CALCIUM SUPPLEMENTS IN THE DIET OF NESTLING TREE SWALLOWS NEAR ACID SENSITIVE LAKES

We quantified supplemental sources of calcium in the diet of nestling Tree Swallows (Tachycineta bicolor) near acid sensitive lakes in northwestern Ontario. Among the calcium-rich items brought to nest boxes by adult swallows and ingested by nestlings, fish bones (particularly flat pieces) were most numerous, followed by crayfish exoskeleton, clam shell, and bird eggshell in that order. Only 5% of the items eaten were longer than 10 mm, whereas over 29% of the items found in nest boxes were in this size class. We found significantly fewer calcium-rich items in the stomaches of nestlings from acidic lakes than in those from reference lakes, suggesting that items suitable for ingestion may be scarcer there. However, Tree Swallows flew at least 50 to 650 m from their nest-site lake to obtain calcium-rich items. Because lake acidification has reduced numbers of animals providing calcium for Tree Swallows in regions in which many lakes are atmospherically acidified, the availability of calcium sources for nestlings may be seriously affected.