Key points
- Many years of scientific observation and research show that temperate birds nest in the spring, and their breeding seasons rarely vary by more than a few days from year to year for any given species
- In contrast, scientists know little about the breeding seasons of tropical birds. The information that does exist suggests that species may shift their breeding times by up to one month at a given site.
- The recently published results of a five-year field study show that this variation has been drastically underestimated in tropical mountains, where — in some cases — whole chunks of the community can breed in opposite seasons in response to shifting climates and resource availability.
In 2014, Felicity Newell joined the Florida Museum of Natural History as a doctoral student, then promptly left the country in search of a tropical spring. It's a concept she started thinking about while doing biological surveys in Honduras. There, a colleague told her about the work of Alexander Skutch, a renowned ornithologist who spent 20 years studying the breeding habits of birds in Costa Rica. Based on this work, he became "convinced that the birds … have a definite nesting season, and its beginning coincides with the return of spring."
Costa Rica is a tropical country 9 degrees north of the equator, and as such, it does not have the spring and winter seasons associated with temperate regions. Its climate is balmy year-round, which — Skutch noted — gave people "…the impression that, with tropical birds, singing and nesting continue freely throughout the year."
Skutch combined the observations he'd made over 20 years with those from other naturalists and determined that this was not the case. Tropical birds, it seemed, primarily nested when their resources were abundant. For birds that eat insects, called insectivores, this occurred from March through June, aligning with the spring breeding season of birds further north. Hence, the concept of a tropical spring.
Back in Honduras, Newell was intrigued. She read up on similar studies to see what had been learned since Skutch's initial discovery, which he'd published in 1950. But research on the breeding habits of tropical birds was scarce. Much of the work that had been done was anecdotal, lacking data on rainfall and other climate variables and often combining all the observations from a particular country or region. That meant that any local variation in breeding times would be effectively invisible.
"I realized we actually didn't know how the whole ecosystem worked at all," Newell said. So she decided to conduct her own study, developing a new project from scratch on tropical montane birds with colleague Ian Ausprey; both Newell and Ausprey are now asisstant professors at Texas A&M University, and were advised by study co-author Scott Robinson, the Ordway eminent scholar at the Florida Museum.
They found a spot in the cloud forests of northern Peru where they lived with local families and communities and collected data for the next five years.
"It was a massive undertaking," Newell said. "We'd leave before it was light and spend 12 hours in the fields, hiking up mountains in the mud and rain, often getting back after dark. We worked with over 20 field assistants, many from across Peru and Latin America."
An important decision they had to make early on was how to go about monitoring birds and how to distinguish between those that were breeding and those that weren't. The obvious and most direct strategy would be to locate nests, which Newell had gotten good at while studying migratory songbirds in the Appalachians . This type of work is long and arduous, but it initially suited Newell just fine.
"I'm kind of obsessed with nests. Actually, that was the original goal of the study, to spend more time looking for nests. But then I realized I didn't even know when the birds were nesting, and I didn't have 20 years to find out like Alexander Skutch." She also had a lot more ground to cover. While Skutch had mostly stuck to one spot, Newell and Ausprey were sampling on eight separate mountains over an area spanning more than 60 miles.
So they settled on mist nets, a reliable and widely used tool among ornithologists everywhere, though they still made note of any nests that they could find.
After netting a bird, they looked for any signs of recent breeding, such as the downy feathers of juvenile birds or a cloacal protuberance on males, which would indicate their readiness to mate. If they had a female, they'd search for a brood patch, which forms when they "lose the feathers on their abdomen, which becomes more vascularized to help incubate the eggs," Newell said.
Newell wanted to know when tropical birds were nesting, but she also wanted to know what environmental factors were correlated with breeding. So she, Ausprey and the field assistants also measured the abundance of flowers and fruit, recorded rainfall and collected insect biomass data by "vigorously beating the nearest shrub several times over a sweep net."
At the end of five years, they'd captured and released more than 8,000 birds, documented nearly 4,000 breeding events (such as the presence of juveniles), collected 48,000 insects and found 318 nests.
Finally, they conducted analyses to determine how all of these factors influenced each other. The results caught Newell off guard.
"I wasn't expecting this amount of variation," she said. In temperate regions, she explained, the start of the nesting season for a given species can vary annually by three to five days. Research done in the tropics suggested breeding times could vary by a few weeks, potentially even a month. What Newell found was much more than that.
"There was a massive amount of variation, and not just in individual species. Whole chunks of the community were shifting, both spatially and temporally in different ways."
Tropical environments may not have a true spring, but they do have pronounced wet and dry seasons. Skutch found that most of the birds at his site nested at the tail end of the dry season, just as the rains were beginning to return, stirring plants from their torpor as they busily transformed water into fresh, new leaves, and insects turned those leaves into chitin, hemolymph and viscera. Skutch likened this flush of activity to spring, but the reality is more nuanced and complex.
To tease apart the subtleties, Newell divided the birds into three groups based on their diet: nectarivores, frugivores and insectivores.
The first two groups had fairly predictable patterns that aligned with previous data. Nectar drinkers, like hummingbirds, built nests at the beginning of the dry season, when bird-pollinated flowers were in bloom. Fruit-eating birds, such as tanagers, built their nests during the wet season, when fruit was abundant.
But insectivores couldn't seem to make up their mind.
"They might breed in June in one year, and if the next year was dry they would breed in May, so they're varying temporally by about a month," which isn't particularly unusual, she said. "However, within 60 miles of each other, one insectivore community might breed in May and the other community might breed in October."
That was unusual. No other study had shown that tropical birds could put off breeding by nearly half a year.
The point at which a community seesawed between seasons was strongly correlated with insect abundance, to the extent that Newell could pinpoint the average weight of insect biomass at which the scales tipped; 43 milligrams per square meter was the magic number. If the average weight of insects in a square meter remained adequate during the dry season across decades, insect-eating birds would nest as rain tapered off; anything below that number, and the whole community would flip to breed at the beginning of the rainy season, as Skutch had found in Costa Rica.
In the tropics, the abundance of insects is directly tied to rainfall, a fact that Newell knows firsthand. In a separate analysis of the data she'd collected in Peru, she was again caught off guard when the results showed that tropical insects seem to have a Goldilocks preference for the amount of rainfall. Too little is obviously bad, but too much rainfall can similarly reduce their abundance and biomass.
"Intermediate rainfall is where their biomass is highest," she said. Similar to tropical birds, little is known about the biology of tropical insects. Newell's results were the first to show that too much rainfall can be detrimental to their abundance, and even now, the functional relationship between the two remains unclear.
This leaves both insects and the animals that eat them especially susceptible to global climate change. Long-term studies have documented declines in many tropical bird species , especially insectivores, and changes in insect biomass could explain some of these declines.
The team's work has also shown deforestation in the Andes Mountains has jeopardized the existence of hundreds of birds that are specially adapted to the green-tinted gloom created by densely packed tropical trees.
The authors published their study in the journal Global Change Biology.
