Variation in the hematocrit of a passerine bird across life stages is mainly of environmental origin

The heritability of the hematocrit in adult, breeding pied flycatchers Ficedula hypoleuca is examined in a southern European population across seven years to see the consistency, or lack thereof, of patterns found with the trait at fledgling age where no significant heritability could be detected. While the across-years repeatability of the trait in adult, breeding birds was low but significant, heritabilities based on adult parent-adult offspring regressions controlling for assortative mating and full-sibling comparisons did not differ significantly from zero. Neither were heritability estimates affected by selection on fledgling hematocrit, as it was unrelated to local recruitment. There was no relationship between fledgling and adult hematocrit, suggesting that both behave as phenotypically plastic, different traits. This is the first study testing for the heritability of a physiological trait in birds with data from adult, free-ranging individuals. Field studies on birds have provided information on variation in genetic parameters in natural populations, most dealing with heritability of morphological or life history traits (Merila ̈ and Sheldon 2001, Artacho et al. 2005). Few studies have addressed genetic variation in physiological traits in adult individuals (Boulinier et al.