Over the next eight weeks, Neotropical migratory birds will return. Some will move on; others will stay and nest locally.
On a good morning, birders can count 30 or more species feeding in the tree tops. But seeing many species together, often in feeding flocks in the same trees, raises an interesting ecological question: How can they all occupy the same place at the same time?
A basic ecological principle states that two or more species cannot occupy the same niche at the same time. The result is competition, and only one species can prevail.
This is where careful observations can resolve an ecological puzzle. A classic example of multiple species occupying the same space comes from the boreal forests of the northeastern United States. Five species of small, insectivorous warblers live and nest among these conifers.
Careful observations, however, reveal that though these species do inhabit the same trees, they use them in different ways.
In the 1950s, ecologist Robert MacArthur recorded the amount of time each species spent in different parts of the trees. Though they used the same trees, they divvied up the parts of the trees they used.
Cape May warblers, for example, spent most of their time foraging in the highest, outermost branches. Yellow-rumped warblers spent most of their time in the lower branches close to the ground.
Black-throated green warblers used the outer branches of the trees’ midsection. Blackburnian warblers concentrated on the outer branches from the middle to the top of the tree. And bay-breasted warblers spent most of their time on the interior branches in the middle of the trees.
Though all five warblers spent most of their time in the same trees, they minimized competition by using different parts of the trees to find food.
Another obvious example of multiple species seeming to occupy identical niches is Darwin’s finches on the Galapagos Islands. In this case, 13 species are distributed among the islands and differ in body size, bill shape and size, and behavior. The seed-eating species have various sized bills.
Those with smaller bills eat smaller seeds; those with bigger bills eat bigger seeds. The woodpecker finch uses cactus spines or small twigs to extract insect larva from rotten wood. The warbler finch has a small warbler-like bill and eats insects. The cactus finch eats the seeds, fruits, nectar and pulp of prickly pear cactus.
Though several of Darwin’s finches may use the same habitats, they minimize competition by eating different foods.
Woodpeckers, nuthatches, and brown creepers partition their foraging behavior in a similar manner. They all hunt for insects and spiders on the furrowed bark of trees, but they do so from different perspectives.
Woodpeckers hitch their way up tree trunks and have different sized bills so downy woodpeckers, for example, eat smaller prey than pileateds.
Nuthatches typically work their way down tree trunks so their point of view is quite different from woodpeckers’. And creepers typically spiral up tree trunks, then fly down to the base of the next tree.
These different perspectives give these species a unique look at the same places. Several species of wading birds also often forage together, especially along coastal areas.
Great blue herons, great egrets, snowy egrets, and even night-herons can sometimes be seen feeding together. The bigger birds obviously are capable of eating bigger prey, but they also vary what they eat. Great blue herons eat fish, frogs, turtles, snakes, and small mammals and birds.
Great egrets eat mostly fish and other aquatic prey. Snowy egrets eat smaller fish, crustaceans, and other invertebrates. And yellow-crowned night-herons prefer crabs, crayfish, and mollusks.
Waders reduce competition by eating different prey of varying sizes. Backyard bird feeding stations can attract as many as a dozen species at any one moment.
In this case, however, competition is minimized or even eliminated because the supply of food is unlimited. Only when feeders run out of food does competition ensue, and then bigger birds such as blue jays and red-bellied woodpeckers prevail.
Birding is more than learning names. It’s also about studying behavior to understand each species’ natural history.