Clutch and egg size in marine and freshwater common terns <i>Sterna hirundo</i>

Clutch and egg sizes in birds are affected by intrinsic and environmental factors. Clutch size is affected by food availability and predation risk, while egg size is influenced by parental age and environmental conditions during both the breeding and pre-breeding period. In the Common Tern Sterna hirundo , egg size decreases with the laying sequence, and the survival rate of the last-hatching chicks is lower than that of their siblings. We studied the differences in clutch size, egg volume, and egg volume reduction in Common Terns inhabiting marine and freshwater habitats. Clutch size varied in the studied regions. Egg size differed annually, but the greater volume reduction of second- and third-egg was found in marine habitats and among late clutches, regardless of the year. Egg volume and reductions were correlated with females’ body characteristics.