Spatial and temporal variation in heterophil-to-lymphocyte ratios of nestling passerine birds: comparison of blue tits and great tits.

Environmental factors affecting trophic conditions act as stressors on nestling altricial birds. Access of parental birds to a sufficient supply of food in a limited period of the nestling stage differ in time and space, depending on nesting habitat, prey density and weather conditions. Heterophil-to-lymphocyte ratio (H/L) is considered as a reliable indicator of prolonged stress reaction in birds. In this study we examine if variation in H/L shows consistent spatio-temporal patterns in nestlings of two parids, blue tit Cyanistes caeruleus and great tit Parus major. We found that blue tit nestlings had on average higher H/L than great tit nestlings, which corresponds with the ecological sensitivity of these species. In both species H/L was higher in a poor parkland habitat than in a high quality forest habitat. In nestling blue tits, higher H/L values occurred in years characterized by more extreme weather conditions and worse caterpillar availability. Such consistent patterns of variation in the H/L ratio of nestling blue tits and great tits suggest that, when age-dependent effects are controlled, the ratio can be used as an indicator of physiological stress that is generated by food-related stressors differing in space and time. In particular, elevated H/L ratios are indicative of human-induced changes in the structure of breeding habitats

Comparison of the forest site and the parkland site with respect to inter-annual variation in the peak abundance of tree-canopy caterpillars, as measured by the peak amount of caterpillar frassfall.

<p>Comparison of the forest site and the parkland site with respect to inter-annual variation in the peak abundance of tree-canopy caterpillars, as measured by the peak amount of caterpillar frassfall.</p

Comparison of blue tit and great tit nestlings in relation to inter-site variation in mean H/L ratios.

<p>Sample sizes are shown as the number of nestlings and, in parentheses, as the number of broods.</p

Results of a mixed model ANCOVA for the H/L ratio of nestling blue tits, with wing length as covariate and year and site as factors.

<p>Results of a mixed model ANCOVA for the H/L ratio of nestling blue tits, with wing length as covariate and year and site as factors.</p

Comparison of blue tit and great tit nestlings in relation to inter-annual variation in mean H/L ratios.

<p>Sample sizes are shown as the number of nestlings and, in parentheses, as the number of broods.</p

Results of a mixed model ANOVA for the H/L ratio of nestling great tits, with year and site as factors.

<p>Results of a mixed model ANOVA for the H/L ratio of nestling great tits, with year and site as factors.</p

Descriptive characteristics of May weather in Łódź in the study years.

<p>Descriptive characteristics of May weather in Łódź in the study years.</p