Constraints imposed by migrating warblers on breeding warbler signal space

Joanna M. Sblendorio, Maarten J. Vonhof and Sharon A. Gill, Biological Sciences, Western Michigan University, Kalamazoo, MI, Maarten J. Vonhof, Institute of the Environment and Sustainability, Western Michigan University, Kalamazoo, MI

Background/Question/Methods
Signals evolve under multiple selective pressures, including conspecifics, heterospecifics and the environment. With respect to heterospecifics, co-existing species could compete for signal space, but instead inhabit well-defined acoustic niches, which minimize signal overlap among males of different species. While partitioning of signal space has been envisioned as the result of competition among syntopically breeding species, reports of species singing during migration suggest additional, unexplored constraints on song detection and selection for partitioning in signal space. We hypothesized that migrant warblers constrain signal space available to resident, breeding warblers and that patterns of competition vary over time depending on the migrants present in the community. We recorded songs of 30 warbler species, including 19 migrant species, in southwest Michigan to analyze acoustic niches, and downloaded eBird data to determine temporal co-occurrence among warblers.
Results/Conclusions
Migrant warblers were present for much of the breeding season, with a maximum of 28 warbler species co-occurring over this period. Within the overall community, most species inhabited distinct acoustic niches, with only 11% of warbler pairs overlapping each other, yet migrant-breeding niche overlaps were twice as common as overlaps between breeding species. The degree of pairwise overlap ranged from 0 to 41%. On average, breeding species experienced 13.3% niche overlap from migrants (range: 0-81%), and 20.7% overlap from other breeding species (range: 0-49%). Our study is the first to explore whether migrants influence partitioning of acoustic space and finds that transient members of communities may have important influences on competition for acoustic space and signal evolution.