Fleeing Winter: A Songbird’s Journey

The yellow warbler (Setophaga petechia) breeds across North America in the spring and summer but prefers a warm winter with temperatures between 60 and 80 degrees Fahrenheit. Weighing in at about 10 grams, roughly the mass of two quarters, these showy birds are known for a sweet song (which can be recognized by the mnemonic sweet, sweet, sweet, oh-so-sweet) they repeat up to 10 times a minute. Their bright yellow plumage and wide geographic distribution make them favorites for birders and park enthusiasts alike. Because they favor wooded habitats near wetlands and streams and the edges of forests, they’re particularly common locally in Pittsburgh’s Schenley and Frick Parks.

When fall approaches, yellow warblers escape the colder temperatures and lack of insects, their main food source, by migrating thousands of miles to Central and South America, mostly at night. After much scientific debate, it seems that this drive to migrate is a result of their ancestors escaping harsh seasonal conditions, dispelling the idea that these long-distance migrants have tropical ancestors who gained an advantage by traveling north to breed. This pattern—an annual escape to cope with seasonal change followed by a return to their ancestral grounds to breed—gave rise to a striking group of birds we associate with the temperate summer, species such as the yellow warbler, who play out their little lives across incredible distances.

“Why are these birds so faithful to territories in the winter, particularly when they’re coffee plantations?”

We can now follow migrating birds to their wintering grounds with miniature tracking technology, such as light-level geolocators that use daylight to estimate location. These trackers are typically small, fitted on the rump of a warbler with negligible effect, and able to collect global positioning data for about a year. The trickiest part is the need to recover the tiny devices by recapturing the same birds after their annual migrations.

Yellow warblers spend about 10 percent of the year breeding, 60 percent migrating, and 30 percent defending a winter territory. Within their wintering grounds in southern Costa Rica, I led a banding effort in coffee plantations and neighboring forested habitats. Over 12 years, I banded 442 yellow warblers; 92 of them were recaptured multiple times within the same coffee plantation.

Why are these birds so faithful to territories in the winter, particularly when they’re coffee plantations? It seems that some yellow warblers are choosing coffee plantations over adjacent forest habitats to overwinter. During my 12-year collecting effort, 429 of the 442 yellow warblers captured (about 97 percent) were in coffee plantations, despite employing twice the amount of trapping in forested habitats.

This tendency to faithfully winter within coffee shrubs is consistent across 13 of the 20 migrant bird species that overwinter in southern Costa Rica. Coffee plantations are also preferred habitats for 81 year-round bird species. Yet, it remains a mystery exactly why coffee plantations are preferred for so many bird species. We do know that the numerous species encountered in the coffee plantations typically are associated with warmer and drier climates, and, like the yellow warbler, often have large geographic ranges. These climatic preferences and large geographic ranges may give these species the necessary adaptability to thrive there.

This surprising amount of biodiversity in coffee plantations pales in comparison to adjacent forests, where 157 year-round species were banded. Some of these forest-dependent bird species are threatened or endangered due to deforestation. But habitats with fewer and less vulnerable species, such as Costa Rica’s foothill coffee plantations, are not necessarily free from threats and are no less worthy of our protection. Unfortunately, many coffee plantations are being converted into cattle pastures due to economic pressures, and we don’t know how the bird species that have learned to thrive in these plantations will fare in the new pastures. In the case of the yellow warbler, this land conversion in their wintering home might be what determines if we get to see them in a favorite local park in the spring and summer.