During reproduction, animals must balance investments into current reproduction with investments into self-maintenance and future reproduction. This trade-off can be examined by evaluating short-term physiological costs incurred during a reproductive event, such as changes in body condition, or increases in stress hormones, metabolism, and oxidative stress. Alternatively, changes in blood cell counts such as the heterophil-to-lymphocyte (H: L) ratio that reflect both physiological stress and immune activation, may prove to be an important marker of parental effort. In this study, we tested whether the H: L ratio could be a potential marker of parental effort in two seabird species: the king (Aptenodytes patagonicus) and macaroni (Eudyptes chrysolophus) penguin. As central place foragers, penguins’ reproductive performances are markedly affected by their foraging performances. Here, we investigated whether penguins’ H: L ratio was a good marker of parental foraging performance (effort and success), breeding effort (chick growth), and breeding success (chick survival). We did not find any significant association between metrics of foraging performance and parental H: L ratios. However, in king penguin, parental H: L ratio increased from incubation to chick-brooding, suggesting a higher cost to raising chicks. Females of both species showed a slight, but non-significant, increase in their H: L ratio after one foraging trip. Taken together, our results suggest that the H: L ratio may better reflect longer-term changes in parental effort between breeding stages rather than short-term variations at the scale of a foraging trip.