Examining body mass and physiological parameters in breeding adults may provide valuable insight into parental efforts provided by males and females at a given stage of breeding. Here, we examined body mass and physiological variables (hemoglobin and cholesterol concentrations, leukocyte profile) in males and females in a small pelagic seabird, the European Storm Petrel (Hydrobates p. pelagicus) during the incubation period. We expected females to be in poorer physiological condition compared to males because of their assumed higher investment (production of costly egg, with all other parental activities performed being similar). Contrary to our expectation, we did not find sex differences in the hematological values, indicating similar non-resource–based costs of reproduction. Also, we found females in good nutritional state, being heavier and having higher cholesterol concentration than males. Although we are not able to explicitly identify mechanisms responsible for the pattern observed, we suggest that common assumptions about female-biased efforts during the pre-laying and/or equal male and female parental efforts during incubation periods in the European Storm Petrel require verification.

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