TREE SWALLOW DIET IN RELATION TO WETLAND ACIDITY

We examined 12 825 prey of tree swallows (Tachycineta bicolor) northeast of Sudbury, Ontario, to observe the importance of aquatic food and the influence of wetland pH on swallow diet during the breeding season. Items of aquatic origin constituted most of the biomass in diets of both nestlings and adults. Molluscs and Ephemeroptera (mayflies), whose distributions in the study wetlands were strongly influenced by pH, were eaten by swallows, particularly nestlings. The major change in nestling diet in relation to wetland acidity was a decrease in mayflies and a corresponding increase in aquatic Diptera at wetlands of lower pH. The number of aquatic taxa eaten by nestlings was lowest at acidic wetlands. The total biomass of emerging insects captured at acidic wetlands was not lower than at wetlands of higher pH, and in fact tended to be greater. The rate at which swallows fed their young and the amount of food fed to nestlings were not related to wetland pH or other wetland characteristics. Nevertheless, the reduced availability of calcium-rich foods at acidic wetlands and the change in types of food available may play a role in the reduced reproductive success of swallows at acidic wetlands.