We see a variety of birds crowding around the bird feeders now that it’s getting colder and snowier — some small in size, some larger, some that are closely related but still using the same resource. When there is plenty of food in the feeder, there is no problem sharing the largess, but when the feeder is empty (or there is no feeder), different rules apply.
As a general rule, most animals competing for resources in a given environment (like the backyard) tend to specialize in how they find food: woodpeckers drill holes in the bark seeking out wood-boring insects; nuthatches comb in the bark upside down, looking under it the lower edges for prey that may hide there, while brown creepers explore the same bark right-side up exploring the top edges of the bark; chickadees and often downy woodpeckers as well, explore the periphery of branches and twigs from both the rightside up and upside down positions.
But what happens when individuals of two closely related species that specialize in finding food in very similar ways try to occupy the same backyard?
You might predict that the bigger, faster, stronger, or craftier of the two species would displace the other. But you might be surprised to find that often the two species can coexist if they are different enough in size — often by a factor of two (i.e., twice as heavy, twice as long or large in girth). A few examples:
Red-breasted vs. White-breasted Nuthatches
Red-breasted Nuthatches are about the same size as a chickadee, and weigh about half of what a White-breasted Nuthatch does. The two species overlap in their winter range in the continental U.S., but Red-breasted Nuthatches prefer coniferous forests and pine seeds, while White-breasted Nuthatches prefer to forage in deciduous trees, as well as conifers.
How do they coexist? Probably by where they hunt and how they use their chisel-shaped bill to excavate cracks in the tree bark.
Tufted Titmouse vs Black-capped Chickadee
Tufted Titmice overlap with Black-capped Chickadees in the northeastern part of the U.S. and with Carolina Chickadees in the southeastern part of the U.S. Like the nuthatches, there is about a two-fold difference in size, with Tufted Titmice weighing about twice what the diminutive chickadees weigh.
All three species are fond of sunflower seeds and peanuts and are frequent bird feeder visitors, as they switch from a strictly insectivorous diet in the spring through summer to a mostly seed diet in the winter. Titmice prefer the largest seeds offered at a feeder, and in the absence of feeders will typically feast on beech nuts and acorns, which they can drill more effectively with their broadened, sturdy bill than a chickadee can. Thus, differences in size (and bill shape) enables these two species to split up the food resources by seed size and type.
Downy vs Hairy Woodpecker
What is particularly interesting about these two species is that they are not at all closely related, yet they have such similar plumage patterns. Hairy Woodpeckers weigh 3-4 times more than Downy Woodpeckers and definitely dominate them when they are present together on a feeder. Other than suet and peanuts, both woodpecker species excavate under bark for insect larvae and pupae, although Hairys prefer to excavate along the main trunk and major branches of a tree, and Downys forage higher and on more peripheral and smaller diameter limbs. In addition, Downy Woodpeckers often forage on tall perennials, like goldenrod and mullein, for insects that infest the stems or seeds of those plants.
So what advantage is it for a Downy Woodpecker to look like its much larger relative?
Using data from Project FeederWatch, several scientists determined that because Downy Woodpeckers look like the larger and more pugnacious Hairy Woodpeckers, they were able to displace more species at a feeder than would be predicted for a bird their size, even driving away larger-bodied Cardinals, finches, sparrows, and nuthatches. Thus Downy Woodpeckers mimic Hairys to gain a feeding advantage, just as Viceroy butterflies mimic Monarchs to avoid being eaten.
Check out the fascinating graphic created by a Cornell ornithologist’s analysis of some of the Project FeederWatch data, to see the winners and losers when bird species interact at feeders. If you hover over a particular species, the graphic shows negative interactions (being displaced) in red and positive (doing the displacement) ones in blue.