Today I saw a gruesome sight: a herring gull picked a blue crab out of the bay at low tide and began eating it alive. It was sort of gross, and I felt sorry for the crab, but hey, a gull’s gotta eat.

This leads into this week’s column: what do birds eat?

You can tell what a bird eats mainly by looking at its bill. Each bird’s beak has evolved over time to be perfect for its diet. Darwin based his theory of natural selection on this; he studied finches in the Galapagos Island and noticed that each species had a different bill and a different diet. The finch that ate fruit had the thickest bill. The finch that ate grubs from the ground had a long, pointy beak and so on.

The bird with the most unusual beak is the pelican. Pelicans use their bills to scoop up fish. Cormorants also have a small pouch under their chins; they are also fish eaters. Ducks and spoonbills, who have flat, broad bills, also eat small insects and decaying material (called detritus) from the water. Wading birds like herons and egrets use their long, pointy bills to stab and catch fish.

Birds of prey have pointy, curved beaks to rip apart the rodents they catch. Ospreys and bald eagles eat more fish than mice, but their bills are similar to their mammal-eating cousins.

The wading birds’ beaks reflect how deep they have to dig in the mud for the small marine animals and insects that they eat. A short beak, like a plover’s or sandpiper’s, means dinner is just below the surface. A long beak, like a dowitcher’s or ibis’s, shows that the bird has to dig a little deeper.

Songbirds’ bills also reflect what they eat. A bird with a triangle bill, like a cardinal or sparrow, usually is a seed eater. A bird with a small beak is an insect eater. Warblers and flycatchers (natch) are good examples of this type of bird.

One of the most unusual birds is the crossbill. This bird has an overlapping beak, which is used to open pine cones. Of course, there is a legend as to the “real” reason the bill is so odd: the story goes, these birds were trying to help Jesus off the cross back in the year 33. I guess it didn’t work, because Christians still celebrate Good Friday and Easter.

But we won’t worry about those holidays for a few months yet.