Causes and consequences of egg size variation in Swallows Hirundo rustica


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Relationships between egg mass, egg composition, hatchling mass, female characteristics and environmental conditions during egg formation were investigated for Swallows Hirundo rustica in CentraI Scotland. Egg dimensions were highly correlated with egg masso 60% of variation in egg mass originated from differences between females. Egg mass was repeatable (repeatability=0.54) for individuaI birds but was not related to other female characteristics such as measures of structural size. Egg mass was not related to clutch size, hatchability or position in the clutch. Eggs contained more lipid iffavourable environmental conditions prevailed during egg formation. Heavier eggs contained more lipid and lean dry component. A sample of eggs was hatched in a incubator so that hatchlings could be matched with the eggs from which they emerged. Egg mass explained 60% ofvariation in hatchling masso 79% ofvariation in hatchling mass was explained by the first component in a principal component analysis which used egg mass, length and breadth to describe egg size. Heavier eggs produce larger hatchlings which generally grow faster and have greater survival across 34 studies of non-passerine birds. Although larger passerine eggs produce hatchlings which are initially larger and grow faster (n=7 studies) this does not lead to increased survival. Hirundines lay relatively small eggs for their body size although production oflarger eggs was predicted to have little extra cost for a Swallow and should be advantageous as larger hatchlings would be more likely to survive periods of food shortage.