Confronting the winds: orientation and flight behaviour of roosting swifts, Apus apus

Swifts, Apus apus, spend the night aloft and this offers an opportunity to test the degree of adaptability of bird orientation and flight to different ecological situations. We predicted the swifts' behaviour by assuming that they are adapted to minimize energy expenditure during the nocturnal fligh t and during a compensatory homing flight if they become displaced by wind. We tested the predictions by recording the swifts' altitudes, speeds and d irections under different wind conditions with tracking radar; we found an agreement between predictions and observations for orientation behaviour, b ut not for altitude and speed regulation. The swifts orientated consistentl y into the head wind, with angular concentration increasing with increasing wind speed. However, contrary to our predictions, they did not select alti tudes with slow or moderate winds, nor did they increase their airspeed dis tinctly when flying into strong head winds. A possible explanation is that their head-wind orientation is sufficient to keep nocturnal displacement fr om their home area within tolerable limits, leaving flight altitude to be d etermined by other factors (correlated with temperature), and airspeed to s how only a marginal increase in strong winds. The swifts were often moving 'backwards: heading straight into the wind but being overpowered by wind sp eeds exceeding their airspeed. The regular occurrence of such flights is pr obably uniquely associated with the swifts' remarkable habit of roosting on the wing.