Egg-size variation in the Imperial Cormorant: on the importance of individual effects

SVAGELJ, W. S.; QUINTANA, F.

THE CONDOR

COOPER ORNITHOLOGICAL SOC

Año: 2011 vol. 113 p. 528 - 537

0010-5422

Although several studies have analyzed some of the components of egg size variation in birds, approaches simultaneously splitting whole phenotypic variation and analyzing their causes are lacking. We partitioned and analyzed the phenotypic variation in egg size in 1588 eggs from 572 clutches of the Imperial Shag Phalacrocorax atriceps during three breeding seasons (2004 to 2006) at Punta León, Argentina. We estimated repeatability and phenotypic plasticity in egg size, investigating the effects of year, laying date, body size and body condition on inter- and intra-clutch egg size variation. Egg size exhibited a large amount of variation where the largest egg was more than twice as heavy as the smallest. Both egg-size and mean egg-size repeatabilities (0.761 and 0.894, respectively) are amongst the highest values reported for any bird species. Most inter-clutch variation was due to inter-individual differences, being weakly related to laying date and unrelated to year, body size or body condition. Egg size decreased with laying order. This general pattern was not related to year, laying date, body size or body condition. Proximate constraints did not explain either inter- or intra-clutch variation. There was no obvious adaptive benefit of intra-clutch variation because the effect of intra-clutch egg-size variation in brood reduction was negligible. However, the egg size was positively related with the survival time of the last (third) chick. Thus, to invest in a large third egg would benefit females of the Imperial Shag, a brood reducer seabird, by keeping the last chick alive for a long period of time.