THE FUNCTION OF FLIGHT FORMATIONS IN GREYLAG GEESE ANSER-ANSER - ENERGY-SAVING OR ORIENTATION

Twenty five formations of Greylag Geese Anser anser were photographed from immediately below to eliminate perspective distortion, and the la teral and longitudinal displacements of the birds relative to each oth er were measured, We scaled the photographs and used measurements of b ill to tail and wing span made on 15 freshly shot birds to convert the lateral displacements to wing-tip spacings, The birds flew on average with an overlap of their wing tips (median = 17 cm) which corresponde d very closely with the overlap expected (16 cm) from an aerodynamic m odel which predicted the position which maximized energy savings, Howe ver, the variation in positions was large, and only 17% of birds actua lly flew in the optimum range, The mean saving in induced power averag ed across the distribution of positions was 26.5%, and the contributio n to total flight costs was a reduction of 4.5-9%, This saving was gre ater than we found previously in the Pink-footed Goose Anser brachyrhy nchus. There was no correlation between position in the skein and long itudinal displacement, as would be anticipated if the birds were equal izing the savings across the skein, This does not mean costs were not equalized because other mechanisms are possible, There was no correlat ion between depth and wing tip spacing which does not support the orie ntation/communication hypothesis. Body size and thus flight costs mag be a factor influencing the function of formation nights.