DOI: 10.5061/dryad.vmcvdncs3
title: High resource overlap and small dietary differences are widespread in food-limited warbler communities

description: 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.
description: We sampled insectivorous bird species in two diffusely
 competitive communities. In summer, redstarts and six other species were
 sampled in seasonally flooded, bottomland hardwood forest sites at Bogue
 Chitto National Wildlife Refuge and Honey Island Swamp State Park in
 southern Louisiana, 22 May - 6 July 2006. Both locations are protected,
 relatively natural bottomland hardwood forests, containing small, sluggish
 streams, cypress-tupelo swamp, and less inundation-tolerant hardwood
 species at slightly higher elevations. The other abundant insectivorous
 bird species studied in this site were Acadian Flycatcher
 <i>Empidonax virescens</i>, Hooded Warbler <i>Setophaga
 citrina</i>, Kentucky Warbler <i>Geothlypis formosa</i>,
 Prothonotary Warbler <i>Protonotaria citrea</i>, Swainson’s
 Warbler <i>Limnothlypis swainsonii</i>, and White-eyed Vireo
 <i>Vireo griseus</i>. These species comprise all of the most
 common small insectivorous birds at this site, and for which we could
 obtain at least seven stomach samples. We sampled
 wintering redstarts along with four other species of arboreal,
 insectivorous warblers in Jamaican wet-limestone forest: Black-throated
 Blue Warbler <i>Setophaga caerulescens</i>, Black-and-white
 Warbler <i>Mniotilta varia</i>, Northern Parula
 <i>Setophaga americana</i>, and Worm-eating Warbler
 <i>Helmitheros vermivorum</i>. These five species were the
 only migrant, arboreal insectivores common in the habitat. This habitat is
 native, historically widespread, floristically diverse, and structurally
 complex, with diverse tree types including canopy emergents, dense ground
 cover and shrubs, vines and vine tangles, and epiphytes including
 bromeliads and orchids. The first of these sites was located on a cattle
 ranch near Betheltown in Westmoreland Parish (15 February - 15 March
 2017). The second site was located near the Windsor Research Station in
 Trelawney Parish (16 - 31 March 2017), within Cockpit Country.
 We included two systems more similar to classical competition
 studies, in which a pair of species exhibits interspecific aggression. In
 both communities, redstarts were subordinate to the other competitor.
 Breeding redstart and Least Flycatcher <i>Empidonax
 minimus</i> diets were sampled at a northern hardwood forest site in
 the Hubbard Brook Experimental Forest, NH, primarily in spring 1982, but
 additional data for redstarts were available from 18 May - 1 July
 1981-95. Hubbard Brook is in the White Mountain National Forest, and is
 dominated by northern hardwoods—predominantly American beech
 <i>Fagus grandifolia</i>, sugar maple <i>Acer
 saccharum</i>, and yellow birch <i>Betula
 alleghaniensis</i> (Sherry & Holmes 1985).
 Lastly, we sampled wintering redstarts and non-migratory,
 socially dominant, Yellow Warblers <i>Setophaga petechia
 petechia</i> (Sherry <i>et al.</i> 2020, Powell
 <i>et al.</i> 2021), the two most abundant parulids in
 mangrove habitat, at the Font Hill Reserve, Luana Point, St. Elizabeth
 Parish, Jamaica. The site where the stomach samples were obtained is
 adjacent to the coast and comprised overwhelmingly of black mangroves
 <i>Avicennia germinans</i>. Sampling took place from 13
 January - 14 March 2014. Field and Lab Methods—The
 majority of individuals’ diets were sampled during morning hours to help
 ensure full stomachs resulting from active feeding. Northern hardwood
 forest and Jamaican black mangrove birds were located by sight and sound
 and were obtained using shotgun or mist nets. The stomach was removed
 immediately in the field, perforated, and preserved with its contents in
 70% ethanol. We obtained gut samples in the bottomland hardwood forest and
 Jamaican wet-limestone sites, as in shaded coffee habitat (Sherry
 <i>et al.</i> 2016), using the emetic antimony potassium
 tartrate to induce regurgitation (Poulin <i>et al.</i> 1994).
 Birds were then placed in a waxed paper-lined box for 15 minutes,
 released, and gut samples retrieved for preservation in 70%
 ethanol. Stomach samples were examined in 70%
 ethanol-filled petri dishes using an Olympus SZH binocular dissecting
 microscope at 75-650X magnification. Samples were thoroughly searched for
 diagnostic arthropod fragments (such as heads, elytra, mandibles, legs,
 and wings). Sclerotized body parts of most arthropods make identification,
 at least to a certain taxonomic level, straightforward.
description: Any data on non-animal prey (e.g., fruit/seeds) is not
 included.