Discussion
The singing behaviour of territorial male Bermuda Vireos was related to
their breeding activities. Although the Bermuda Vireo can be heard
year-round, our study suggests that males become more conspicuous during
the breeding season by ascending to higher song perches, before
returning to their usual haunts in the understory vegetation for the
remainder of the year. Males used discrete songs more extensively than
rambling songs year-round, and discrete song rate did not differ between
the breeding and non-breeding seasons. During the breeding season,
however, males with nesting duties sang fewer songs than males without
nesting duties, and males caring for nestlings produced the fewest songs
of all. Song perch height was higher during the breeding season than
during the non-breeding season, for breeding males without nesting
duties than for breeding males with nesting duties, and when males sang
discrete songs rather than rambling songs.
White-eyed Vireos, like several other species in the genus Vireo ,
have a species-typical song and a longer, faster, run-on song (Lawrence
1953; Graber 1961; Nolan 1960; Nolan 1962; James 1978; Bradley 1980;
Robinson 1981; Gomez-Montes and Moreno 2008; Hedley 2016); for V.
griesus , the former and latter song types are known as “discrete
songs” and “rambling songs,” respectively. That the production of
discrete song did not differ between the breeding and non-breeding
seasons suggests that discrete song functions, at least partially, in
year-round territory defence. Similar patterns of song production have
been described for non-migratory tropical birds that also defend
year-round territories (Tobias et al. 2016). The rambling song was
generally rare, as it is in continental White-eyed Vireos (Bradley
1981), and we were unable to compare its rate of production between
seasons. Nevertheless, several observations support a territorial
defence function for rambling song. It is produced in the non-breeding
season and, compared to discrete songs, is produced lower in the canopy,
where male-male interactions typically occur (Liu 2004). Although
anecdotal, Bermuda Vireos in our study sang rambling songs during
several close-quarter countersigning exchanges with neighbouring males.
We note, however, that one male also directed rambling song towards a
female moments before copulating with her, suggesting that rambling
songs might also function in a breeding context.
Our findings also provide evidence that Bermuda Vireo song functions in
mate attraction. During the breeding season, males without nesting
duties usually were unaccompanied by a female and spent this time
singing discrete songs at a high rate, whereas males with nesting duties
were most often accompanied by a female and sang significantly fewer
discrete songs. Similar singing patterns have been described in Bell’s
Vireos (Vireo bellii ; Nolan 1960), Yellow-throated Vireos (James
1984), and Warbling Vireos (V. gilvus ; Howes-Jones 1985), and for
several avian taxa beyond the Vireonidae (Powlesland 1983, Hayes et al.
1986, Staicer et al. 2006, Foote et al. 2017). Our findings that song
perch height was higher during the breeding season than during the
non-breeding season, and higher among breeding males without nesting
duties than among breeding males with nesting duties, provides further
support that Bermuda Vireo discrete song functions to attract mates.
Males with no nesting duties performed lengthy song bouts of discrete
song whilst remaining stationary on higher branches in the tree crown,
before repeating this behaviour at another elevated and frequently
visited perch in the territory. These behaviours have also been
described for unmated males in Blue-headed Vireos, Yellow-throated
Vireos, South Island Robins (Petroica australis ), and Chipping
Sparrows (James 1978; Powlesland 1983, Liu and Kroodsma 2007); these
researchers suggest that singing from elevated perches increases an
unmated male’s visual and acoustic conspicuousness to prospecting
females. Three anecdotal observations provide further support that
singing from elevated perches is a mechanism for attracting prospective
females: (1) breeding pairs travelled primarily in the understory, (2)
nests were never built in the canopy, but, rather, from forked branches,
usually 2–3 meters above the ground, and (3) males often returned to
canopy perches after their mate disappeared, typically following nest
failure. The tendency of males to select higher perches when singing
discrete song versus rambling song might be because discrete song is
louder than rambling song (Bradley 1980); the combination of being
louder and being sung from higher perches may reflect a history of
selection for maximizing signal transmission distance (Sprau et al.
2012, Podos and Sung 2020).
Most of the nest predation in our study occurred during the nestling
care stage, which is consistent with the predictions of the nesting
stage hypothesis (Morton et al. 1993, Burhans et al. 2002). Given the
increased risk of nest predation during nestling care, and the fact that
songs can alert predators to nearby nests (Ellison and Ydenberg 2019),
it is perhaps not surprising that Bermuda Vireos produced the fewest
discrete songs during this time. Similar declines in vocal activity
during nestling care, compared to other stages of the nesting cycle,
have been documented in species spanning multiple avian familes,
including House Wren (Troglodytes aedon , Wilson and Bart 1985),
Yellow-throated and Blue-headed Vireo (James 1999), Golden-cheeked
Warbler (Bolsinger 2000), Chipping Sparrow (Liu and Kroodsma 2007), and
Common Reed Bunting (Emberiza schoeniclus , Brunner and Pasinelli,
2010). Our observation of song resurgence during the fledgling stage
(Figure 4) appeared to occur because males often used discrete song to
guide fledglings throughout the natal territories and to bring them
close after securing a food item. There have been no observations of
predation of adult or fledgling Bermuda Vireos, which spend most of
their time in thicket understories. Thus, in addition to its established
role in passerine song learning (Nowicki et al. 1998), the increased
singing rate of males during fledgling care possibly reflects this low
predation risk, further supporting the nesting stage hypothesis.
Future research should attempt to further distinguish the functions of
the two main song types used by Vireo species, as has been done
in the two-category singing system of North American parulids (Spector
1992). The many observational studies that preceded our work not only
brought to light the ubiquitous nature of the two-category vireonid song
system, but also provide a list of vireonid species that can serve as
candidates for hypothesis testing. Future research could also test the
effects of feather moult on singing rate. In Bermuda, August marks the
peak of feather moult in vireos (M. Mejías pers. obvs.), when most
adults were seen hastily feeding while missing some or all their tail
feathers. Feather moult is one of the most energetically expensive and
time-consuming life stages in birds (Rohwer et al. 2009, Kulaszewicz and
Jakubas, 2015), and could also explain the decline of song in August,
with males prioritizing intensive foraging over vocalizing, as observed
in moulting Blue-headed and Yellow-throated Vireos (James 1999).
In conclusion, we found that male Bermuda Vireos are year-round singers
that alter their singing behaviour in relation to breeding stage. Our
results provide support for the territory defence and mate attraction
hypotheses of passerine song, and suggest that nesting birds reduce
their production of conspicuous songs during nestling care when their
nests are at the greatest risk of predation.