Brood-guarding behaviour in Cory’s Shearwaters Calonectris diomedea

Abstract

Brood-guarding (or the continual attendance at the nest by one parent) has been relatively little studied in altricial birds. Parental investment in brood-guarding is often highly variable within a species, and the study of such variability may contribute to the understanding of the functions and regulation of this behaviour and of the tradeoffs involved in the choice between attending the nest and leaving to forage. In some colonial birds, it has been found that early nesting pairs attend their chick for longer than later nesting counterparts, giving rise to the synchronisation hypothesis that suggests that early pairs prolong broodguarding in order to reduce the probability of nest predation by a dilution effect. In this paper, for the first time we test the prediction that burrow-nesting colonial birds subject to little predation pressure should not display a seasonal decline in brood-guarding duration. The growth assistance hypothesis suggests that brood-guarding may allow the provision of frequent small meals and the efficient use of energy by chicks with poor homeothermic capabilities, resulting in improved early chick-growth. Finally, the chick-protection hypothesis predicts that chicks in more exposed nests should be brood-guarded for longer. Data collected at two Cory's Shearwater Calonectris diomedea colonies situated in contrasting environments supported the synchronisation hypothesis, as there was no seasonal trend in brood-guarding duration. Contrary to the growth assistance hypothesis, chicks brood-guarded for longer periods did not have an improved growth (in one colony there was even a negative effect of brood-guarding on early chick development). Finally, we found no difference in brood-guarding between nests with contrasting levels of exposure to potential predators and weather. Despite confirming the prediction of the synchronisation hypothesis, more research is needed to identify the main factors underlying the variability of brood-guarding observed in this and other studies.

Parent–offspring dietary segregation of Cory’s shearwaters breeding in contrasting environments

“…At SG, shearwaters performed much longer trips, reaching in most cases the Western African shelf, also an important upwelling zone (Davenport et al 2002). This last result indicates that the food availability around SG may be scarce, which agrees with previous studies that reported lower feeding frequencies (Granadeiro et al 1998b) and longer foraging trips at this colony (Catry et al 2009;Paiva et al 2010b), when compared to BE. Indeed, the length of foraging trips is known to be largely influenced by the local food availability (Uttley et al 1994) and by the distance to the most profitable areas (Paiva et al 2010c).…”

“…Mixed-effects ANOVAs were used to test for differences in d 13 C and d 15 N between adults and their offspring, between colonies and the interaction between both, with nest used as a random factor. Although the hatching dates of Selvagem Grande and Berlenga are generally similar (Catry et al 2009), the age of our sampled chicks was unknown, and we considered that wing and tarsus length should give us a reasonable indication of chick age at both colonies (Granadeiro 1991). An analysis of covariance (ANCOVA) was used to test for differences in chick weight between the colonies, using wing length as a covariate.…”

Moult strategies of Cory’s Shearwaters Calonectris diomedea borealis: the influence of colony location, sex and individual breeding status

“…Individuals nesting on Selvagem Grande generally make much longer foraging trips than birds nesting on Berlenga (Granadeiro et al 1998;Catry et al 2008). This is presumably a result of the lower productivity of the ocean in pelagic waters and, possibly, because of increased intraspecific competition in pelagic waters in comparison to waters over the continental shelf, where birds face less competition and the smaller colony may be limited by nesting site rather than food availability.…”

“…This is presumably a result of the lower productivity of the ocean in pelagic waters and, possibly, because of increased intraspecific competition in pelagic waters in comparison to waters over the continental shelf, where birds face less competition and the smaller colony may be limited by nesting site rather than food availability. In fact, birds on Selvagem not only make longer foraging trips during incubation (Catry et al 2008), but also they can engage in a dual strategy of mixed short and long foraging trips during chick rearing (Granadeiro et al 1998). These activities are indicative of a shortage of resources in the vicinity of the colony (Chaurand and Weimerskirch 1994;Weimerskirch et al 1997).…”

“…Hence, these sites are well-suited for a study of the influence of environmental conditions on moult strategies. Shearwaters nesting on Berlenga are close to the productive waters over the continental shelf, while those at Selvagem Grande live in a presumably less productive environment, which forces them to make longer foraging trips, both during incubation and chick-rearing, in order to obtain food (Granadeiro et al 1998;Catry et al 2008). The nearest Cory's Shearwater colony to Berlenga is many hundreds kilometres away, which suggests that birds from that colony face little intraspecific competition.…”