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