Breeding stage
During the breeding season (April 2019 – August 2019), we revisited
subjects in the afternoons (1300–1700 h) to document their breeding
activities. We spent a maximum of 30 min searching for a given male, and
usually found them on account of their loud vocalizations and small
territories. We followed located males at a minimum distance of 5 m and
categorized them into one of six breeding stages: (1) no nesting duties,
(2) nest building, (3) egg stage, (4) nestling care, (5) fledgling care,
or (6) non-breeding (non-breeding status was assumed for all males
between December 2019 and January 2020; Mejías 2021). We defined nest
building as the stage when vireos are adding materials to a nesting
branch until a nest is completed. We defined the egg stage as the period
after nest completion, when the vireos are engaged in egg laying and
incubation; the egg stage terminates at hatching. Nestling care is the
stage when nestlings are seen inside the nest cup and the parents are
actively feeding or brooding them. Fledgling care is the stage when the
young are outside the nest and being fed by their parents.
Because we were unable to colour-band every female, it was difficult to
determine reliably whether subjects were paired or unpaired, as has been
done in some previous studies (Liu and Kroodsma 2007, Brunner and
Pasinelli 2010). Separation between male-female pairs occurs in our
study system. For example, one of our subjects that was seen with his
colour-banded mate was subsequently observed in his territory 4 days
later building a new nest with a new unbanded female, and his original
colour-banded mate was never seen again. For males that were paired to
unbanded females, a temporary absence of the female (i.e., 2-3 weeks)
therefore could mean that she was replaced by another unbanded female
following a period in which the male was unpaired, or it could simply
mean that we were unable to find her and that they had remained paired
throughout. To avoid the ambiguity of assigning paired and unpaired
status, we instead categorized males as with or without “nesting
duties.” During the breeding season, we considered males to have
nesting duties if they were engaged in nest building, the egg stage,
nestling care, or fledgling feeding, and to be without nesting duties if
they were not engaged in any of the above nesting behaviours with a
female. We often could not see a female accompanying a male that was
without nesting duties, but we refrain from categorizing such males as
”unpaired.”
Nests were located during
afternoon sessions by following vireos as they carried nest material or
food for nestlings. Bermuda Vireos rear one brood per season, with pairs
making up to five breeding attempts if previous attempts fail (Mejías et
al. 2021). For this reason, some of the focal males experienced the same
nesting stage multiple times throughout the breeding season. Whenever
possible, we identified the causes of nest failure. The black rat
(Rattus rattus ), Great Kiskadee (Pitangus sulphuratus ),
and Argentine ant (Linepithema humile ) were abundant across study
sites and are known predators of Bermuda Vireo eggs and chicks (Mejías
2021). Predation from ants was obvious because swarms would usually take
several days to consume eggs and nestlings. Kiskadee or rat predation
was not observed directly, but these potential predators were often
observed near nests a few days before the sudden and complete
disappearance of eggs or nestlings. Once a nest was inactive due to
fledging or predation, we recorded its location with the same GPS unit.
We imported the GPS coordinates of all nest and singing localities into
ArcMap 10.7.1 and used the “generate near table” to measure the
distances (1 m resolution) between each subject’s nest and its various
singing locations.