Growth of Stonechats Saxicola torquata from Africa and Europe: an analysis of genetic and environmental components

Abstract

The postnatal growth of the European subspecies of the Stonechat Saxicola torquata rubicola and the African subspecies Saxicola torquata axillaris and their hybrids was studied to investigate differences in growth rates. Comparisons were made between subspecies, between genetically related chicks within subspecies and between individuals within sibling groups. Genetic and environmental effects on variation and their impact on the growth curves were evaluated. We compared posthatching increase in body mass and wing-length between chicks raised under identical conditions and between chicks raised under different photoperiod and rearing conditions. Significant differences in body mass increase between the two subspecies were found for all parameters except for the inflection point of the growth curve. Differences in wing-length occurred during the late posthatching development and were significant only for asymptotic size. However, interactions between independent variables masked the effects on some parameters. Furthermore, our data showed that the components of the total genetic variability are differently attributed to each level of comparison: we found the form of the growth curve and asymptotic size significantly different between subspecies; size at a given day and asymptotic size were genetically different between sibling groups within subspecies; age at the inflection point of the growth curve showed most variability between individuals within groups of related siblings. Cross-breeding experiments showed that the growth parameters in the hybrids were intermediate, providing further evidence for a genetic basis of the observed differences. In this particular case, the between-subspecies variability reflects genetically adjusted adaptation rather than phenotypic responses to different ecological conditions. The phenotypic responses of growth to differences in “environment” have been investigated in groups of birds reared under different conditions (hand-reared v parent-reared chicks; European v equatorial photoperiod). The rearing method affected only size parameters and not the form of the growth curve of body mass. Significant differences were found for maximum body mass, final body mass and asymptote. Photoperiod had no effect on growth.