Breeding biology of tree swallows in relation to wetland acidity

We examined aspects of the breeding biology of tree swallows (Tachycineta bicolor) in relation to the pH, presence or absence of fish, and dissolved organic carbon concentration in 51 wetlands adjacent to nest boxes northeast of Sudbury, Ontario. Wetland pH was positively related to swallow clutch size, the total volume of eggs in a clutch (clutch volume), the number of fledglings per successful nest, and several measures of nestling size and growth. Swallows breeding near wetlands where fish were present laid earlier, but had smaller eggs and clutch volumes and slower growth of primary feathers than swallows breeding near fishless wetlands. Organic carbon concentrations were poorly related to the reproduction of swallows. These results are discussed in relation to possible mechanisms for the effects of wetland acidity on avian reproduction.