The function of flight formations in Greylag Geese Anser anser; energy saving or orientation?

Abstract

Twenty five formations of Greylag Geese Anser anser were photographed from immediately below to eliminate perspective distortion, and the lateral and longitudinal displacements of the birds relative to each other were measured. We scaled the photographs and used measurements of bill to tail and wing span made on 1 5 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 corresponded very closely with the overlap expected (1 6 cm) from an aerodynamic model which predicted the position which maximized energy savings. However, the variation in positions was large, and only 17% of birds actually flew in the optimum range. The mean saving in induced power averaged across the distribution of positions was 26.5%, and the contribution to total flight costs was a reduction of 4.5-9%. This saving was greater than we found previously in the Pink-footed Goose Anser brachyrhynchus. There was no correlation between position in the skein and longitudinal displacement, as would be anticipated if the birds were equalizing the savings across the skein. This does not mean costs were not equalized because other mechanisms are possible. There was no correlation between depth and wing tip spacing which does not support the orientation/communication hypothesis. Body size and thus flight costs may be a factor influencing the function of formation flights. 2 8 0

First records of lesser black-backed gullsLarus fuscuscrossing the Sahara non-stop

“…All our observed gulls flew under wind support, some of them with tailwind allowing them to double their migratory speed (Table 1). Migrating in formation flight might have saved extra energy, because this decreases the overall costs of migration compared with solitarily migrating birds by about 3–5% (Lissaman and Schollenberger 1970, Cutts and Speakman 1994, Speakman and Banks 1998, Weimerskirch et al 2001).…”

V-like Formations in Flocks of Artificial Birds

“…For relatively large birds, which are the ones that have been observed to group into V-like formations most often, this seems to be the main reason for formation flight [8,6,2,18,21]. The second, competing line of reasoning has been that flying in a somewhat skewed position relative to the bird in front is crucial for an individual's orientation, in addition to allowing unhindered visual communication and therefore helping avoid collisions.…”

Size and Composition of Foraging Flights in Two Species of Piscivorous Colonial Birds: Limited Evidence for Intra- or Interspecific Information Transfer

“…Provisioning adults make more trips [30] and carry more mass so the cost of flight is greater. This is supported by the flight formations observed during inbound flights (personal observations); groups were typically arranged in column, V, or J formations [31], which provide more energy savings than flying alone [27][28][29][30][31]. Energy savings may also explain why incoming group sizes tended to be larger during late chickrearing, when adults would be carrying larger and heavier prey to feed their young.…”

“…Group formation in returning flights may be for navigation or communication, but it is unlikely that this is necessary when returning to the breeding colony. Alternatively, returning birds may fly in groups to benefit from the energy savings of flying in formation [27][28][29][30][31]. Provisioning adults make more trips [30] and carry more mass so the cost of flight is greater.…”