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.