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 Eigh t performance. Using an optical range finder, we measured flight tracks of Eleonora's falcon (Falco eleonorae), a species breeding in the Mediterranea n region and specialised for hunting autumn passage bird migrants, when com muting between their nesting colony and offshore hunting areas (straight tr ansportation flight) and when searching for prey (transecting and searching flight). Airspeed during searching flight was significantly slower than du ring straight transportation and transecting flight, but there was no signi ficant difference in airspeed between the latter two flight modes. Straight transportation flight was significantly faster than predicted minimum powe r speed. Also, during straight transportation flight, the falcons responded to head- and tailwinds by increasing their airspeed when flying into the w ind. However, they did not shan any significant airspeed adjustments with r espect to the angle between the track and the heading, as would be expected in birds trying to maintain a constant track direction. Mean sustainable c limb rate (during greater than or equal to 240 s) was 1.4+/-0.31 m s(-1) (m ean +/- 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-carryi ng capacity and maximum sustained horizontal flapping flight speed. The mea n wingbeat frequency during powered climbing Eight was 4.68 Hz, which was u sed to estimate the mass-specific muscle work. When falcons were leaving the colony for offshore hunting, they gained alti tude by slope-soaring when there was an onshore wind. We formulated a simpl e criterion for the required gliding-flight rate of climb during an initial slope-soaring episode when minimizing the energy cost of reaching a certai n altitude far out over the sea (which is where the prey is to be found). T his climb rate was 0.36 m s(-1), and our observations indicated that the fa lcons experienced climb rates above this value when soaring in slope-lift.