Do Birds in the Tropics Have a Breeding Season—or Do They Just Mate All of the Time?

Here’s the way we’re used to thinking about bird breeding in North America: As our days get longer and temperatures rise, neotropical migrants arrive to produce and raise chicks alongside our resident bird species, taking advantage of abundant flowers, fruits, and insects. When those resources dwindle, they head back south to enjoy favorable weather and prepare for their return trip to the breeding grounds.

But what about species that don’t make long-distance migrations—the ones that spend the year in perpetual warmth and twelve-hour days, always surrounded by abundance? Do they still have a breeding season, or do tropical birds crank out babies year-round? 

In some regions, they do—but not the way you’re thinking. Take the Amazon rainforest, which boasts year-round leaf cover, consistent precipitation, and ample bugs, fruits, and flowers. Even here, individual birds do not constantly reproduce, as nesting is likely too time- and energy-intensive to be a sustainable full-time job. “There are some families that show some variation, some that are more associated with the wet season, and some that are associated with the dry season,” says Louisiana State University ornithologist Phil Stouffer, who has observed fascinating patterns in Amazonian breeding. “But there’s a lot of birds that don’t show much seasonality at all.”

Among them are the Wedge-billed Woodcreeper and the Common Scale-backed Antbird. For these common Amazonian species, Stouffer found evidence of breeding activity in every month of the year. What’s more, some individual birds even tried nesting six months after a previous nest attempt, occasionally interrupting their molt to do so—an unthinkable concept for most temperate species. It would be like a Northern Cardinal breeding in February and then again in August, pausing in its naked-headed molt to raise young. “It flies in the face of having an annual period when you would breed,” Stouffer says. 

The Amazon may be particularly forgiving of this spontaneous approach to breeding; its environmental conditions are relatively stable, and food is widely available throughout the year. Stouffer suggests that this allows Amazonian species to be more flexible than their temperate counterparts. “In the absence of any particular period being impossible to breed, the birds don’t really have a mechanism that shuts them off,” he says.

Not all tropical ecosystems are so stable, however. From lush Andean mountains to scorching Venezuelan plains, many tropical regions experience dramatic environmental swings from season to season. In turn, tropical birds have adapted to breed when conditions are favorable and lay low when food is scarce.

If this sounds similar to temperate ecosystems, that’s because it is. “Reproduction is strongly tied to seasonal variation in food resources,” says Felicity Newell, an ecologist at Texas A&M University who studies the environmental factors influencing the timing of breeding in the tropics. When food is abundant, birds breed; when meals are scarce, birds don’t. This pattern generally holds throughout the world from pole to pole. It’s why Scarlet Macaws in the Peruvian rainforest, for example, breed during the wet season, typically from January to April.

Despite the shared big-picture strategy, Newell says that tropical and temperate ecosystems still have key differences that influence bird breeding. Outside the tropics, variation is driven by day length, which directly relates to the strength of the sun and thus the plant growing season. In equatorial regions that get about the same amount of sunlight year-round, a different factor heavily influences the environment: rainfall.

The timing and intensity of the wet and dry seasons directly impact the abundance of the fruit, flowers, and insects that tropical birds need to reproduce. These foods all tend to be most abundant in the spring and summer in North America, but in the tropics, they can peak at different times.

Rainfall patterns in turn create seasonal spikes in breeding activity that, while predictable at a given site, vary widely from place to place. For example, in arid Venezuelan thorn and scrub forests, all bird communities have found that wetter is better—they time their nesting to avoid the harsh dry stretch from November to April and take advantage of the explosion of fruit, flowers, and insects when rain returns in mid-May. But in the mountains of Costa Rica, flowers peak around November, in the early dry season, so hummingbirds do not breed in the early wet months from April through June alongside fruit- and insect-eaters. 

Even local differences in rainfall can have major impacts on tropical breeding behavior. In the Peruvian Andes, for instance, Newell has found that communities of insectivores separated by as few as 60 miles will breed at almost completely opposite times of year as rainfall varies with local topography. By sampling precipitation and insect abundance and tracking down nests over five years in the field, she discovered that birds in rainier mountains bred in May, late in the wet season, when their six-legged prey were still abundant. By contrast, insectivores in drier patches bred earlier in the wet season, in October, before the landscape dried out and bugs became scarce.

“Climate typically varies at much larger spatial scales,” Newell says. “Here, it’s just varying at really small spatial scales, and the birds are responding to the availability of the resources.”

Such patterns—or, in the Amazon, the lack thereof—might be common across tropical communities. But many tropical ecosystems remain poorly understood. Newell is collaborating with researchers from around the world to continue piecing together the cause and effect of tropical breeding—research that may be important for understanding why many tropical species are experiencing sharp declines even in intact habitat.

All told, there is no unified tropical breeding season. Instead, the incredible ecological diversity of the tropics has given rise to a fascinating mosaic of breeding strategies that scientists are only just beginning to comprehend.