Flight performance during hunting excursions in Eleonora’s falcon Falco eleonorae

Among birds, falcons are high-performance flyers, in many cases adapted for aerial hunting and hence suitable targets for investigating limits to flight performance. Using an optical range finder, we measured flight tracks of Eleonora’s falcon (Falco eleonorae), a species breeding in the Mediterranean region and specialised for hunting autumn passage bird migrants, when commuting between their nesting colony and offshore hunting areas (straight transportation flight) and when searching for prey (transecting and searching flight). Airspeed during searching flight was significantly slower than during straight transportation and transecting flight, but there was no significant difference in airspeed between the latter two flight modes. Straight transportation flight was significantly faster than predicted minimum power speed. Also, during straight transportation flight, the falcons responded to head- and tailwinds by increasing their airspeed when flying into the wind. However, they did not show any significant airspeed adjustments with respect to the angle between the track and the heading, as would be expected in birds trying to maintain a constant track direction. Mean sustainable climb rate (during ≥:240 s) was 1.4±0.31 m s⁻¹ (mean ± S.D., N=13), which is rather a high rate for a bird the size of an Eleonora’s falcon. The climb rate was used to calculate maximum load-carrying capacity and maximum sustained horizontal flapping flight speed. The mean wingbeat frequency during powered climbing flight was 4.68 Hz, which was used to estimate the mass-specific muscle work.

When falcons were leaving the colony for offshore hunting, they gained altitude by slope-soaring when there was an onshore wind. We formulated a simple criterion for the required gliding-flight rate of climb during an initial slope-soaring episode when minimizing the energy cost of reaching a certain altitude far out over the sea (which is where the prey is to be found). This climb rate was 0.36 m s⁻¹, and our observations indicated that the falcons experienced climb rates above this value when soaring in slope-lift.