Mysterious attendance cycles in Cory's shearwater, Calonectris diomedea: an exploration of patterns and hypotheses

Abstract

Cory's shearwater logger periodicity seabird Selvagem Grande social interaction synchrony Several species of seabirds show cyclic patterns of attendance at their nesting colonies. We examined the patterns of variation in the numbers of Cory's shearwater at three colonies (two oceanic and one located on the continental shelf), including the world's largest, at Selvagem Grande, Madeira, Portugal and considered several hypotheses concerning their causal mechanisms. At Selvagem Grande, cycles were exceptionally marked and regular, with a periodicity ranging from 7.8 to 11 days, and involved both breeders and nonbreeders. In contrast, variation in numbers was aperiodic at a nearby and much smaller colony (Selvagem Pequena), and also at the colony located off the Portuguese coast (Berlenga Island). We found no relationships between number of birds ashore and environmental variables such as wind direction and speed or lunar cycle. Cycles did not seem to be driven by oscillations in food availability or accessibility, given that they did not correlate with daily chick growth rates (which were acyclic) or diet. Despite their regularity, cycles were slightly out of phase in different sectors of Selvagem Grande, which suggests that social interactions at the colony could act as an entrainment agent for an endogenous rhythm, and so cycles are probably more likely to occur in large and dense colonies. Observations are consistent with the hypothesis that cycles facilitate social interactions by maximizing the probability of encounters at the colony. However, the exact mechanisms through which these remarkable cycles are controlled are still completely unknown, and clearly further research is needed. Ó

Acoustic activity across a seabird colony reflects patterns of within‐colony flight rather than nest density

“…Additionally, the number of birds in the colony is likely to be highest during incubation before breeding birds fail and leave, and before prospecting males, which are highly vocal, leave the colony (James 1985). Secondly, the relationships between acoustic activity and environmental variables in our study are consistent with those reported previously (Granadeiro et al 2009, Buxton & Jones 2012, Borker et al 2014). An example of this is the significantly reduced acoustic activity on moonlit nights, a wellknown phenomenon thought to be a predator avoidance mechanism (Bretagnolle et al 2000, Oppel et al 2014.…”

“…Secondly, the relationships between acoustic activity and environmental variables in our study are consistent with those reported previously (Granadeiro et al . , Buxton & Jones , Borker et al . ).…”

Factors affecting mortality of shearwaters stranded by light pollution

“…Given the remarkable sexual size dimorphism of the species (Thibault et al ., ; see Supporting Information Appendix S1), one would expect a greater sensitivity of male chicks to adverse conditions (Genovart et al ., ). However, and despite parental food delivery being less frequent on full‐moon nights (Klomp & Furness, ; Riou & Hamer, ; but see also Granadeiro et al ., , ; Mougin et al ., ), we did not detect differences in body condition between sexes in two fallouts differing in moon phases (Fig. ).…”

“…Our first specific goal was to assess body condition of fledglings attracted to lights throughout the fledging period, taking into account gender, year, bird status (successfully released/dead) and/or plumage characteristics (abundance of down). Parental food delivery and feeding frequency decrease sharply at the end of chick‐rearing period in petrels (Warham, ), and on full‐moon nights (Klomp & Furness, ; Riou & Hamer, ; but see also Granadeiro, Burns & Furness, ; Granadeiro et al ., ; Mougin, Jouanin & Roux, ). The majority of petrel species are sexually size dimorphic, males being larger and heavier than females and, consequently, more sensitive to adverse conditions (Cockburn, Legge & Double, ; Kalmbach & Benito, ).…”

Estimating population size of a nocturnal burrow-nesting seabird using acoustic monitoring and habitat mapping

“…Vocal activity of burrow-nesting seabirds at colonies is dependent on many environmental factors and thus varies considerably within nights and over the breeding season Granadeiro et al 2009). Instantaneous vocal activity is therefore unlikely to accurately predict nest density, which has affected acoustic population assessments made by human observers in the past (Bolton et al 2010).…”