It’s time to rethink the nature of warblers

In the tangled green of Bocas del Toro, Panama, where mangroves knit into coral-lined shorelines and the rainforest hums with unseen life, a golden spark flits through the shadows. It’s not sunlight piercing the canopy, but a bird: a prothonotary warbler (Protonotaria citrea), its lemon-yellow chest glowing against the darkness of these saltwater swamps. And as it hops low through the tangle of prop roots that so characterize these mangroves, it stops to feed on banana left over from a troop of white-faced capuchin monkeys that had dragged their spoils of the mornings foraging expedition into the secluded recesses of this forest.

For those who know this species from North America’s wooded swamps and bayous, this winter behavior is easy to overlook. But that tiny moment in the tropics could hint at a much larger story—one that might help explain why warblers are among the most colorful songbirds on the continent.

Every spring, North America’s forests erupt with color and song as warblers return from their wintering grounds. From the fiery throat of the Blackburnian warbler (Setophaga fusca) to the clean sky-blue of the cerulean warbler (Setophaga cerulea), these migratory birds offer a pageant of color that belies their fragile size and weightless silence outside the breeding season.

Unlike birds that remain year-round, warblers are travelers. Most breed in the U.S. and Canada, where they are studied relentlessly by armies of ornithologists and casual birdwatchers alike, but spend more than half the year in Central or South America. This North American microscope has long framed the way we study and understand warblers. But a closer look at their coloration suggests that our northern bias may be blinding us to what’s really shaping these birds.

Among birds, it’s well known that bright red, orange, and yellow plumage is derived from a family of pigments called carotenoids. These pigments are not produced internally. Birds must consume them, usually in the form of colorful fruits, to deposit the carotenoids in feathers. Consider the northern cardinal (Cardinalis cardinalis) for a moment. These familiar birds of the eastern deciduous forests are the brilliant red of their namesake because of their seasonal love affair with dogwood and sumac berries, among others. Like the northern cardinal, this is why fruit-eating birds like tanagers and orioles are so boldly colored, while most insectivorous species are subdued, cloaked in browns and grays that also serve as camouflage.

At first glance, warblers break this rule. They are insect specialists during the breeding season, feeding on caterpillars, flies, and beetles. Yet their colors rival any tropical frugivore. It’s a biological contradiction unless their winter diets aren’t what we think they are.

Emerging research, along with field observations in places like Bocas del Toro, suggest that many warblers do indeed diversify their diets during the non-breeding season. Insects become harder to find as the dry season when these birds are spilling into the neotropical rainforests, for instance, and many of these migratory species have been observed consuming berries and fruits all across the tropics. These fruits, often rich in carotenoids, could serve a dual function: filling bellies and fueling plumage.

This idea that tropical fruit might be helping warblers produce their breeding colors opens a new way of thinking about these birds. For decades, ornithologists have attributed warbler color to sexual selection: brighter males are more successful in attracting mates and therefore pass on their vivid traits. But that hypothesis often stops at the North American forest edge each autumn, as birds migrate south and disappear from most scientific scrutiny.

If diet is driving coloration and tropical fruit is a key ingredient, then the tropics aren’t just a vacationing ground for warblers. They’re where the raw materials to build a warbler are gathered. This reframes their identity, in my opinion. Perhaps they are not insectivorous northern birds that winter in the tropics, but rather tropical birds that breed in the north.

It also emphasizes how little we truly know about their lives outside the breeding season.

If warblers rely on tropical fruits to maintain or enhance their plumage, then this brings into question our notions of avian conservation. In 2019, the journal Science dropped a bombshell study showing that some three billion less birds exist in North America today than in 1970. Many of these birds are migratory species, those who spill out of the north and overwinter in the tropics such as warblers.

Such knowledge now underscores the importance of full cycle conservation, approaches that protect migratory species not just where they breed, but across the entire landscape of their lives. Warblers may be most visible to us in spring, but their survival depends just as much on what happens in the fruit-filled forests of winter.

For me, I can no longer see these birds as anything but tropical emissaries that choose to come north to breed at the peak of insect abundance. The newly hatched offspring of every species of songbird except the American goldfinch feed exclusively on insects when they are first born. Even hummingbirds, who we think to be inextricable from the nectar of flowers, require significant amounts of insect derived protein at this time of the year. But once this stage of life is over, and birds mature, their lives, their diets, their nutritional needs change dramatically. And so should our perspectives about life on Earth.