Morphology of the wings, legs and tail of three coniferous forest tits, the goldcrest, and the treecreeper in relation to locomotor pattern and feeding station selection

Abstract

Functional morphology of the wings, legs, and tail of Parus ater, P. montanus, P. cristatus, Regulus regulus, and Certhia familiaris is analysed and compared, and correlations are sought with locomotion pattern and feeding station selection. These species are treated together because they are sympatric, occur in the same habitat, and partly overlap in feeding station selection, as well as in food selection. Where possible, the selective advantage of a feature is judged in terms of the energy savings that it makes possible with the respective locomotion pattern. P. ater has relatively low weight, low wing loading and long wing span. It is adapted to slow flight and high manoeuvrability. P. montanus has high wing loading, relatively short wings and long tail. It is not well adapted to manoeuvrable flight, but more to a clinging and climbing behaviour. P. cristatus has the highest wing loading of the species. It has short and broad wings, and rather short tail in relation to the body size. It is not adapted to slow manoeuvrable flight. It hops about on the branches or ground to a greater extent than the other species. Further, by its mere size, it is more adapted to low temperatures than the other species. R. regulus and C. familiaris have the lowest wing loadings and shortest arm wings in relation to the total length of the wings. C. familiaris also has relatively long span. The longer the span the lower the induced power, which forms a big part of the power consumption in hovering. The induced power per unit body mass is lowest in C. familiaris (1.13 W kg-1), and lower in R. regulus (1.19 W kg-1) and P. ater (1.21 W kg-1) than in P. montanus (1.28 W kg-1) and P. cristatus (1.31 W kg-1). The ratio (length of hand wing) / (length of arm wing) is 3.0 and 2.9 in C. familiaris and R. regulus, respectively, and 2.4- 2.6 in the other species. The shorter the arm wing is in relation to the total length of the wing, the more proximally the main mass of the wing will be located, and the less the inertial power and inertial loads on the wing skeleton become. The inertial power is another power drain in hovering. Therefore, because of their low wing loading and short arm wings, R. regulus and C. familiaris are particularly well adapted to slow flight and to hovering. P. ater and R. regulus are partly migratory and, therefore, should benefit more by long wing span than the other species. In fact, P. ater has relatively long span while R. regulus has not. The relatively short span in R. regulus is probably an adaptation for manoeuvrability and practicability in the dense vegetation where it usually forages. As related to body size, C.familiaris has the shortest legs, longest tail and toes, and longest and most curved claws, features that are obvious adaptations to climbing locomotion. The reduction of the leg length in the course of adaptations for climbing has affected the tibiotarsus most and the femur least. It is especially important for the tibiotarsus to be short to minimize the muscle force needed for clinging on a vertical trunk, and also to shorten the legs with least loss of step length. The tail is used as support in climbing (although not during the latter part of the power stroke). The longer the tail is, the less the horizontal force between claw and bark becomes, and, hence, the less the energy expenditure during the power stroke in climbing. The three tits have almost the same relative leg length. R. regulus has the longest legs in relation to body size. P. ater, P. montanus and C.familiaris have relatively short tarsometatarsus. Further, P. ater and R. montanus have long muscle lever arm of the flexor of the tarsometatarsus. Both characters are adaptations for hanging under branches and/or for climbing. Birds with need of rapid leg movements should have a short lever arm for the flexor muscle of the tarsometatarsus. During foraging R. regulus, P. cristatus and C.familiaris use their legs more for hopping, which requires speed of leg movements, and less for hanging than do P. ater and P. montanus, and they also have shorter muscle lever arms than the latter two species. In C.familiaris the short tarsometatarsus thus is adapted for hanging whereas the short lever arm of the muscle force is not. When the bird is hanging under a perch with 45° flexion of the tarsometatarsus relative to the tibiotarsus, then the muscle force (of M. tibialis anticus) per unit body mass is about 36, 44, 45 and 54 % larger in R. regulus than in P. cristatus, P. ater, C.familiaris and P. montanus, respectively. The corresponding differences between P. cristatus and the three latter species are 6, 7 and 14%. A clustering process was used to illustrate more clearly the phenetic resemblances among the species regarding the morphology of the locomotor apparatuses. As regards the wing skeleton the tits form a group, and C.familiaris is more similar to the tits than to R. regulus. The wing-form phenogram shows that P. montanus and P. cristatus resemble each other most, that P. ater is intermediary between these tits and R. regulus, and that C. familrsis most similar to R. regulus. The phenogram based on the form of the leg and foot shows that the tits resemble each other most and that R. regulus and C. familiaris are rather unlike the tits and also very unlike each other. In the tits, the skeleton of the legs is more diverse than that of the wings, and seems to have been subjected to more divergent selection pressures than the wing skeleton. As regards aerial locomotion in the three tits, the adaptation to different niches, and hence to different flight patterns, have led to divergent evolution of the wing feathers rather than of the wing skeleton. The pattern for all five species together is most diverse in leg characters and wing form and least diverse in the wing skeleton.