Lipid metabolism, begging behaviour and nestling obesity in a pelagic seabird

Summary

1.  Several alternative explanations have been proposed to account for the evolution of nestling obesity in a variety of avian groups. These predict quite different patterns of fat storage, utilization and food solicitation by chicks of different ages, yet surprisingly few studies have measured changes in begging behaviour or use of fat stores during chick development.

2.  We tested the applicability of these hypotheses for Manx shearwaters Puffinus puffinus, by direct measurement of changes in plasma lipid metabolites (non-esterified fatty acids and β-hydroxybutyrate) and begging behaviour throughout the chick’s development until fledging.

3.  Chicks metabolized fat reserves during periods of parental absence throughout their development but there was no evidence that even the poorest fed chicks were close to exhausting these reserves during the period of mass growth. Chicks begged more vigorously during the period of mass loss at the end of the nestling period than at any previous stage of their development, and mobilization of fat stores was also at a maximum during this period in well-fed chicks, coinciding with a decrease in food provisioning by parents. Increases in lipid metabolite levels occurred at an earlier age in poorly-fed chicks than in well-fed chicks and were followed by a marked decline only in poorly-fed chicks, indicating that they were exhausting their fat stores.

4.  These findings are incompatible with several of the proposed explanations for nestling obesity and mainly support the hypothesis that fat stores fuel chicks through a period of several weeks of reduced food provisioning prior to fledging, when parents are unable or unwilling to maintain earlier levels of food provisioning, despite increased solicitation from chicks.