Waders flying with folded legs – which species, when, where and why?

Waders normally fly with their legs stretched out under the tail, with their feet pointing backwards. Whereas there are a few published records of waders flying with their legs folded forward (like a passerine), with legs and feet tucked in under the belly feathers, the causes and consequences of waders flying with folded legs are poorly understood. Through searching the literature, asking ornithologists in person and through public calls, and from searching photos on the internet, we retrieved 90 records of 17 different species where one or more individuals flew with folded legs. It was most common in Spotted Redshank Tringa erythropus (in up to 10% of all birds) and Common Redshank T. totanus, but was also observed in short-legged species such as Dunlin Calidris alpina and Common Ringed Plover Charadrius hiaticula. The behaviour was recorded in most months of the year, in different age classes, in single birds as well as in flocks, at different ambient temperatures, and in both migratory flights and local movements. Whereas reducing heat loss at low temperatures is a likely, but not omnipotent, explanation for waders flying with folded legs, aerodynamics and leg muscle fatigue may also affect how waders keep their legs in flight.