The Adaptive Syndromes of Two Guilds of Insectivorous Birds in the Colorado Rocky Mountains

Abstract

This paper analyzes the foraging behaviors of a community of insectivorous birds breeding in a high—altitude willow floodplain. The eight species belong to two guilds. Active searchers are represented by the gleaning guild (warblers): Wilsonia pusilla, dendroica petechia, and Geothlypis trichas; passive searchers are represented by the fly—catching guild (flycatchers): Empidonax oberholseri, Empidonax tralli, Empidonax difficilis, Contopus sordidulus, and Nuttallornis borealis. The paper focuses on the following general questions: (1) What are the different components, and their correlations, of foraging behavior in the eight species? (2) What are the overall adaptive syndromes associated with predatory tactics of active and passive searchers? (3) Are the predictions of several models of optimal foraging supported by the results of the present study? Adaptive syndromes (coordinated sets of characteristics, including the specific manner of resource utilization, and an array of other, related adaptations) are described for each species. Warblers forage primarily by gleaning, concentrating on stationary prey. Their velocity (average number of perches per minute) is >10.0, their search intensity (perches visited prior to an attack) is >5.0, and their foraging intensity (number of attacks per minute) is >1.7. They have an attack radius (mean distance from perch to prey) of <0.6m. Members of the gleaning guild may thus be characterized as active searchers (they look for prey difficult to find) but passive pursuers (they attack prey easy to catch). In addition, gleaners exhibit a narrow range of preferences in habitat structure. Flycatchers forage primarily by hovering or sallying, concentrating on prey available via aerial attack. They have both a velocity and search intensity of <4.0, and a foraging intensity of <1.8. They have attack radii ranging from 1 m to >7.5 m. Members of the fly—catching guild may thus be characterized as passive searchers (they look for prey easy to find) but active pursuers (they attack prey difficult to capture). Flycatchers show a broad range of preferences in habitat structure. For all species considered together there is an inverse correlation between body size and velocity, and a positive correlation between body size and attack radius. Three core adaptations identify the adaptive syndromes of actively and passively searching predators. Active searchers have high velocities, high search intensities, and a foraging repertoire associated with passive prey. Passive searchers have low velocities, low search intensities, and a foraging repertoire associated with active prey. Additional characteristics also differentiate the two adaptive syndromes. Search/pursuit ratios critical to numerous optimal foraging models are impossible to measure reliably in the field; thus the concept of active and passive searchers is proposed as a meaningful and readily quantifiable substitute. The data presented here exhibit poor fit with the predictions of several selected models. Predictions concerning differences in diet specialization, the extent of territorial and foraging overlap, and the prevalence of sexual dimorphism are not confirmed. In general, predictions concerning morphological size gradients are upheld; those concerning habitat specialization are confirmed if habitat is measured in structural terms, but are not supported when measured by vegetation type.