Effects of supplementary feeding on provisioning and growth rates of nestling Puffins Fratercula arctica: Evidence for regulation of growth

We used a supplementary feeding experiment to investigate the existence of a causal relationship between the nutritional status of nestling Puffins and the subsequent rates of food provisioning by their parents. Chicks given supplementary food received less frequent meals than controls, with the effect that the total amount of food received by the experimental group (parental delivery plus supplementary food) was similar to the amount delivered by parents in the control group. This indicated compensatory regulation of provisioning by parents in the experimental group. Increases in body size and body mass were virtually identical in the two groups until shortly before fledging, when supplemented chicks attained higher body mass than controls. This difference appeared to result mainly from the fact that unmanipulated chicks normally receive little food from their parents towards the end of the nestling period; supplements thus constituted a large proportion of chicks' normal daily food intake over this period, and compensatory reduction in food delivery by parents did not completely match the extra food given to chicks in the supplemented group. If growth rates of chicks were usually limited by poor food supply and adults delivered food to the chick at the maximum rare permitted by food availability irrespective of the chick's immediate nutritional requirements, then regulation of food delivery by parents would not be expected. The fact that adults were regulating food delivery to their offspring below the maximum attainable rate suggests that in this study at least, nestling growth rate was not limited by a scarce and unpredictable food supply.