The Crazy Stuff Birds Eat

Yellow-rumped Warbler on poison ivy / © Bryan Pfeiffer

Note: This is a version of my regular column in Northern Woodlands.

FOR MANY OF US who watch their behavior, birds are what they eat — or at least are predictable in their diets: Kingfishers catch fish. Hummingbirds drink nectar. Robins eat earthworms. Flycatchers catch flies.

A Yellow-rumped Warbler eating poison ivy fruits in Massachusetts (November 2014) / © Bryan Pfeiffer

Except when they don’t.

Plenty of birds go off their diets, breaking the rules or notions we often harbor about what they eat.

I’ve seen a Herring Gull crush and swallow an Arctic Tern chick, and a Bald Eagle nibble on the leg of a road-flattened Northern Leopard Frog. I’ve watched a Snowy Owl try to catch fish or ducks at sea, and a Great Blue Heron unearth and swallow a gopher. What may seem unsavory to us, is usually an avian delicacy.

Migration drives big changes in diet. Besides requiring raw determination and innate navigational skills, migration takes calories, lots of them, stored as fat and burned as fuel on long-distance flights. Insects are a fine source of protein and fat for southbound songbirds every fall.

But insects become scarce during the fall migration, which would seem to be a problem for classic insect eaters such as flycatchers and warblers. So the insectivores go for the next best thing: fruit. In one study of 69 bird species stopping on Block Island, Rhode Island, all but one (Winter Wren) had fruit in their droppings. Least Flycatchers, Blackpoll Warblers, Red-eyed Vireos and scores of other self-respecting insectivores find essential fuel in the fruits of viburnum, bayberry, pokeweed, Virginia creeper, and other shrubby plants and vines.

Gram for gram, fruit is lower in lipids and protein than insect tissue. Even so, some birds fare well at the fruit bar, particularly avian omnivores such as tanagers and sparrows, which tend to eat fruit year-round and can maintain body mass and conditioning on a fruit diet during migration.

The classic insectivores in the study, however, did not fare as well when fruit constituted a greater portion of their diet. Yet they often don’t have much choice. On the journey south, songbirds might stop where fruit is the only option. If you were weak and hungry with miles to go, you might settle for an apple if your hamburger weren’t available.

A Red-eyed Vireo on the invasive Japanese barberry on Monhegan Island, Maine (September of 2014) / © Bryan Pfeiffer

For many migrants on their way south, the eastern seaboard offers a banquet of fruits such as rose hips, bittersweet, and poison ivy. Bayberry, a waxy source of fat for otherwise insectivorous Yellow-rumped Warblers and Tree Swallows, grows in abundance along the east coast. But the coast is also where a good many people live. Ornithologists now recognize the value in keeping these shrubby swaths of habitat (even some invasives) from being replaced by lawns or housing or shopping malls. For many songbirds, it’s not only about the breeding and wintering grounds; it’s also about the fruity turf that lies between.

What may seem odd to us, of course, usually isn’t so odd for birds. Nature has a way of toppling our assumptions. Those of us in the Northeast, for example, who see warblers and flycatchers turning from insects to fruit in autumn, might have it backwards. After all, many of those birds came here to nest and to feed their young from our spring flush of black flies, mosquitoes, and inchworm caterpillars. They visit for three or four months of the year – and then they’re gone, back to the tropics to eat fruit.

So a songbird that we know as insectivorous may, in fact, be little more than a fruitarian that in spring prefers to eat bugs.

Reference:

Parish, J.D. Patterns of frugivory and energetic condition in Nearctic landbirds during Autumn migration. The Condor.  v. 99, no. 3, p. 681-697.