Seabirds exhibit considerable behavioural flexibility in foraging investment in order to meet the nutritional needs of their chicks during variable environmental conditions. Although regulation of offspring provisioning is generally thought to be related to species-specific constraints imposed by central place foraging, some studies suggest different responses within the same species linked to local differences in foraging conditions. Under adverse conditions, seabirds are expected to be less flexible because they must secure their own survival chances first before investing in current reproduction. Short-ranging single-prey loaders are expected to show large intra-specific variation in time spent on foraging because their mode of foraging is energetically expensive, and because they face restricted possibilities to increase the numerical prey input to the colony compared to multiple prey loaders. In this study, we examined if and how the single-prey loading Sandwich Tern Thalasseus sandvicensis varies colony attendance based on the nutritional status of their chick as well as parental body condition in two study colonies. The proportion of time that a chick was left unattended at the colony negatively correlated with chick body condition, suggesting that the parents tried to counterbalance poor feeding conditions by investing more time in foraging. Energy transport rates to the chicks (corrected for time spent away from the colony) and body condition of the chicks were similar in both colonies. However, at Zeebrugge, where adults were in poor body condition, parental non-attendance was much lower than on Griend, even when chicks were in poor condition. Still, our results suggest that parental nest non-attendance in Sandwich Terns is merely a corrective response to food loss to kleptoparasitic gulls in order to meet the nutritional status of the chick, although an effect of adult body condition could not be excluded. |
