Food is as important to wild animals as it is to humans, providing the energy needed for motion, growth, reproduction, and, in the case of warm-blooded critters like birds, maintaining a suitable body temperature. Large portions of a bird’s time and effort goes toward finding and consuming food, and the reliable availability of a suitable diet largely determines where a species can survive. Moreover, the diets of birds can influence entire ecosystems; predation helps controls the populations that are being preyed upon. It’s not too much of a stretch to say that an ecological system is nothing more than the sum of the eating that goes on within it.
Add to this the fact that, from the perspective of a human observer, foraging is often the most interesting aspect of bird’s behavior, and you can see why understanding the avian diet is a major focus for ornithologists. But it’s not always easy to tell what is on the menu for a species you’re studying.
Without access to the amazing optical equipment that birders today enjoy, ornithologists in the 19th and early 20th centuries relied on a shotgun as standard field equipment, and the few identification guides that were available for birds assumed that you were holding a specimen in your hand. Unsurprisingly, this approach extended to the study of bird ecology: older accounts of birds are replete with studies of digestive tract contents, with the recent diet of scores or even hundreds of dissected birds broken down by type of insect, fruit, or seed.
The method has its merits, yielding precision and indisputable results, and the results of such historical studies are still useful. However, resourceful observers of today prefer not to slaughter the objects of their observation, but they can still produce surprisingly good assessments of the diet of wild birds.
Often birds leave behind evidence of what they’re eating. Famously, hawks and owls routinely expel the indigestible parts of their prey (depending on species, that means anything from beetle wings to bones, teeth, and hair) in the form of so-called “pellets.” Tossed up like cats’ hairballs, pellets accumulate beneath favored perches, and pulling them apart readily yields a look at the bird’s diet. Biologists skilled in identifying the tiny bones of rodents can produce a precise picture of what hawks and owls are catching.
Bird feces can work the same way, especially in the case of birds dining heavily on fruit. Full of water and simple sugars and starches, fruit and berries travel rapidly through a bird’s gut, emerging at the other end still retaining the distinctive color and texture of skins, pits, and flesh. In situations where multiple kinds of berries are available, a little attention to bird poop will let you determine which kinds of fruit are most appealing to birds. (And it has persuaded me that our catbirds eat more of our modest blueberry crop than my wife and I do!)
One recent twist in the diet analysis of birds of prey comes from the proliferation of “nest cams” — video cameras set up to focus on a hawk or owl nest. Running 24 hours a day, such cameras show the full gamut of domestic behavior by these birds and often allow precise identification of the prey species being fed to the nestlings. One such camera, for example, focused on the nest of a pair of peregrine falcons in the Cincinnati area, revealed a surprising fondness for yellow-billed cuckoos and Virginia rails. Neither species is very common, and their disproportionate representation in the diet of these birds suggests that these species were either especially easy for the adult falcons to capture, or strongly preferred for some reason (perhaps they simply taste good?).
The debris a feeding bird leaves behind can be as informative as what emerges from the bird’s digestive tract: I keep current on the diet of the down-Island gull population by checking what kinds of shells are accumulating on the sea wall near the drawbridge. At various points in the year, bay scallops, clams, whelks, and spider crabs dominate the mix; I’m sure that the ratio of prey species accurately reflects what the gulls are eating and also gives a hint of the seasonal movements or changes in abundance of these shellfish in the shallow waters of the Lagoon and outer harbor.
But most enjoyable for me is actually observing birds eat. With stealth, patience, and good optics, it’s often possible to glean surprisingly good information on avian diets. Loons and diving ducks, for example, capture their prey underwater but typically swallow it on the surface. Often the process involves manipulation of the prey item, breaking it up or getting it oriented for easy swallowing. That may afford a momentary glimpse of, say, the spindly legs of spider crab or the distinctive caudal fin of a mummichog. It’s knowledge that adds to my understanding of the Island’s ecology; it’s also a rewarding moment of insight into the details of a bird’s hidden life.