Feeding rate reflects quality in both parents and offspring: a longitudinal study in common terns

Offspring provisioning can act as a proxy of resource acquisition and vary with parental sex and age. Age-related variation can arise from individual experience and senescence, but also from selective disappearance of poor-quality parents. Distinguishing between these processes and quantifying their effect on the resource acquisition and fate of individual chicks requires longitudinal monitoring of known-age individuals, which is still rare. In our longitudinal study, we observed offspring provisioning of common terns, Sterna hirundo, across a 6-year period and analysed provisioning behaviour from both a parental and offspring perspective. Using repeated measures of provisioning of individual parents, our analyses showed that provisioning did not increase with age, but that parents that were observed at older ages provisioned more, suggesting selective disappearance of parents that provisioned less. Parental provisioning was higher in males than females and increased with brood size in both sexes. For offspring, energetic acquisition declined with hatching order and increased with age. Acquisition from the mother increased faster with chick age than that from the father, and mothers distributed their provisioning more evenly across chicks of different hatching order. Parental age, however, did not affect the energetic acquisition of the offspring. The early energetic acquisition rate of chicks predicted their fledging success, but not fledging mass. When decomposing effects on energetic provisioning and acquisition rate into effects on feeding rate, prey energetic density and prey size, we found that all arose from variation in feeding rate. Overall, these results therefore show that both parents and offspring vary in quality, which is reflected in their feeding rate.