High resource overlap and small dietary differences are widespread in food-limited warbler communities

Although both interspecific competition and coexistence mechanisms are
central to ecological and evolutionary theory, past empirical studies have
generally focused on simple (two-species) communities over short time
periods. Experimental tests of these species interactions are challenging
in complex study systems. Moreover, a number of studies of ‘imperfect
generalists’, consistent with Liem’s Paradox, raise questions about the
ability of evolved species differences to effectively partition niche
space when resources vary considerably across the annual cycle. Here we
used a recently developed theoretical framework to combine past research
on population-level processes with observational data on resource use to
test for ongoing interspecific competition and understand the nature of
resource overlap. We compared species diet overlaps and differences in
several distinctive communities centred on a focal species, the American
Redstart Setophaga ruticilla replicated both spatially and seasonally, in
combination with documentation of population regulation to assess the
ability of similar species to partition dietary niche space and limit
interspecific competition. Our results document high dietary overlap in
most of the communities studied, with only subtle differentiation
consistent with known species-differences in foraging behaviour and
morphology. These findings are largely consistent with species foraging as
imperfect generalists. However, in contrast to past studies, the high diet
overlaps observed here during times of inferred resource scarcity were
driven by low value prey taxa (e.g. small ants) and did not involve truly
‘private’ resources. All of these factors increase the potential negative
impacts of interspecific competition and limit the ability of these birds
to avoid competition if conditions further deteriorate, either seasonally
or at longer intervals.