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Photo © Keith Williams
Photo © Sharon Sorenson

These Birds Have Grit

Nest monitor Sharon Sorenson shared an interesting photo and question with us this month. She asked, “Why did a Tree Swallow nest contain 14 tiny mussel shells?”

Tree Swallows may eat mollusk shells for the grit, which helps them grind up their food in a muscular part of the stomach called the gizzard (a function performed by teeth in mammals). But many other bird species obtain grit from sand, small stones, and other similar items. So why do swallows prefer mollusk shells as a form of grit? Perhaps it is because they contain calcium and other minerals, and maybe a tasty morsel as well.

It is not well-known how swallows eliminate the grit, but given that these shells were heavily soiled when found, and parents usually carry away fecal sacs, we suppose that they must have been cast up as indigestible material.

We’ll never know all the secrets of birds, but we love trying! Keep asking puzzling questions, and we’ll answer them if we can.

Cornell Lab of Ornithology

Cornell Lab of Ornithology