Benefits of mass reduction for commuting flight with heavy food load in Leach’s storm‐petrel, <i>Oceanodroma leucorhoa</i>

Abstract

When rearing chicks, Leach’s storm‐petrels ( Oceanodroma leucorhoa ) commute between foraging areas and breeding colonies with heavy food loads. At this time they should maximize the size of energy‐supplying organs in response to increased energy expenditure but minimize total body mass to decrease the energetic cost of flight. Nineteen storm‐petrels were killed to examine the changes in body composition and the masses of energy‐supplying organs in birds that were incubating and rearing chicks. Parents lost a mean of 7.95 g in body mass between the stages of incubation and chick‐rearing mainly via a loss of skin including subcutaneous adipose tissue, and a small fraction of heart and digestive organs, which are considered energy‐supplying organs. This mass loss actually enables them to decrease flight cost by 14.4%. The benefits of decreasing flight costs by reducing total body mass are greater than if the energy‐supplying organs of birds are enlarged only.