Introduction
Bird song is diverse among species, yet also varies within species (Mejías et al. 2020, Rose et al. 2022). One hypothesis for explaining intraspecific variation is that song serves multiple functions, including territory defence, where males sing to announce occupancy of utilized space (Szymkowiak and Kuczyński 2017, Mejías et al. 2021, Wheeldon et al. 2021), and mate attraction, where males sing to attract females (Kroodsma 1984, Catchpole and Slater 2003, Sockman et al. 2005). In some species that produce multiple song types, different acoustic displays have different intended receivers (Spector 1991, Demko et al. 2013, Janes et al. 2017). In several genera of New World warblers (Parulidae), for example, structurally complex songs among males tend to be sung later in the breeding season and exchanged with rival males, whereas songs that are simple in structure and stereotyped among males are sung primarily in the presence of females early in the breeding season (Ficken and Ficken 1962, Staicer 1989, Janes et al. 2017). These findings suggests that one type of song is instrumental for male-male competition and the other for female attraction, and therefore could drive the observed structural variability in songs of the same species.
Several hypotheses exist to explain variation in singing behaviour and its effects on conspecific receivers. For example, during the breeding season, male Great Tits (Parus major ) that are removed from their territories and replaced with speakers broadcasting their song experience fewer territorial intrusions than controls that are removed and replaced with speakers broadcasting silence (Krebs 1977). This finding provides strong support for the territory defence hypothesis. In line with the mate attraction hypothesis, vocal output can also be higher among unpaired males than paired males (Staicer et al. 2006, Liu and Kroodsma 2007), with bachelor males singing at higher rates at dawn (e.g., Savannah Sparrows, Passerculus sandwichensis ; Moran et al. 2019) or males reducing daytime song once paired (e.g., Chipping Sparrows, Spizella passerine ; Liu and Kroodsma 2007). Among paired males, nesting stage might also influence singing rate. Predation, the most significant cause of nest failure in songbirds, can be affected by the vocal displays of parents and offspring (Ricklefs 1969, McDonald et al. 2009; Haff et al. 2015). Parents singing near the nest can reveal nest locations to eavesdropping predators, which may explain why some male birds sing less frequently when approaching or sitting on nests (Nice 1930, Bolsinger 2000, Chiver et al. 2007). According to the nesting stage hypothesis, the risk of nest predation changes with nesting stage and is highest during nestling care (Morton et al. 1993, Burhans et al. 2002). The cost of nest failure may also increase during later nesting stages because parents have invested more and have less time remaining to re-nest (Slagsvold 1984, Verhulst and Nilsson 2008). Under the nesting stage hypothesis, birds may assess these risks and reduce their singing when the costs of nest loss are highest, such as during the nestling stage.
Breeding stage can also affect song perch height, which is an important component of singing behaviour. During the breeding season, many songbirds use elevated song perches (Castrale 1983, Rodenhouse and Best 1983, Hallworth et al. 2008), yet, despite decades of such observations, few studies have tested for a relationship between breeding stage and song perch height. Some studies have focused on the consequences of song perch choice on song transmission (Mathevon et al. 2005, Barker and Mennill 2009, Mennill et al. 2009), predation risk (Duncan and Bednekoff 2006, Campos et al. 2009), and foraging success (Greig-Smith 1983, Guilfoyle et al. 2002). While these factors undoubtedly are related to song perch height among breeding birds, singing from higher perches can also increase the probability of a male being detected by a prospecting female (Petit et al. 1988, Beck and George 2000, Hallworth et al. 2008). In Chipping Sparrows, song perches are higher among unpaired males than paired males, consistent with the hypothesis that males adjust their song perch height according to their breeding stage and whether they are actively seeking a mate (Liu and Kroodsma 2007).
The White-eyed Vireo (Vireo griseus ) is a small songbird inhabiting shrublands and thickets in the southeastern United States. Only males sing, and their songs are described as fast and robotic in delivery (Adkission and Conner 1978, Borror 1987). Two distinctive song types are recognized, discrete song and rambling song (Bradley 1980). The discrete song type is short (ca. 1 s) and comprises highly modulated elements, including chips, buzzes, and whistles delivered in a fixed sequence; individual males have at least 10 discrete song variants in their repertoires (Borror 1987). The rambling song type is a long (up to ca. 10 s) warble comprising discrete song elements and harsh scolding elements delivered in an unpredictable sequence and at a faster rate than for discrete songs. Previous observations suggest that discrete songs function primarily in territory defence and rambling songs function primarily in interactions with females (Bradley 1980), though their usage among seasons, breeding stages, and social contexts remains unquantified. A non-migratory subspecies known as the Bermuda White-eyed Vireo (V. g. bermudianus ) or “chick-of-the-village ” (hereafter Bermuda Vireo) is endemic to the Bermuda archipelago (Bangs and Bradlee 1901, Mejías 2021). Like the continental form, it sings both discrete and rambling songs.
The overall goal of this study is to gain insight into the function of song in male Bermuda Vireos by testing whether song production and song perch height are associated with breeding stage. First, we quantify the number of discrete and rambling songs used during breeding and non-breeding seasons. Since Bermuda Vireos maintain year-round territories, a song type that is confined to the breeding season suggests that it is used primarily for acquiring a mate, whereas a song type that is produced consistently throughout the year suggests that it functions in territory defence. Second, to investigate the mate attraction and nest predation hypotheses, we test whether vocal output is associated with a male’s nesting status within the breeding season. We predict that males without nesting duties sing more than males with nesting duties (i.e., mate attraction), and that song rate is lowest during the nestling care stage when nests experience the highest predation risk (Morton et al. 1993, Burhans et al. 2002). Third, we test whether song perch height is related to breeding stage. Since singing from higher perches should increase the probability that a male is detected by a distant prospecting female (Liu and Kroodsma 2007), we predict that song perches are higher during the breeding season than the non-breeding season, for males without nesting duties than for males with nesting duties, and when males sing rambling songs that are thought to function in female attraction rather than discrete songs that are thought to function in territory defence (Bradley 1980).
Methods