Song of the Riverbank Warbler Myiothlypis rivularis: male constraints and female territorial signaling

Birds defend territories by engaging in potentially costly behaviours, such as vocalizations and aggressive interactions. Songs are usually good proxies of individual quality, thus could help guarantee access and exclusivity to food, sexual partners and nesting sites. We described territorial and vocal behaviours of the Riverbank Warbler Myiothlypis rivularis, a year-round territorial wood-warbler (family Parulidae), and tested for relationships between territory size and individuals' vocal attributes. From October 2018 to April 2019, we banded 14 males and 9 females in the Atlantic Forest of south Brazil. We estimated territory sizes of nine pairs and recorded 10 and 5.4 h of 9 males and 6 females vocalizations, respectively. In male songs, a tradeoff between trill rate and frequency bandwidth indicates mechanical vocal constraints in the production of both parameters. Territory size did not relate to male song parameters, which could signal individual, rather than territorial, quality. However, females with broader bandwidth songs occupied smaller territories. Although the reduced sample size, which precludes generalization, this result may be a first indication of female song acting on territoriality in this species. Overall, this work presents new data on Riverbank Warbler and contributes to the knowledge on vocal tradeoffs in male songs and both male and female territorial behavior for wood-warblers. In addition, we highlight the need for further studies focused on female song, which like male song could be associated with territory defense.