What determines the elevational ranges of tropical birds?
The relative importance of species interactions and physiology

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Slopes of San Pedro in the Kosnipata Valley, where we have completed
hundreds of surveys to understand how bird communities change
with elevation.
Ecologists have long been challenged to understand the ecological factors that maintain species range limits. Montane regions allow us to study range limits for many species simultaneously because habitats and species composition change abruptly along elevational gradients.  Tropical mountain ranges, because of their high species diversity and density of range limits at small spatial scales, have enormous, mostly untapped potential as study systems for understanding species distributions and the ecological factors that maintain them.  

Our research project utilizes a large-scale elevational gradient in the Peruvian Andes to test alternative ecological mechanisms that could determine range limits of montane birds.  We begin with estimating the fundamental or physiological niche of species along the gradient based on key thermoregulatory parameters, including tolerance of thermal conditions and associated costs of thermoregulation. This describes a species’ idealized distribution if it were limited only by the range of physical conditions it could tolerate.  Using these distributions as a baseline, we then use surveys to determine whether species show different realized distributions along the gradient.  Using a combination of field experiments and intensive data collection on life-history and behavioral traits, we target specific biotic constraints that could be acting on species range limits, including interspecific competition, mutualistic interactions, and nest predation.  
The Black-spotted Bare-eye (Phlegopsis nigromaculata) is
one of hundreds of species of antbirds in the Amazon that
do not reach Andean cloud forest at 1500 m. Are these species
physiologically constrained to hot and humid low-elevation

climates? Basic physiological traits of most tropical bird species 
are still unknown.

At each of three stations, we have set up a metabolic physiology lab to
measure physiological traits of captured birds. These  data allow us
to estimate physiological niches of species along the mountainside.
This study builds on an extensive addition to an existing database of the distributions of birds of Manu National Park, where one can find >1000 bird species along a single gradient from 400-3400m -- more species than all of North America.  We are examining a number of questions with this research: (1) What are the thermal tolerances and thermoregulatory costs of tropical montane birds, how do they vary with elevation, and do species' distributions reflect physiological constraints imposed by thermal environments and hypoxia? (2) Does interspecific aggression underlie "species replacements" along elevational gradients, constraining the realized distributions of closely related species? (3) Does nest predation in tropical montane regions reinforce elevational limits of birds? (4) Do mutualistic interactions among species generate discrete communities, where range limits of species providing mutual benefits coincide along gradients?

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