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 ofParus ater, P. montanus, P. cristatus, Regulus regulus, andCerthia familiarisis 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. aterhas relatively low weight, low wing loading and long wing span. It is adapted to slow flight and high manoeuvrability.P. montanushas 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. cristatushas 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. regulusandC. familiarishave the lowest wing loadings and shortest arm wings in relation to the total length of the wings.C. familiarisalso 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 inC. familiaris(1.13 W kg^-1), and lower inR. regulus(1.19 W kg^-1) andP. ater(1.21 W kg^-1) than inP. montanus(1.28 W^kg-1) andP. cristatus(1.31 W kg^-1). The ratio (length of hand wing) / (length of arm wing) is 3.0 and 2.9 inC. familiarisandR. 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. regulusandC. familiarisare particularly well adapted to slow flight and to hovering.P. aterandR. regulusare partly migratory and, therefore, should benefit more by long wing span than the other species. In fact,P. aterhas relatively long span whileR. regulushas not. The relatively short span inR. regulusis probably an adaptation for manoeuvrability and practicability in the dense vegetation where it usually forages. As related to body size,C.familiarishas 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. regulushas the longest legs in relation to body size.P. ater, P. montanusandC.familiarishave relatively short tarsometatarsus. Further,P. aterandR. montanushave 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 foragingR. regulus, P. cristatusandC.familiarisuse their legs more for hopping, which requires speed of leg movements, and less for hanging than doP. aterandP. montanus, and they also have shorter muscle lever arms than the latter two species. InC.familiaristhe 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 inR. regulusthan inP. cristatus, P. ater, C.familiarisandP. 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, andC.familiarisis more similar to the tits than toR. regulus. The wing-form phenogram shows thatP. montanusandP. cristatusresemble each other most, thatP. ateris intermediary between these tits andR. regulus, and thatC. familrsismost similar toR. regulus. The phenogram based on the form of the leg and foot shows that the tits resemble each other most and thatR. regulusandC. familiarisare 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...