Inter- and intraclutch egg-size variation in the Imperial Shag (Phalacrocorax atriceps): on the importance of individual effects

SVAGELJ, W. S.; QUINTANA, F.

Maturín , Venezuela

Congreso; VIII Neotropical Ornithological Congress; 2007

Neotropical Ornithological Society

Although several studies have separately analyzed some of the components of egg size variation in birds, studies simultaneously decomposing whole phenotypic variation and analyzing their causes are scarce. We decomposed the phenotypic variation in egg size in 1588 eggs from 568 clutches of the Imperial shag (Phalacrocorax atriceps) monitored during three consecutive breeding seasons (2004, 2005 and 2006) at Punta León, Argentina. Egg size of the Imperial shag exhibited a large amount of variation where largest egg was more than twice as heavy as the smallest. Extremely small and extremely large eggs had reduced hatchability, suggesting that egg size was under stabilizing selection. Most of the total phenotypic variation in egg size (76.1%) corresponded to variation among females (repeatability). Despite substantial variation within clutches (22.5%), egg-size was mostly inflexible within individual females (plasticity) across years (1.4%). Female identity was the main factor determining egg size. Egg size decreased with laying order; this pattern was inflexible. Ultimate causes (female-energy savings) appear to explain the observed pattern of intra-clutch egg-size variation; females benefit by investing fewer resources in those eggs with lower likely survival due to hatching asynchrony resulting in consequent brood reduction in this species.