Soaring and intermittent flap-gliding during migratory flights of Northern Bald Ibis

Abstract

Migratory journeys represent an energetic challenge for many long-distance migrants. The choice of flight times, migration route, altitude, travelling speed, wingbeat patterns, soaring as well as formation flight can all affect the energy expenditure for the journey. We monitored the flight patterns of two Northern Bald Ibises (Geronticus eremita) equipped with data loggers while crossing the Alps during a human-led migration from southern Germany to Tuscany. We observed that the birds used an intermittent flapping pattern, where phases of active flapping flight were regularly interrupted by short gliding phases. As a result of intermittent flight, the effective wing beat frequency was 13–20% lower than the observed wing beat frequency of 4.0 s−1. When local conditions allowed, the birds gained altitude through circling in thermal updrafts. During those circling bouts, gliding on the outstretched wing was predominant, though active wing flapping was still observed. Overall, the two birds spent 19 and 22% of the time soaring on the outstretched wing, accruing during that time 26 and 28% of the altitude gain required for the crossing of a major mountain range. This shows that, apart from formation flight, northern bald ibis use at least two more strategies—thermal soaring and intermittent flap-gliding—for improving energy economy during migratory flights.