Reduced take-off ability in robins (Erithacus rubecula) due to migratory fuel load

Recent studies have shown that large fuel loads in small birds impair flying ability. This is the first study to show how migratory fuel load affects flying ability, such as velocity and height gained at take-off in a predator escape situation, in a medium-distance migrant, and whether they adjust their take-off according to predator attack angle. First-year robins (Erithacus rubecula) were subjected to simulated attacks from a model merlin (Falco columbarius), and take-off velocity and angle were analysed. Robins with a wing load of 0.19 g cm⁻² took off at a 39% lower angle than robins with a wing load of 0.13 g cm⁻², while velocity remained unaffected. The robins did not adjust their angle of ascent in accordance with the predator's angle of attack. Since many predators rely on surprise attacks, a difference in flight ability due to varying fuel loads found in migrating robins can be important for birds' chances of survival when actually attacked. Received: 28 October 1998 / Received in revised form: 12 January 1999 / Accepted: 30 January 1999     

Zebra finches,<Emphasis Type="Italic"> Taeniopygia guttata</Emphasis>, with elongated tails delay taking flight during group take-offs

Zebra finches with tail-elongations left perches during induced take-offs consistently later than unmanipulated controls. Latency in take-off was the same for both a short pintail-elongation (27% of finch long-axis) and an extreme pintail-elongation (49% of finch long-axis). There was no evidence for artificial pintails increasing energy expenditure, as predicted by flight aerodynamics models. Neither flight metabolic rate determined using the doubly labelled water technique, nor resting metabolic rate or food intake rates were affected. Hence, the energetic costs of elaborate pintails seem likely to be small and perhaps biologically trivial. In contrast, pintails appear to confer a manoeuvrability cost, as found in previous studies of long tails. Latency in take-off may be induced by an increased risk of collision with conspecifics during group take-offs and landings. Alternatively, latency may represent a change of predator-avoidance strategy, because taking off within the group does not minimise predation risk when handicapped by tail-elongation. The effects of tail-elongation are likely to be context-dependent and may differ between solitary species and more gregarious species that fly or forage in flocks.Communicated by W. Wiltschko