Females can differentially deposit the immune factor lysozyme into eggs based on conditions of local breeding density and laying order.

We collected 80 eggs from Great Cormorants (Phalacrocorax carbo) and then analyzed whether the level of lysozymes in the eggs is related to breeding density and laying order.

Between clutches, the level of lysozyme in eggs is positively related to breeding density; while within a clutch, the level of lysozyme is positively related to the laying order.

When parents breed under conditions of high density, they allocate more lysozymes to their offspring, a trait adaptive to the local environment. That the increase in the level of lysozymes is a function of the laying order seems a necessary condition to mitigate the hierarchy among siblings for improving the survival of the entire clutch.