Recording effort and a general description of singing behaviour
Bermuda Vireos were vocally conspicuous amongst the island’s woodland avifauna. Counter-singing among neighbouring males was common and we observed this at both the edge and interior of a singer’s territory. Our fieldwork produced 430 15-min recordings across the breeding (April 2019–August 2019; N= 374 recordings) and non-breeding seasons (December 2019–January 2020; 56 recordings), equating to 6,450 min. We obtained more recordings of focal males during the breeding season (mean ± SD: 31 ± 3 recordings per male; range: 26–37 recordings; N = 12 males) than the non-breeding season (5 ± 0.30 recordings; 5–6 recordings; 11 males) because our winter residency on the island was limited compared to the summer months, and because the breeding season was subdivided into five stages. Subjects produced at least one song in 349 (81%) of the 430 recordings. A total of 17,682 vireo songs were detected from the recordings, and, of these, 16,818 (95%) were discrete songs and 864 (5%) were rambling songs. Males produced both song types during the breeding and non-breeding seasons; discrete song rate was fairly constant throughout the breeding and non-breeding season, with the lowest rates recorded in August (Figure 3). We noted that August also marked the onset of feather moult in our subjects, where males with missing tail feathers spent more time feeding quietly than singing.
Male Bermuda Vireos used multiple song perches throughout their territories and vocalized at varying distances from their nests. Seldom did males vocalize while either sitting inside the nest cup or while perched on the rim. Occasionally, incubating males sang a couple of discrete songs, with noticeably longer pauses between songs. These were often followed immediately by the female returning to the nest and relieving the male, which then resumed steady bouts of discrete song away from the nest. In general, breeding males used song perches away from the nest (mean ± SD: 18 ± 18 m; range: 0–86 m). With respect to perch height, males accompanied by a female or engaged in nesting duties often alternated between singing and flutter-hopping amongst understory perches. In contrast, males in the breeding season with no nesting duties usually performed stationary song bouts from exposed canopy perches.