Grit ingestion by nestling Tree Swallows and House Wrens

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Abstract
We examined the stomach contents of 200 Tree Swallow (Tachycineta bicolor) and 187 House Wren (Troglodytes aedon) nestlings for grit. Grit, composed mainly of small stones, sand, and mollusk shells or fragments, was found in 35% of the swallow and 31% of the wren stomachs. Grit ingestion started about 3 d following hatching in wrens and about 5 d after hatching in swallows. After reaching a peak, it declined in both species towards fledging. The mean number of grit particles was 10.2 in swallows and was significantly lower than that of 22.3 in wrens. The mean mass of grit particles was 17.2 mg in swallows and was significantly higher than that of 6.2 mg in wrens. The number and mass of grit particles were significantly correlated with age in swallows; only the latter correlation was significant in wrens. Analysis of a few adult stomachs suggested that grit ingestion was similar to that of nestlings.

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