Males in domestic and laboratory populations usually display a lower immuno
competence than females due to differential action of steroid hormones, but
information for natural populations is scant. Here we show that male Magel
lanic penguins Spheniscus magellanicus had a significantly lower cell-media
ted immunity at hatching of their chicks than females. This was assayed thr
ough the phytohemagglutinin injection assay, a standard method of measuring
T-cell-dependent immunocompetence in birds. Breeding phenology affected co
ndition at laying and cell-mediated immunity at hatching for males, with hi
gher values for late-breeding males. In females, condition at laying declin
ed with breeding date, with no link with immunocompetence at hatching. Earl
y breeding males may incur date-dependent costs in terms of condition and s
ubsequent immunocompetence.