Treecreeper Climbing; Mechanics, Energetics, and Structural Adaptations Author(S): R
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Nordic Society Oikos Treecreeper Climbing; Mechanics, Energetics, and Structural Adaptations Author(s): R. Åke Norberg Source: Ornis Scandinavica, Vol. 17, No. 3 (Jul., 1986), pp. 191-209 Published by: Blackwell Publishing on behalf of Nordic Society Oikos Stable URL: http://www.jstor.org/stable/3676828 Accessed: 17/06/2010 12:41 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=black. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Nordic Society Oikos and Blackwell Publishing are collaborating with JSTOR to digitize, preserve and extend access to Ornis Scandinavica. http://www.jstor.org ORNISSCANDINAVICA 17: 191-209. Copenhagen1986 Treecreeper climbing; mechanics, energetics, and structural adaptations R. Ake Norberg Norberg,R. A. 1986.Treecreeper climbing; mechanics, energetics, and structuralad- aptations.- Ornis Scand. 17: 191-209. The mechanicsand energeticsof tail-supportedclinging and climbingby Certhiafa- miliarison verticaltrunks were analysedfrom still photographsand cine films. A complete movement cycle, or stride, consists of the power stroke and a floating phase. One stride analyzedin detail was 66 mm long and took 0.1375 s. The bird moved abouthalf the stridelength in the powerstroke which took about0.0625 s. In the beginningof a stridethe birdaccelerates with 20 m s-2 (= 2 g), and the feet exert a force on the trunkabout 3.2 times the birdweight Mg. Its maximumvelocity is about 1 m s-' verticallyupwards and 0.24 m s-' horizontallytowards the trunk.The energy cost of one strideis 0.028 J, and 84% of this is for movingvertically upwards, while the rest goes to movinghorizontally (6%), movingthe legs (5.6%), and rotatingthe body and head (4.4%). The metabolicpower due to work done in a strideis 0.2 W, whichis 0.91 the basal metabolicpower BMP. Duringforaging the birdclimbs 0.1 m s-~, and has a stridefrequency of 1.5 s-', and a poweroutput for locomotionof 0.043 W, or 0.19 BMP. The strongclaw curvature,long hind-claw,and gradedlength of fore-toesare adaptationsto climbing.Factors are consideredwhich affect optimal tail length in tail-supportedtrunk-climbing birds. Certhiahas a longer tail in relationto body size than have woodpeckers,a differencerelated to differencesin locomotor habits. R. Ake Norberg,Dept. of Zoology, Univ. of Goteborg,Box 250 59, S-40031 Gote- borg, Sweden. Bock and Miller and 1. Introduction (1942), (1959), Spring (1965), Riger (1972). The mechanics of trunk-climbing with tail Among boreal-forest passerine birds, the Treecreeper tail support, and structural adaptations to this climbing Certhia familiaris exhibits the most extreme special- mode have been treated by Winkler and Bock (1967), ization in foraging locomotion mode. Its behavioural U. M. Norberg (1979), and R. A. Norberg (1981, 1983). and morphological adaptations to trunk-climbing repre- I also compared the energetics of various locomotion sent a remarkable evolutionary convergence with wood- modes of forest birds, including vertical trunk-climbing. peckers Picidae, whereas its mode of food extraction Owing to the Treecreeper's habit of climbing upwards differs from those of most woodpeckers, in that its bill is while searching for food in trees, and then flying down- not adapted for pecking. The Treecreeper, like wood- wards to the next tree, its flight between trees becomes peckers, use the tail for support during climbing, a habit very cheap in energy. Most of the work done against the associated with structural adaptations different from force of gravity during the ascending climb can be re- those involved in trunk-climbing without tail-support, covered during the descending flights as gravitational as practised by nuthatches Sitta spp., Darwin's Wood- potential energy is used for horizontal progression be- pecker Finch (the Galapagos Tool-using Finch) Cama- tween trees. This enables the bird to use glidng or par- rhynchus pallidus, and others. tially powered flight, which is energetically much Structural adaptations for trunk-climbing in birds cheaper than horizontal, fully-powered, flight (R. A. have been treated by Scharnke (1930), Richardson Norberg 1981, 1983). With this mode of locomotion in ? ORNIS SCANDINAVICA 13* ORNIS SCANDINAVICA 17:3 (1986) 191 forest, most work is instead done during the ascending The tracings of this particular stride (Fig. 1) was used climb in trees. And since a Treecreeper may easily climb in all mechanical and energetic analyses. I located the vertically upwards several kilometers in the course of a centre of mass of the Treecreeper by eye on the tracings day's foraging, there must have been strong selection from the film (Figs 3-5). The length scale in the figures for an efficient climbing gait and for structural adapta- was calibrated using length from bill tip to tail tip as tions to it. measured on a dead Treecreeper (120 mm; Appendix I therefore address some questions relating to the 1). I measured it in a straightened posture, but not max- mechanics and energetics of tail-supported climbing, as imally stretched, made to resemble the attitude taken observed in Certhia familiaris: (1) How does the bird near the end of the power stroke, as traced in Fig. 1:5. move to prevent falling away from the trunk when both I did not catch the particular Treecreeper that I feet leave it simultaneously at the end of the power filmed and on which I based the kinematic description. stroke? (2) What are the magnitude and direction of the Instead I use masses and measurements from other forces between bird and bark during stationary clinging birds as set out in text and tables. I dissected one Tree- to vertical as well as sloping trunks, and during acceler- creeper and weighed the body, head, and the upper, ation at the start of a hop? (3) What is the energy cost of middle, and lower leg, and the foot separately. Using climbing and how is it divided between various com- the percentage weights of the various parts I then esti- ponents of movement? (4) What is the efficiency of this mated the weights of these elements of an average-sized climbing gait, i.e. what is the ratio between useful work 9.1 g bird for use in calculations (Tab. 1 and Appendix and total work done in vertical climbing? (5) How do 1). Therefore the mechanical and energetic analyses the claws hold their grip to the bark? (6) What factors presented may not be strictly accurate for the particular determine optimal tail length and distance of the bird's bird and stride shown in Fig. 1. But I regard the results centre of mass from tree trunk in tail-supported tree- as representative for Certhia familiaris in general. climbers? (7) How does tail length in Treecreeper com- Equations used appear in the illustrations. Further pare with those in woodpeckers? procedures and methods are explained where used. The S. I. system of units is used throughout, except where otherwise stated. To facilitate evaluation I also 2. Material and methods express accelerations in terms of g (acceleration due to gravity), The following mechanical and energetic analyses are forces in terms of weight Mg of the bird, and powers in based on cine films and photographs of Certhia famil- terms of BMP (bird's basal metabolic power, or rate). iaris climbing on vertical trunks of spruce Picea abies and pine Pinus sylvestris. All recorded climbing oc- 3. Climbing movement pattern, or gait curred on the lower parts of old tree trunks, where the bark was rough and with good attachment for the Tree- One complete movement cycle, or stride, is shown in creeper. Figs 1 and 2. Stride length is the distance travelled in a I filmed and photographed Treecreepers in the wild complete cycle of leg movements. This one is 66 mm near active nests in coniferous forest. Motion films were long, takes 0.1375 s, and consists of two distinct phases, taken 50 km ENE of Goteborg (Stockasen) in SW Swe- the power stroke (or propulsive or effective stroke) and den in June 1974 and still photographs from a blind at a the floating phase (or recovery stroke), each about nest 40 km E of Goteborg (Almas) in June 1975. I used 0.0625 s in duration. a 16 mm Path6 Reflex cine-camera, run at 80 frames s-l, and with a 135 mm lens, and a 35 mm Leica equipped 3.1. Powerstroke camera with Telyt-R 1:6.8, 400 mm, a 25 mm extension ring, and 1 ms electronic flash lighting. The use of col- The power stroke is the part of the stride during which our film (Kodachrome) greatly facilitated tracing from the Treecreeper moves actively with claws still attached the motion films since the brown bird and green back- to the bark. At start of the power stroke the Tree- ground contrasted much better than different shades of creeper moves obliquely upwards and towards the grey would have done on a black-and-white film.