“They’re not your birds,” our guide said.
I was with a Maine Audubon group in Costa Rica a few years ago. We were watching a chestnut-sided warbler that nests in Maine and winters in the tropics. One of our group exclaimed how nice it was to see “our” birds in Costa Rica. With a sincere smile, our guide gently reminded us that our birds are their birds most of the year.
And so they are. A majority of American songbirds are neotropical migrants. That is, they nest in the northern latitudes but winter in the tropics.
That raises an obvious question. If there’s enough food for them down there all winter, why do they bother to nest up here? Fundamentally, it’s a math problem.
First, North America is larger than Central and South America combined. Up here in the north, they have room to spread out. Down there, they must cram together with the abundant birds that live in the tropics year-round. There is enough food to sustain everybody in winter, but not enough to start and feed a growing family. Too much competition.
Second, seasons don’t change much near the equator. The food supply stays static most of the year. In the northern latitudes, you may have noticed all the insect life that springs forth in May, then dwindles by October. Neotropical migrants can simply make more babies up north, where there is seasonally more food and less competition.
Most birds that head for the tropics eat insects, including most warblers, flycatchers and thrushes. Birds with a diet dependent on seeds and vegetation may migrate, but most don’t leave the country.
Sparrows and blackbirds go just far enough south that snow doesn’t bury their food supply. The song sparrow is our most common Maine sparrow. Some of them migrate no farther south than Portland. Finches that feed in treetops all winter simply wander until they find the food they’re looking for.
This is where things get a little tricky. When all those North American breeders return to the tropics in autumn, they still must sort themselves out so that there’s enough food for everybody. Take warblers, for instance.
Some warblers seek habitats like those up north. Some change their diets and adapt to a different habitat and food supply in winter. Such varying strategies affect where they go and how they distribute themselves.
Final destinations for neotropical migrants include the Caribbean Islands, Mexico, Central America and the northern countries of South America. Some even find southern states to be suitably hospitable. In short, some warblers migrate farther than others.
Yellow-rumped warblers have the most adaptable diet, switching from insects to berries for much of the winter. Many go no farther than New Jersey. But their winter range extends through all the Caribbean Islands and every country in Central America.
The winter range of black-and-white warblers begins in North Carolina but extends as far south as Ecuador. Pine warblers are dependent on pines year-round. Birds that spend the summer in the white pines of Maine spend the winter in the yellow pines of southern states.
January palm warblers can be found in Delaware, and most stay in the country. A few linger among Caribbean palm trees and the coast of Central America. Nashville warblers go no farther than southern Mexico.
Oddly, black-throated green warblers reach South America. The winter range of black-throated blue warblers stops in Panama. Cape May warblers don’t even get south of the Caribbean.
Maine’s ubiquitous common yellowthroat winters all across the southern states, extending its range through the Caribbean Islands and most of Central America, but not to South America. Likewise, Maine’s magnolia warblers and northern parulas don’t reach South America.
Ovenbirds, yellow warblers, chestnut-sided warblers and bay-breasted warblers reach Venezuela and Colombia. Tennessee warblers make it as far as Ecuador. Canada warblers and American redstarts reach the lowlands of Peru. Blackburnian warblers head for elevation in the Andes Mountains as far as Bolivia.
And the champion neotropical migrant? Blackpoll warblers. When they leave New England, they head straight for South America, completely bypassing Central America and the Caribbean.
Our guide was right. All these warblers may be our close neighbors in summer, but they span a vast area during the rest of the year. We share “our” birds with a lot of other people.
