Body Size Differentiations In Male Common Stonechat (Saxicola Torquata Axillaris) Sub-Populations In Different Open Habitats In Kenya.

Abstract

Studies on how body size and shape of animals are influenced by selective pressures such as habitats, mates, foraging and migration, are of key importance in understanding the ultimate causes promoting biometric and morphological diversification. In ornithology, body size is a fundamental measure used to understand many ecological and behavioral issues including foraging, sexual selection, dominance relationships between and within species, and evolutionary processes. The goal of this study was to assess body size differences of sedentary Common Stonechat (Saxicola torquata axillaris) populations that occupied different habitats in Kenya. The final aim was to identify the best linear predictor of body size for this species. Specific objectives included determining if body size differences of subpopulations of the species could be recorded in different habitats occupied the species, and which linear predictor can be used to predict the species’ body size. Studies were carried out in four habitats: forest glades and edges of a tropical rainforest (Isecheno, Kakamega); farmlands interspersed with fallow lands (Kinangop and Ol Kalou); pure croplands without fallow lands (Timau in Buuri and Mataara in Aberdares); and the forest edges of a coastal montane forest (Taita Hills). One hundred and thirty nine male Common Stonechats were captured using decoy traps in their territories. Their early morning songs (dawn choruses were recorded as well). Each individual had four biometrics taken which included body mass, wing, head and tarsus lengths. A strong correlation is assumed between body mass and some linear dimension, such as wing or tarsus length, with the presupposition that the linear dimension reflects the size component of mass. Body mass was used as a measure of size since it has vital importance with respect to forte of supporting structures (skeletons), the demands on the muscular system in locomotion and, the need for food. One-way analyses of variance showed significant differences of Common Stonechat biometrics between the habitats (wing length,F(3,135) = 105.69, p < 0.05, head length, F(3,135) = 4.87, p < 0.05, tarsi length, F(3,135) = 5.01 p < 0.05, and body mass, F(3,135) = 4.74, p < 0.05). Such biometric differences could be related to habitat adaptations and, ultimately, to survival. Linear regressions of the wing, head and tarsus with body mass showed the head length to be the best linear predictor for body size. The largest birds were found in the farmlands interspersed with fallow land in Kinangop and Ol Kalou (16.28±0.02grams) and the smallest birds were found in the forest edges of the Taita Hills montane forests (16.13±0.02grams) and in the forest glades and forest edges of the tropical rainforest in Kakamega (16.14±0.02grams). This study found that the choice of linear measurement that best expresses body size in different passerine species can differ from the conventional wing or tarsus lengths depending on the species in question. These results also suggest that habitat type has an influence on the body size of the Common Stonechat. (Competition for territories and predation could be some of the ecological factors leading to differences in body size). Further research on microhabitat features are now needed to understand the morphological differences recorded here.