Fly or die: the role of fat stores in the growth and development of Grey-headed Albatross Diomedea chrysostoma chicks

Chicks of albatrosses, like other Procellariiformes, become independent at a mass similar to their parents but during growth attain a peak mass some 3 0% or more greater, before losing mass prior to fledging. The current views are that this high peak mass represents chicks storing fat reserves as an energy sink, or as an insurance against periodic food scarcity, or as a con sequence of natural stochastic variation in provisioning rate. We analysed growth and body composition of Grey-headed Albatross Diomedea chrysostoma c hicks at Bird Island, South Georgia in 1984 and 1986, two years of very dif ferent food availability. In 1984 when overall breeding success was only 28 % (the lowest in 20 years and less than half that in 1986), chicks were sig nificantly smaller in terms of peak mass (by 37%), primary length (by 25%), liver, lung, heart and kidney size (by 18-34%) and fat (by 75-80%) but not significantly different in terms of skeletal (tarsus, culmen, ulna, sternu m) or muscle (pectoral, leg) size. Despite these differences, there were so me important similarities in the patterns of growth in both years. Up to th e attainment of peak mass, most of the growth of organs and of skeletal str uctures was completed and little fat was deposited. In the remaining part o f the chide-rearing period, feather growth and acquisition of fat stores we re undertaken. Thus Grey-headed Albatross chicks begin to acquire substanti al fat stores only during the later part of the development period; this is contrary to the predictions of any of the existing hypotheses concerning p rovisioning patterns and the role of fat stores in Procellariiformes. We pr opose that the deposition of fat in the later stages of chide growth is an adaptation to: (a) ensure against energy demands and/or nutritional stress affecting the quality of flight feathers (many of which. are not renewed fo r up to three years after fledging); and (b) provide an energy reserve for chicks to use in the critical period immediately after independence.