Adaptive Loss of Mass in Thick-Billed Murres

Mass loss of breeding Thick-billed Murres, Uria lomvia, was examined.Adult breeders lost a significant amount of mass from incubation to chick hatching (0.032 kg in 1988 and 0.063 kg in 1989).Regression of the timing of mass loss during incubation and brooding revealed that body mass was maintained constant during incubation, was lost rapidly soon after chick hatching, and remained constant at the lower level thereafter.Murres made more than twice as many foraging trips per clay while feeding chicks than while incubating eggs, and therefore increased the amount of time spent in flight while brooding the chick.The stepwise mass loss is interpreted as adaptive in increasing flying efficiency as murres spend a greater amount of their time in flight during chick brooding.Murres may experience as much as a 25% of BMR per day energetic savings, or a reduction in the mechanical cost of flight of 9.0% to 9.5% by losing the observed 4.4% to 6.2% of incubation body mass.Murre mass loss during breeding was interpreted as adaptive in reducing the energetic cost of reproduction.