GROWTH OF STONECHATS SAXICOLA-TORQUATA FROM AFRICA AND EUROPE - AN ANALYSIS OF GENETIC AND ENVIRONMENTAL COMPONENTS

The postnatal growth of the European subspecies of the Stonechat Saxic ola torquata rubicola and the African subspecies Saxicola torquata axi llaris and their hybrids was studied to investigate differences in gro wth rates. Comparisons were made between subspecies, between genetical ly related chicks within subspecies and between individuals within sib ling groups, Genetic and environmental effects on variation and their impact on the growth curves were evaluated, We compared posthatching i ncrease in body mass and wing-length between chicks raised under ident ical 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 th e inflection point of the growth curve, Differences in wing-length occ urred during the late posthatching development and were significant on ly for asymptotic size. However, interactions between independent vari ables masked the effects on some parameters. Furthermore, our data sho wed that the components of the total genetic variability are different ly attributed to each level of comparison: we found the form of the gr owth curve and asymptotic size significantly different between subspec ies; size at a given day and asymptotic size were genetically differen t between sibling groups within subspecies; age at the inflection poin t of the growth curve showed most variability between individuals with in groups of related siblings. Cross-breeding experiments showed that the growth parameters in the hybrids were intermediate, providing furt her evidence for a genetic basis of the observed differences. In this particular case, the between-subspecies variability reflects genetical ly adjusted adaptation rather than phenotypic responses to different e cological conditions. The phenotypic responses of growth to difference s in ''environment'' have been investigated in groups of birds reared under different conditions (hand-reared nu parent-reared chicks; Europ ean nu equatorial photoperiod). The rearing method affected only size parameters and not the form of the growth curve of body mass, Signific ant differences were found for maximum body mass, final body mass and asymptote. Photoperiod had no effect on growth.