The immune system plays an important role in fitness, and interindividual variation in immunocompetence is due to several factors including food supply.

Seasonal variation in food resources may therefore explain why immunocompetence in bird nestlings usually declines throughout the breeding season, with chicks born early in the season receiving more food than chicks born later, and thereby possibly developing a more potent immune system. Although there are studies supporting this hypothesis, none has been experimental.

We performed an experiment in the kittiwake Rissa tridactyla by manipulating the food supply of pairs that were left to produce a first brood, and of pairs that were induced to produce a late replacement brood.

If food supply mediates, at least partially, seasonal variations in chick immunocompetence, non‐food‐supplemented chicks would show a stronger seasonal decline in immunocompetence than food‐supplemented chicks.

Food supplementation improved humoral immunocompetence (the production of immunoglobulins Y), but not T‐cell immunocompetence (phytohaemagglutinin, PHA response). T‐cell immunocompetence of food‐supplemented and non‐food‐supplemented chicks decreased through the season but to a similar extent, whereas the humoral immunocompetence of non‐food‐supplemented chicks decreased more strongly than that of food‐supplemented chicks.

Our results suggest that the seasonal decline in humoral immunocompetence can be explained, at least partly, by variations in food supply throughout the breeding season.