Evolution: Hawks learn on the fly to swoop up before perching

Swooping upwards before landing on a perch helps larger birds to avoid any hazardous loss of control when perching, according to a study of four hawks published in Nature. This braking behaviour is learned through experience.

Perching at speed is one of the most demanding flight behaviours for birds, and is one that has eluded most autonomous vehicles. Smaller birds may touch down by hovering, but larger birds typically swoop up to perch—probably because their size prohibits hovering and because swooping upwards transfers kinetic energy to potential energy before a collision.

Graham Taylor, Marco Klein Heerenbrink, Lydia France and colleagues observed the swooping trajectories of four Harris’s hawks (Parabuteo unicinctus) to investigate this behaviour. Three of the birds were inexperienced juvenile males that had flown only short distances previously and the other was an experienced adult female. The authors collected trajectory data from 1,585 flights with perches set 5, 7, 9 or 12 metres apart. The young birds flew directly between the perches by flapping their wings for the first few flights. However, they soon adopted the indirect swooping behaviour of the experienced bird. They swooped by jumping forwards into a dive with several powerful wingbeats, which transitioned into an unpowered climb followed by an upward manoeuvre that ended with the body almost vertical and wings outstretched as the feet connected with the perch.

After analysing the data, the authors conclude that, although swooping behaviour does not necessarily minimize energy expenditure, it helps birds to reduce the distance they fly under hazardous conditions after effectively stalling in the air when perching and may aid visual fixation on the landing.