How Rainfall Shapes Tropical Bird Breeding in Peru

For years, scientists assumed that bird breeding followed straightforward patterns. In cooler climates, birds generally nest in spring according to a set timetable. Tropical regions, however, are different. As these places remain warm throughout the year, scientists once believed birds could reproduce at any time.

Early research by Alexander Skutch proved otherwise. Birds still followed a form of “tropical spring”, driven by food availability rather than temperature.

New research from the Florida Museum of Natural History and Texas A&M University now indicates that this system is much more complicated than previously expected.

How tropical birds select their breeding season

Felicity Newell and her colleagues set out to discover how breeding in tropical birds truly operates. They found that earlier research often lacked detailed information, particularly on rainfall and food availability.

“I realized we actually didn’t know how the whole ecosystem worked at all,” Newell said.

The researchers therefore carried out a five-year study in Peru’s Andes Mountains. They examined birds, insects, plants and rainfall together, enabling them to trace the links across the full food web.

Their findings also revealed that tropical ecosystems are strongly shaped by rainfall, rather than temperature or daylight length.

Life in Peru’s cloud forests

The team conducted its work at eight mountain sites. While the locations experienced differing levels of rainfall, their temperatures were similar. This allowed the researchers to isolate the effects of rain on living systems.

“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.”

They caught thousands of birds, recorded insect measurements, and monitored flowers and fruit. The resulting data provided a detailed view of how climate and food supplies vary over time.

Rainfall drives the whole food web

The researchers discovered that rainfall has a crucial role in tropical ecosystems. It determines when plants develop, insects emerge and birds reproduce.

Rainfall effectively triggers a chain reaction: it first influences plants, then insects, and ultimately birds. Plants need rain to produce flowers and fruit, while insects rely on those plants.

Birds depend on both plants and insects. This relationship helps explain why bird species breed at different times.

Different birds use different strategies

Nectar-feeding birds, including hummingbirds, breed when flowers are in bloom. Birds that eat fruit reproduce when fruit is plentiful. These schedules remain relatively consistent from year to year.

Insect-eating birds, by contrast, behave very differently. Their breeding periods shift with insect availability, which is itself determined by rainfall.

“They might breed in June in one year, and if the next year was dry they would breed in May,” Newell said. “However, within 60 miles of each other, one insectivore community might breed in May and the other community might breed in October.”

As a result, bird communities living close to one another may follow entirely different breeding schedules.

A hidden tipping point in nature

Among the most significant results was evidence of a tipping point. Once insect populations fall below a particular level, birds abruptly alter their breeding season.

Scientists found that minor rainfall differences can produce major changes in bird behaviour. This demonstrates that ecosystems are not necessarily stable; instead, they can change rapidly when conditions shift.

Insect-eating birds account for most tropical bird species, and they rely heavily on insect populations to feed their chicks.

However, insects are highly sensitive to rain. Insufficient rainfall can dry them out, while excessive rain also lowers their numbers. The study found that insects favour moderate levels of rainfall.

“Intermediate rainfall is where their biomass is highest,” Newell said.

Since insects respond rapidly to changing conditions, insect-eating birds do so too. This leaves them more exposed to climate change.

Climate change is changing the rules

The study underlines how climate change could affect tropical ecosystems. Rainfall patterns are becoming increasingly extreme, with drought affecting some places and heavy rain affecting others.

Such changes can diminish insect populations and interrupt breeding cycles. During dry years, birds may sometimes forgo breeding because there is too little food.

The impact is particularly pronounced among insect-eating birds, which already display substantial variation in breeding timing.

A warning for the future

The research suggests that tropical ecosystems are more sensitive than scientists previously recognised. Small changes in rainfall can influence entire food webs.

Insect-eating birds could serve as early warning indicators. Changes in their breeding patterns may point to more fundamental problems within the ecosystem.

The findings expand understanding of how climate change is reshaping life in tropical ecosystems, showing that even reproductive timing can change in unexpected ways.

They also emphasise the close connections within natural systems, in which changes to rainfall and insect populations can spread through whole bird communities.