By Christa L. LeGrande-Rolls and Jonathon J. Valente
Linked paper: Conspecific attraction for conservation and management of terrestrial breeding birds: Current knowledge and future research directions by Jonathon J. Valente, Christa L. LeGrande-Rolls, James W. Rivers, Anna M. Tucker, Richard A. Fischer, and Matthew G. Betts, Ornithological Applications
In 2012, we were working on a large project aimed at identifying the factors that affect population growth for Wood Thrush (Hylocichla mustelina). We had some evidence that their distributions were limited by forest fragmentation, and there was a growing body of research indicating that many breeding passerines exhibit conspecific attraction, an inclination to select habitat near other individuals of the same species. Thus, we hypothesized that fragmentation could limit the ability of Wood Thrush to move efficiently through a landscape and gather information about conspecific breeding locations. Our first step in testing this hypothesis was to verify that Wood Thrush do indeed exhibit conspecific attraction, so we ventured into the forests of Southern Indiana and simulated Wood Thrush presence using hand-painted bird decoys and song broadcasts in unused, but presumably quality habitat.
Despite strong evidence for conspecific attraction for many migratory songbirds, we found no difference in Wood Thrush use of locations where we presented cues versus control sites with no simulated bird decoys or songs; Wood Thrush settlement was random with respect to our treatments. Surprised by this, we asked ourselves why the experiment worked for other songbirds but not for Wood Thrush. We sat down as a team and brainstormed plausible explanations. Did we not present the right type of vocalizations? Is there something unique about our study region? Maybe Wood Thrush simply don’t use conspecific presence to identify habitat. When we turned to the literature for clues, we found substantial variability among experiments in the way conspecific presence was simulated, contradictory evidence for conspecific attraction in the same species, and even data indicating birds could be repulsed by simulated conspecific presence. It became clear that there was not going to be a straightforward explanation for the results from our Wood Thrush experiment.
We thus decided to conduct a formal meta-analysis to aggregate the information across these previously conducted conspecific attraction studies and test for signals in the data that might not be obvious in a simple literature review. We aggregated our list of questions into four major hypotheses: that conspecific attraction in terrestrial breeding birds is mediated by characteristics of species, simulated cues, sites, and how researchers measure the response. Under these broad hypotheses, we identified 19 potential variables that could affect whether a bird uses the presence of conspecifics as a factor in choosing a location to settle during the breeding season. Surely, for example, a migratory bird, which must spend its time wisely upon arrival to its breeding grounds, is more likely to use cues of other birds to find quality habitat, instead of trying to figure that out completely on its own. The results from our meta-analysis indicated that the birds studied in the literature tended to exhibit strong conspecific attraction, particularly warblers in the genus Setophaga and sparrows in the genus Ammodramus. Surprisingly, however, there was no substantial support for any of our mechanistic hypotheses regarding factors mediating this attraction.
Understanding how birds select habitat is critical for conserving and managing populations, and effective management involves creating and restoring critical habitat as well as ensuring such areas are used. In certain situations, simulating conspecific presence could encourage use of protected areas, or help expand a species distribution, making populations less vulnerable to catastrophic events or climate change. Yet, despite a decade of calls to improve management efficiency by better understanding factors influencing conspecific attraction, we seem to be no closer to such answers. Given this, we outlined three major themes for future research that we expect will lead to the greatest progress in the field, including evaluating how cue characteristics, life history traits, and spatial context mediate conspecific attraction. We encourage carefully designed experiments that can explore these interactions and measure the different variables within and among species. In doing this, we expect to enhance our understanding of how and when birds use visual or auditory cues, and when they can be used as a tool for conservation and management.