitude and longitude. After controlling for the effects of confounding variables, clutch size was related positively to latitude a nd relative resource abundance (summer AE after controlling for breeding de nsity). Our results are consistent with Ashmole's hypothesis that relative, rather than absolute, levels of resource abundance determine at least some of the variation in clutch size.