Light and thermal niches of ground-foraging Amazonian insectivorous birds

Insectivores of the tropical rainforest floor are consistently among the
most vulnerable birds to forest clearing and fragmentation. Several
hypotheses attempt to explain this pattern, including sensitivity to
extreme microclimates found near forest borders—particularly brighter and
warmer conditions. Importantly, this “microclimate hypothesis” has
additional implications for intact forest under global climate change that
could be evaluated through direct assessment of the light and temperature
environment of terrestrial insectivores. In this study, we harness novel
technology to directly quantify the light and thermal niches of 10 species
of terrestrial insectivores in undisturbed Amazonian rainforest. Loggers
placed on birds (N = 33) and their environment (N = 9) recorded nearly
continuous microclimate data from 2017–2019, amassing >5 million
measurements. We found that midday light intensity in treefall gaps
(~39,000 lux) was >40 times higher than at the ground level of
forest interior (950 lux). Light intensity registered by sensors placed on
birds averaged 17.4 (range 3.9–41.5) lux, with species using only 4.3
(0.9–10.4) % of available light on the forest floor. Birds thus selected
very dark microhabitats—the light environment was >2,200 times
brighter in treefall gaps. Bird thermal niche was a function of ambient
temperature as well as body temperature, which averaged >40.5 °C
but varied among species. Forest floor temperature peaked daily at 27.0
°C, while bird loggers averaged 35.1 (34.5–35.7) °C at midday. The
antpitta Myrmothera campanisona and the antthrush Formicarius colma used
thermal conditions closest to their body temperatures, whereas leaftossers
(Sclerurus spp.) and Myrmornis torquata occupied relatively cool
microclimates. We found no general link between abundance trends and
variation in species-specific light and thermal niches. Rather, all
species occupied remarkably dim and cool microclimates. Because such
conditions are rare outside of the interior of primary forest, these
results support the microclimate hypothesis in disturbed landscapes.
Moreover, strong avoidance to conditions that are becoming more common
under climate change highlights the vulnerability of terrestrial
insectivores even in the absence of disturbance and may be the reason for
enigmatic declines in Amazonia and elsewhere.