Parental body condition affects egg volume in Rhinoceros Auklet but trophic level does not

Abstract

Planktivorous alcid females feeding more on lower trophic prey, i.e. copepods, during the pre-breeding season, lay larger eggs. The Rhinoceros Auklet (Cerorhinca monocerata) is piscivorous, but feeds also on krill in the spring at Teuri Island, Hokkaido, Japan. To examine the factors affecting their egg volume, we measured body size, body condition and the nitrogen stable isotope ratio ( δ 15N, a proxy of trophic level TL) of blood and feathers of male and female parents incubating eggs in three years (2010, 2011 and 2012). In 2011 and 2012, we sampled eggs within one day of laying and measured δ 15N and δ 13C (carbon stable isotope ratio) of yolk, albumen and membrane and those of feath- ers and blood of the female parents. We also sampled stomach contents of adults using water off-loading technique during the laying and incubation period, and determined their prey composition. Female body condition and size as well as male body size positively affected the egg volume index. Birds fed on low (krill) and high (fish) trophic prey during the laying and incubation periods, while individual variation in the δ 15N of blood cells that reflected TL during the pre-laying period did not affect the egg volume index. δ 15N of breast feathers that reflected TL during the molting period also did not relate to egg vol- ume index. δ 15N and δ 13C of feathers and blood of females did not correlate with those of egg yolk, albumen, or egg membrane.