Human-modified habitats amplify seasonal differences for bird flocks in the Amazon

man uses binoculars
Recent LSU doctoral graduate, Cameron Rutt, spent 15 months in the Brazilian Amazon collecting data for his dissertation. Rutt uses his binoculars to track the movement of mixed-species flocks.
Scott Kosiba

(02/09/2021) BATON ROUGE, La.– The Amazon Rainforest is often thought of as a place of perpetual heat and rain. While this may be true most of the year, it is not an unchanging environment. The central Amazon undergoes seasonal changes, which have been found to affect the animals that live there, including mixed-species flocks of forest birds.

The Amazon's seasons have different characteristics than those of the temperate zones, which are usually marked by changing day length and temperature. In the Amazon, however, there are two distinct seasons defined by rainfall – a wet and a dry season.

Cameron Rutt, a recent graduate from the School of Renewable Natural Resources at LSU, spent 15 months in the Brazilian Amazon collecting data for his dissertation. He spent over 500 hours wading through the forests following mixed-species flocks – recording species composition and tracking flock movements with a handheld GPS. 

“My research often looked a lot like glorified bushwhacking,” says Rutt when asked about his time spent in the Amazon. “I would spend the day walking through the rainforest, finding a flock, and then trying my best to follow its every move for the next three hours. And then go off in search of another flock and hit repeat.”

There were three different forest types where Rutt tracked these flocks: primary forest, small forest fragments, and regenerating secondary forest. Primary forests are tall, natural forests of native tree species, where there are no visible indications of human disturbance. In comparison, small forest fragments and regenerating secondary forests have been disturbed because of farming, ranching, or other land developments for human habitation.

Since fragmented habitats appear to be more susceptible to climate variation, this was another variable Rutt sought to explore. He looked for a seasonal difference in flock formation and function in fragmented and recently deforested habitats.

small bird
A variety of different forest birds, such as this Brown-bellied Stipplethroat (Epinecrophylla gutturalis), make up a mixed-species flock.
Cameron Rutt

Changes Found

During the dry season, the study found that flocks made seasonal adjustments – they included more species, generally ranged over larger areas, and were less complex and cohesive. They made these adjustments regardless of forest type.

Rutt points out that it is unlikely that these adjustments were directly caused by climate. Instead, he suspects that the effects are indirect because the changing seasons very likely also dictate food availability and the timing of breeding, which can lead to downstream effects for flock members.

Interestingly, flocks in primary forests were relatively buffered from the effects of seasonality, while seasonal changes were most pronounced for flocks in forest fragments and regenerating secondary forests. Thus, at least for bird flocks, these human-modified habitats appear more susceptible to the changing seasons, which further limits their value and has important conservation implications. 

Climate change will likely amplify the distinction between seasonal changes. The disparity between wet and dry seasons appears to be increasing in the central Amazon — with increased rainfall in the wet season and decreased rainfall in the dry season. Scientists are aware that this change will affect these habitats – the question remains how the rainforest plants and animals will respond.

“It looks like climate change is increasing this difference between the seasons, and as the climate continues to change, it’s likely to disproportionately affect these human-modified habitats,” says Rutt.

This study’s findings were recently published in Ecological Applications and make a valuable contribution to research about mixed-species bird flocks in the Amazon. View the article Seasonal dynamics of flock interaction networks across a human‐modified landscape in lowland Amazonian rain forest (https://doi.org/10.1002/eap.2235) to learn more about Rutt's findings. 

This work would not be possible without support from the Biological Dynamics of Forest Fragments Project and funding by the US National Science Foundation (LTREB 0545491 and 1257340).

By Annabelle Stokes