Food provisioning to nestling shearwaters: why parental behaviour should be monitored?

Does sexual segregation occur during the nonbreeding period? A comparative analysis in spatial and feeding ecology of three Calonectris shearwaters

Sexual segregation (SS) is widespread among animal taxa, with males and females segregated in distribution, behavior, or feeding ecology but so far, most studies on birds have focused on the breeding period. Outside this period, the relevance of segregation and the potential drivers of its persistence remain elusive, especially in the marine environment, where animals can disperse over vast areas and are not easily observed. We evaluated the degree of SS in spatio-temporal distribution and phenology, at-sea behavior, and feeding ecology during the nonbreeding period among three closely related shearwaters: Scopoli's, Cory's, and Cape Verde shearwaters (Calonectris diomedea, C. borealis, and C. edwardsii, respectively). We tracked 179 birds (92 males and 87 females) from 2008 to 2013 using geolocation-immersion loggers and collected the 13th secondary remige (molted in winter) for stable isotope analyses as a proxy of trophic level and diet. The global nonbreeding distribution did not differ between sexes for the three species, but one specific nonbreeding area was visited only by males. Cory's shearwater males remained in areas closer to the colony in a larger proportion compared to females and returned earlier to the colony, probably to defend their nests. Males presented a slightly lower nocturnal flying activity and slightly (but consistently) higher isotopic values of δ13C and δ15N compared to females. These differences suggest subtle sexual differences in diet and a slightly higher trophic level in males, but the extent to which sexual dimorphism in bill size can determine them remains unclear. Our study showed that SS in ecological niche in seabirds can persist year-round consistently but at a different extent when comparing the breeding and nonbreeding periods. Based on our findings, we propose that SS in these seabird species might have its origin in an ecological specialization derived from the different roles of males and females during reproduction, rather than from social dominance during the nonbreeding period.

Control of provisioning in grey-headed albatrosses (Thalassarche chrysostoma): do adults respond to chick condition?

Recent studies have been equivocal in determining whether long-lived seabirds are able to respond to chick nutritional status and regulate provisioning rates accordingly. With some exceptions, albatrosses and petrels that feed their chicks every 1–3 days seem able to respond to variability in chick condition. In contrast, those with longer overall feeding periodicity and that frequently show a dual foraging strategy, mixing short trips (which most benefit the chick) with longer, pelagic feeding excursions (during which time adults recover lost condition), tend not to regulate provisioning. We used a combined observational and experimental approach to determine whether feeding rates in grey-headed albatrosses (Thalassarche chrysostoma) were determined by parental foraging proficiency and (or) offspring demand. The results indicated that both factors were important. Adults showed consistent differences in provisioning characteristics, but chick satiation and solicitation also influenced the mass of the meal delivered when adults returned. Given a provisioning schedule characterised by relatively frequent colony visits and with no means of assessing the feeding success of their partner in the interim, the adjustment of meal mass upon arrival appears to be a simple means of tailoring prey delivery to changing chick requirements and is likely to be as effective as alternative strategies suggested for other species of Procellariiform.

Regulation of chick provisioning in the thin-billed prion: an interannual comparison and manipulation of parents

To investigate how thin-billed prions, Pachyptila belcheri, regulate their chick-provisioning behaviour, we compared data from two seasons with different food availability (1992 and 1997) and increased the costs of foraging by using load bands during 1997. The 1992 breeding season was poorer than 1997, but during both seasons parents used a two-part foraging strategy, mixing long trips, when they built up body reserves, with short trips, when they used previously stored reserves and foraged to feed the chick. In 1992, parents alternated long and short trips and spent more time on long trips, whereas in 1997 they performed several short trips before a long trip. As a consequence, provisioning rates were higher in 1997. Similarly, manipulated birds decreased their provisioning rates compared with control birds. During the two seasons, both control and experimental parents kept their body mass constant, and survival rates did not differ between seasons or groups. Using a logistic regression, we examined the factors influencing the decision whether, after a first short trip, to start a long trip or a second short trip. This decision was not influenced by the body condition of the chick, the meal mass, or the duration of the previous foraging trip, only by adult body condition. The results show that parents regulate their provisioning according to their body mass, which is the main factor influencing decisions concerning energy allocation between current reproduction and survival in this small long-lived seabird.