Vocal behaviour of Song and Swamp sparrows upon arrival on shared breeding grounds

Abstract

Closely related species interact often, typically competitively, aggressively, and asymmetrically, with a consistent dominance hierarchy among species. Competition for resources appears to be a rate-limiting step in diversification, but beyond this, we know little about the ecological role of aggression in facilitating or constraining the coexistence of species due to the difficulty in observing natural interactions. To examine how closely related species in a dominance hierarchy aggressively interact, and how those interactions may facilitate coexistence, we documented vocalizations of Song (Melospiza melodia) and Swamp (M. georgiana) sparrows during natural and simulated territory settlement to answer the question: How does vocal behaviour of a dominant species change when first faced with a subordinate competitor on shared breeding territory? Though sample sizes were too low for statistical testing, we saw slightly increased rates of “Swamp Sparrow-like” songs sung by Song Sparrows in relation to Swamp Sparrow presence, as well as trill syllable lengths of Song Sparrow songs approaching average lengths of Swamp Sparrow trill syllables. These trends may suggest syllable sharing or vocal shifts in Song Sparrows as a response to Swamp Sparrow competitors – this vocal convergence may be beneficial in mediating conflicts over shared resources. We also provide novel descriptions of Swamp Sparrow behaviour during settlement on territories overlapping Song Sparrows. Further descriptions of vocal interactions can inform how closely related species interact aggressively, and will contribute to our understanding of how aggression might relate to coexistence on shared territories.