Environmental and genetic variation in the haematocrit of fledgling pied flycatchers Ficedula hypoleuca

We report a field study of the haematocrit of pied flycatcher (Ficedula hyp oleuca) nestlings when close to fledging. First a descriptive study was con ducted of both fledgling and adult haematocrit over 2 years to analyse corr elates of variation ill this trait. Then a swapping experiment was performe d to see whether variation among fledglings had a measurable genetic compon ent. Average fledgling haematocrits were lower than those of their male and female parents. Intraclass correlations among sibships in fledgling haemat ocrit were high in both years, indicating that the estimates of resemblance were inflated, probably by common environmental effects. Fledgling haemato crits were unrelated to date and number of young in the nest. Fledglings wi th a high haematocrit were heavy and had thick breast muscles. There were n o significant relationships between the average fledgling haematocrit and t hose of the adults caring for them. Nest mite ectoparasites negatively affe cted fledgling haematocrit. The haematocrits of adults did not differ betwe en sexes or years and in both sexes were unrelated to breeding date, body m ass, age, clutch size or number of young reared. Females, but not males, ca ring for fledglings in nests infested by mites had a lower haematocrit than those rearing young in mite-free nests. The cross-fostering experiment ind icated that almost all measured variation in haematocrit was explained by t he nest where the bird was reared (67.2% of the explained variance), not by their nest of origin (7.8%), meaning that there was a very small, non-sign ificant resemblance in the haematocrit of genetically related sibs when rea red in different environments while unrelated nestlings reared in the same nestbox had similar haematocrits. The low proportion of variance explained by the familial component may be due to the high connection of haematocrit to fitness.