Soaring flight in the Eleonora's Falcon (Falco eleonorae)

ABSTRACT.-Eleonora's Falcon (Falco eleonorae) breeds in the Mediterranean region and is highly adapted for catching small birds on passage migration between Eurasia and their African winter quarters, which they feed their young. We studied gliding flight behavior of Eleonora's Falcon at a breeding colony located on a small island southwest of Sardinia, Italy. Gliding and soaring flight performance was measured using an optical range finder and evaluated against flight mechanical theory. The male falcon does the majority of hunting and usually sets off from the colony to hunting areas located at high altitude over the open sea to catch prey. To lower the cost of transport and maximize the energy gain from hunting, we show that the birds use vertical winds for soaring when available. The occurrence of rising air changes with wind direction. At north-north-- westerly winds (on-shore), slope lift is available outside the nesting cliffs, and at south-southeasterly winds thermals that form over the island drift out over the sea. Our observations demonstrated the flexibility of flight behavior in relation to the wind situation, and birds thereby make full use of available soaring conditions.

RESUMEN.-Falco eleonorae anida en la region mediterranea y se encuentra altamente adaptado para capturar aves pequenas con las cuales alimenta a sus crias durante el paso migratorio de las aves entre Eurasia y los sitios de invernada en Africa. Estudiamos el comportamiento de planeo de F. eleonorae en una colonia reproductiva localizada en una pequena isla al sudoeste de Cerdena, Italia. Medimos el desempeno en vuelo de planeo y elevacion utilizando un localizador de rango optico y lo evaluamos con respecto a la teoria de mecanica de vuelo. El halc6n macho realiza la mayoria de la actividad de caza y

generalmente se aleja de la colonia hacia Areas de caceria ubicadas a gran altura sobre mar abierto para capturar sus presas. Mostramos que para minimizar los costos de transporte y maximizar la ganancia energetica de la caza, estas aves se elevan utilizando vientos verticales cada vez que se encuentran disponibles. La presencia de corrientes ascendentes cambia con la direccion del viento. Con vientos del nor-noroeste (en la costa), las corrientes ascendentes se encuentran disponibles fuera de los acantilados de nidificacion, y con vientos del sur-sureste las termales de viento que se forman sobre la isla derivan mar adentro. Nuestras observaciones demuestran la flexibilidad del comportamiento de vuelo con relacion a las condiciones del viento, y que estas aves son capaces de utilizar la totalidad de las condiciones disponibles para elevarse.

Eleonora's Falcon (Falco eleonorae) breeds on islands and islets in the Mediterranean Sea and off the North African coast in the east Atlantic (Walter 1979). The breeding season is finely tuned to coincide with peak autumn bird migration; falcons feed their young with small and medium-sized birds on passage migration between Eurasia and African winter quarters.

To catch migrants, the male falcon (who does the majority of hunting for the young) leaves the nesting cliffs for hunting areas at high altitude far off-shore (Rosen et al. 1999). Falcons at that colony usually fly in a northwesterly direction from the nesting cliffs to the hunting area irrespective of the wind direction (Rosin et al. 1999). Far out over the sea, they intercept migrants that probably left mainland Europe in southern France and northwestern Italy. The female remains in the neighborhood of the nest to feed the young and guard them from predators (Walter 1979).

Because energy demands of the clutch and parents are high, the male falcon has to commute several times per day between the colony and off-shore hunting areas. To lower the energy expenditure during transport flights, falcons should use rising air for soaring when available, because gliding flight is energetically cheaper than powered flight (e.g. Baudinette and Schmidt-Nielsen 1974, Hedenstrom 1993). Soaring should also be favorable for parent birds maximizing the rate of energy delivered to the young (sensu Hedenstrom and Alerstam 1995).

Falcons leaving the island for off-shore hunting (i.e. to the northwest) are mainly experiencing two wind situations: tail winds with the warm Sirocco and headwinds with the cool Mistral. Consequently, vertical winds may be available to the falcons as slope lift with on-shore winds and with thermals drifting out over the sea in off-shore winds. We studied the use of soaring flight in Eleonora's falcon in relation to those situations by quantifying flight behavior in the falcons using an optical range finder.

Methods.-Flight behavior of Eleonora's Falcons was studied at Isola di San Pietro, a small island 6 km off the southwest coast of Sardinia, Italy, between 12 to 20 September 1997 and 22 to 30 August 1999. The breeding colony reaches 7 km along the coastline facing towards the northwest and holds -115 pairs of Eleonora's Falcon (Badami 1997, Rosen et al. 1999).