by Carrol Henderson
Snowbirds (n): Residents of northern states and provinces that typically winter in warm southerly regions of the United States and Latin America. Some of them have feathers.
Most Minnesotans have friends and relatives who head south to Arizona, Texas, or Florida for the winter. In recent years Costa Rica has become one of the most popular winter vacation spots in the world. Many birds have used those winter destinations for thousands of years. They are no dummies. They are also "snowbirds," and in ornithological circles, we also refer to them as Neotropical migrants.
We look at a winter trip south as a vacation, but for birds there is an entirely different process at play. The migratory birds that we tend to think of as Minnesota birds—such as the rose-breasted grosbeak, Baltimore oriole, or barn swallow—actually had their origins in the American tropics. When the glaciers receded in North America perhaps 10,000 years ago, many birds, especially those that ate insects, learned to move northward each year with the onset of spring weather. They took advantage of the abundance of insects in spring and summer in northern climates because the insects provided an excellent food source for their chicks during the nesting season. Biologists called it a seasonal protein pulse, which is typically lacking during the nesting season in the American tropics.
Many tropical insects occur in low densities and are so incredibly well-camouflaged that it is very hard for a pair of parent birds in the tropics to find enough insects to feed their young. Neotropical migrants have found an advantage to migrating north to locations like Minnesota where there are lots of insects to feed their young. They only stay north as long as it takes to raise a family. In late summer or early fall, they return to tropical habitats. These birds actually spend more months each year in the tropics than they do in the North.
Neotropical migrants include a fascinating assortment of birds. The tiny ruby-throated hummingbird winters as far away as northwestern Costa Rica. Blue-winged teal may migrate all the way to South America for winter. Other Neotropical migrants include great blue herons, turkey vultures, ospreys, broad-winged hawks, Swainson's hawks, spotted sandpipers, rose-breasted grosbeaks, yellow-throated vireos, great crested flycatchers, and wood thrushes.
Being primarily insect eaters, warblers nest in Minnesota, but they head for the tropics in the fall. They include the black-and-white warbler, Blackburnian warbler, yellow warbler, prothonotary warbler, chestnut-sided warbler, golden-winged warbler, black-throated green warbler, and Wilson's warbler.
Once these warblers arrive in their winter habitats, they face the challenge of finding the forest's well-camouflaged insects. Some warblers cope with this problem in a novel manner. They join mixed flocks of nonmigratory warblers, bush-tanagers, and antbirds to create hunting parties that collectively scour plants in search of insects. They are much more effective in discovering, flushing, and catching insects when they hunt as a gang.
Every migratory bird species has its own particular wintering habitat, diet, and survival strategy. When Swainson's hawks leave the Great Plains, they migrate to northern Argentina's Pampas grasslands, where grasshoppers comprise most of their diet. Bobolinks migrate from grasslands in the Midwest all the way to Bolivia, Brazil, Paraguay, and northern Argentina. Bobolinks have learned to converge on rice fields, where they are widely considered an agricultural nuisance. Purple martins—so popular in the United States that clubs formed just to build nest boxes for them—head to Brazil for the winter. Sometimes purple martins gather in huge concentrations to roost at night in city parks in Brazil, where the local residents consider them a noisy urban nuisance.
Costa Rica is an important wintering site for many warblers, vireos, and thrushes. Baltimore orioles eat a lot of fruit during the winter, feeding on guavas and papayas, which may explain why they are fond of grape jelly and orange slices when they return to Minnesota back yards. Orioles also feed in shade-grown coffee plantations, along with rose-breasted grosbeaks and a variety of warblers. Birds are attracted to shade-grown plantations because the trees are covered with mosses and epiphytes (plants that grow on trees in the tropics), which are crawling with abundant insects.
Along the Pacific Coast of Costa Rica, barn swallows perch by the hundreds on power lines. Yellow, chestnut-sided, and Tennessee warblers winter in the lowlands and foothill forests below the Arenal Volcano. At a higher elevation, flocks of wintering Blackburnian and golden-winged warblers flit through the Monteverde Cloud Forest Reserve.
In January and February, colorful springtime breeding plumage returns to molting snowbirds, such as the golden-winged warbler, rose-breasted grosbeak, and Baltimore oriole. But they aren't ready to migrate back north anytime soon. While Minnesotans are still shoveling snow and chipping ice in the dead of winter, many of our snowbirds are still living a warmer life in Costa Rica. Ospreys and peregrine falcons are soaring above ocean beaches, yellow warblers are foraging for insects among coffee plants, and Tennessee warblers are eating bananas that Costa Ricans set out for them at bird feeders. In March or April, the protein pulse of hatching insects will lure our snowbirds back to Minnesota again.