THE REGULATION OF FOOD DELIVERY TO NESTLING CORYS SHEARWATERS CALONECTRIS-DIOMEDEA - THE ROLES OF PARENTS AND OFFSPRING

In common with other species of Procellariiformes, Cory's Shearwaters accumulate prodigious amounts of adipose tissue during the nestling period, and attain peak weights considerably in excess of adult weight. We conducted two experiments to determine whether this pattern of development could be related to an absence of regulation of food supply to chicks. In the first experiment, chicks were switched between nests at 29 days of age, approximately three weeks prior to peak mass attainment. Serial correlations were then calculated between each chick's weight before switching and both its own weight during subsequent age intervals and the weight of the foster chick during subsequent age intervals. The results supported a parental-determination model of feeding, indicating that the behaviour of the chick had little or no influence on food supply rate. The second experiment, in which chicks were given a supplementary meal of c. 60 g of fresh sardine daily, indicated that parents reduced neither their rate of provisioning nor meal size in response to a reduction in the nutritional requirements of their chicks. Supplements resulted in a doubling of mass growth rate of chicks for the first eight days of supplementary feeding; this was followed by a period of much slower mass growth, but chicks given supplementary food remained significantly heavier than unmanipulated controls. During the latter period, adults at manipulated and unmanipulated nests visited their chicks at similar rates and with similar amounts of food. Slow mass growth of chicks at manipulated nests probably resulted from their refusing to accept food from their parents, and suggested that the rate at which parents supply food under natural conditions may be close to the maximum assimilatory capacity of the chick's digestive tract. The absence of any regulation of food supply in accordance with the behaviour or nutritional requirements of the chick may reflect a low correlation between the nutritional status of a chick at one feeding and its needs at the end of the parent's succeeding foraging trip. Alternatively, an absence of regulation may result from adults supplying their chicks with food at the maximum attainable rate, regardless of the chick's nutritional status.