Haematological indices have been frequently used to investigate the physiological condition of wild birds. In this respect, among many blood parameters, haematocrit has been considered to be a valuable indicator. Many studies, mostly conducted on adult birds, have discussed possible causes of haematocrit variation, pointing out several factors both natural and anthropogenic as being crucial. In this study, we examined the level of haematocrit for Blue Tit Cyanistes caeruleus nestlings in relation to habitat type and inter-year sources of variation. We found that the haematocrit of Blue Tit nestlings varied significantly between study years, showing an increasing trend. We explain this variation by changeable weather conditions, pointing out that temperature is a possible factor. The increase in haematocrit in subsequent study years was accompanied by a decline in the ambient temperature. No significant effect of habitat type was noted in the haematocrit variation. We also found that haematocrit values tended to be positively associated with fledging success. All this confirms that haematocrit may be reasonably used as a one of the indicators of nestling condition.
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