Heterophil/lymphocyte ratios and heat-shock protein levels are related to growth in nestling birds

Growing altricial birds may experience nutritional stress in the nest due to sibling competition, food restriction, or parasites. Nutritional stress may be detected through its effect on nestling growth, although genetic and maternal effects may interfere with its expression. A more direct way of estimating nutritional stress may be through measurements of organismic stress in growing birds. Heterophillymphocyte ratios (H/L) have been proposed as reliable physiological indicators of the stress response. Heat-shock protein (HSP) synthesis is induced by animals in response to various stressors, making the concentration of HSPs a suitable measure of organismic stress. In a study of growing pied flycatchers (Ficedula hypoleuca), we assessed H/L from cell counts in blood smears and levels of two HSPs, HSP60 and HSP70, from the cell fraction of peripheral blood. H/L and the level of HSP60 were significantly positively associated in nestlings of 13 days of age, while the level of HSP70 was not related to the other two measures. Small nestlings with respect to tarsus length on day 7 had higher levels of HSP60 and higher H/L values. Brood means of nestling mass, tarsus length, and wing length shortly before fledging were negatively related to both mean HSP60 level and mean H/L ratio. Brood variances in body mass and wing length on day 13 were positively associated with mean HSP60 levels but not with mean H/L values. Up to 80% of variation in some traits was explained by stress indicators.