Immunity, growth and begging behaviour of nestling Barn Swallows Hirundo rustica in relation to hatching order

In altricial birds, asynchronous hatching often results in a size hierarchy among nestlings. However, laying order and hatching order may affect compo nents of offspring condition differentially. Since parasites can have a hea vy impact on their hosts, strategies may have evolved that allow parents an d offspring to allocate critical resources to immunity depending on hatchin g order. In this study of the Barn Swallow Hirundo rustica we (1) analyse t he relationship between laying and hatching order of eggs, (2) compare morp hological, serological and immunological traits between early- and late-hat ched siblings, and (3) compare the begging behaviour of early- and late-hat ched nestlings. Hatching order strictly reflected laying order. Late-hatche d chicks begged more for food but had smaller body mass throughout the nest ling period. Immunoglobulin concentration and intensity of T cell-mediated immunity were larger in late-hatched nestlings. Large concentration of immu noglobulins in late-hatched nestlings may reflect transfer of immunity from the mother via the egg. Alternatively, in association with larger T cell-m ediated immunity, immunoglobulin production may indicate larger investment in immunity by late-hatched nestlings. Laying order and hatching order may therefore affect components of nestling condition differentially.