Biologists have a set of terms to describe the various ways different species interact with each other. You’re probably familiar with parasitism, when one organism benefits at another’s expense. And you may have heard of mutualism, in which two species work together for each other’s benefit. A less familiar and more slippery term is “commensalism.” Literally, it means “eating together,” and in biology, the term describes interactions in which one species benefits while the other experiences little or no effect.
Some biologists argue that true commensalism, by that strict definition, may not ever exist. And there is generally little point in haggling over whether two species are truly in a commensal relationship. But as a concept, it’s illuminating, especially when applied to relationships between humans and other species: an amazing number of other species have learned or evolved the knack for gleaning crumbs from the human table. Thinking in terms of commensal relationships shifts our attention from ourselves to our neighbors, helping us understand how wildlife views human habitats.
An example of a human commensal, strictly defined, might be the ever-popular cockroach. Presumably cockroaches live in the wild somewhere (certainly many of their close relatives do). But true cockroaches, in the experience of most people, always exploit the shelter and food found in human dwellings, where, as unpleasant as we may find them, the bugs do us no biological harm. Unwelcome they may be, but they’re guests at our table.
Outdoors, the cockroach has a rough equivalent in the house sparrow. Even in its native Europe, this finch associates closely with humans, though millennia ago it obviously must have lived on its own. House sparrows were deliberately imported into this country to suppress insects around barns and stables. This bird still enjoys an active livestock operation (horsetail hairs are a favored material for nest construction), but it also thrives in urban and suburban settings. Indeed, you’ll rarely find a house sparrow away from human-modified habitat. Attics and sheds provide sheltered nesting sites, and a reliably stocked feeding station almost invariably has a mob of these raucous finches hanging around.
Its appetite for flies and maggots still makes it handy around a barn, but the house sparrow is generally a visitor that overstays its welcome. Gregarious, noisy, and aggressive, this species out-competes most other birds for food and nesting sites; you might say the house sparrow is a case of commensalism gone wild, with the guest species flourishing to the point of becoming a nuisance.
At the other end of the spectrum, there are many birds that want nothing to do with humans. The ovenbird, wood thrush, scarlet tanager, and many other woodland birds require large tracts of undisturbed habitat in order to breed successfully; in the fragmented habitat of developed areas, their nests are vulnerable to predators and nest parasites.
A surprising number of birds fall between these two extremes, fully capable of living away from humans but able to make some use of resources that human settlement offers. Successful commensalism, though, requires several qualities in birds. They must have the ability to find and make use of the specific things that humans produce. One example might be berries from the many fruit-bearing shrubs we’ve domesticated as ornamentals (mockingbirds, strongly commensal, typically survive the winter by relying on this food source). Another might be the structural pattern of suburban development, with open areas like lawns juxtaposed to patches of dense cover (this pattern suits American robins to a T, putting nest sites and foraging areas in close proximity).
Commensal birds must also be able to handle frequent disturbance by humans. Disturbance carries a high price for birds: in reacting to humans, birds expend enormous energy and are distracted from more important work, like attracting a mate, incubating eggs, or protecting young.
In some cases, commensal birds simply grow used to us; yard-nesting chickadees, for example, grow almost tame once they realize we aren’t going to try to eat them. Other birds may take advantage of the fact that human activity tends to keep many predatory species away. Carolina wrens, for example, never stop getting worked up by a human near their nest, yet appear to breed quite successfully in yards. Perhaps the presence of humans, while it never stops annoying the wrens, also repels the predators most likely to raid a wren nest, producing an overall benefit for the birds.
If you watch birds around your yard and neighborhood, you’ll quickly learn which species flourish in these areas, and which species are absent. And you can determine what resources — food, shelter, or safety — attract these commensal “dinner guests.” Densely developed areas support only a subset of our local wildlife. But it’s habitat nonetheless, and good habitat, too, from the perspective of those species with the habits and flexibility to adapt to it.