Acoustic signals show immense variation among passerines, and several hypotheses have been proposed to explain this diversity. In this study, we tested, for the first time, the relationships of song structure to phylogeny, habitat type, and morphology in the vireos and allies (Vireonidae). Every measure of song structure considered in this study had moderate and significant phylogenetic signal. Furthermore, two song-constraining morphological traits, bill shape and body mass, also exhibited significant phylogenetic signal. Song length showed the largest within-clade similarity; longer songs were highly conserved in part of the greenlet (Hylophilus) clade, whereas shorter songs characterized the remaining seven genera. We found no differences in song structure among vireonids living in different habitat types. However, vireonids with shorter, stouter bills and larger bodies sang songs with lower minimum and maximum peak frequency, compared with species with longer, thinner bills and smaller bodies. We conclude that Vireonidae song evolution is driven partially by phylogenetically conserved morphological traits. Our findings support the phylogenetic signal and morphological constraints hypotheses explaining structural diversity in avian acoustic signals.

For signal divergence to drive speciation, receivers should perceive structural differences in divergent signals; similar-structured signals from closer relatives are expected to elicit stronger responses than dissimilar signals from distant relatives. Two mechanisms can affect receiver responses to passerine song: (1) sympatric song familiarity and (2) an innate auditory template used to assess acoustic similarity. We examined the role of acoustic similarity by comparing behavioral responses of male Bermuda White-eyed Vireos (Vireo griseus bermudianus) to playback of the songs of allopatric species from across the family Vireonidae. Phylogenetic distance between the focal and stimulus species predicted response strength. Males uttered fewer vocalizations, had fewer speaker flyovers, and remained farther from the speaker during playback of the songs of more distantly related vireos. We then tested whether structural similarity of playback songs, as defined by three phylogenetically conserved song traits, explained these relationships. As predicted, males uttered fewer vocalizations, had fewer speaker flyovers, and remained farther from the speaker in response to more dissimilar songs. Collectively, our results suggest that male Bermuda Vireos perceive and respond to interspecies variation in the phylogenetically conserved song traits of allopatric species of vireos. This suggests that song divergence, and the ability to distinguish divergent songs, reinforces reproductive isolation and competitor exclusion.

The movements and behavior of many taxa of seabirds during the non‐breeding season remain poorly known. For example, although studies conducted in the Pacific and Indian oceans suggest that White‐tailed Tropicbirds (Phaethon lepturus) seldom fly more than a few thousand kilometers from nest colonies after breeding, little is known about the post‐breeding movements and behavior of a subspecies of White‐tailed Tropicbirds (P. l. catesbyi) that breeds on islands in the North Atlantic Ocean. Our objective, therefore, was to use light‐based geolocators to identify the ranges and pelagic activities of White‐tailed Tropicbirds from Bermuda during the non‐breeding periods in 2014–2015 (N = 25) and 2015–2016 (N = 16). Locations were estimated based on changes in light intensity across time, and pelagic activities were determined based on whether geolocators attached to leg bands were wet (i.e., birds resting on the water's surface) or dry (i.e., birds in flight). In 2014, birds spent late summer (July–September) near Bermuda and the British Virgin Islands; by mid‐September, most (N = 17; 68%) birds took a direct easterly route to the Sargasso Sea. In 2015, most post‐breeders (N = 15; 94%) flew east from Bermuda and to the Sargasso before the end of late summer. For both years combined, fall and winter (October–February) ranges extended as far west as North Carolina and as far east as the mid‐Atlantic Ridge. In both years, all birds were located between Bermuda and the British Virgin Islands during the spring (April–May). All birds then flew north to Bermuda in both years, with variations in timing, during April and May. We also found extensive overlap in the ranges of males and females during the non‐breeding season in both years. During the non‐breeding season, White‐tailed Tropicbirds spent 5% of night periods and 41% of day periods in flight in 2014; in 2015, birds spent 8% and 42% of night and day periods, respectively, in flight. Tropicbirds spent more time flying during the day because they hunt by day, detecting prey on the wing by sight. Overall, our results suggest that White‐tailed Tropicbirds that breed in Bermuda are diurnal, nomadic wanderers that range over an extensive area of the Atlantic Ocean during the non‐breeding season.

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Bird song is crucial for attracting mates and defending territories, but different types of song or different singing behaviours may be involved in acquiring or maintaining each resource. Furthermore, male songbirds may adjust when and where they sing throughout the breeding season, depending on their breeding stage. However, such relationships remain untested in several avian taxa. Here, we studied male Bermuda White-eyed Vireos (Vireo griseus bermudianus), a passerine with two distinct song types (discrete and rambling), to test the mate attraction, territory defence, and nesting stage hypotheses. We compare song rate and song perch height among different stages of the breeding season and during the non-breeding season. We show that male vireos produce both song types during the breeding and non-breeding seasons, suggesting dual roles in mate choice and territorial defence. Singing rate did not differ between the two seasons, but, within the breeding season, males without nesting duties had significantly higher song rates than males with nesting duties. Song rate was lowest during the nestling stage, which coincided with the highest rate of nest predation. Song perch height was higher during the breeding season versus non-breeding season, among males without nesting duties compared to males with nesting duties, and when males produced discrete versus rambling songs. Our findings suggest that male vireos increase their conspicuousness to prospecting females by increasing singing rate and song perch height, and that they sing during the breeding and non-breeding seasons to defend year-round territories. Collectively, our study supports the mate attraction and territory defence hypotheses of bird song and suggests that Bermuda White-eyed Vireos adjust their singing rate in response to nest predation risk.