A Peregrine Falcon Polymorph: Observations of the Reproductive Behavior of Falco kreyenborgi

Abstract

-Observations of the reproductive behavior and productivity of the South American Pallid Falcon, historically known as Kleinschmidt's Falcon (Falco kreyenborgi), confirm and amplify findings by Ellis and Peres G. (1983), that F. kreyenborgi may be described in terms of a dichromatic polymorph of F. peregrinus cassini, the South American subspecies of Peregrine Falcon. The pallid form occurs relatively frequently in the eastern zone of the southern tip of South America, where it is locally sympatric with F. p. cassini. This study describes mating behavior of 17 pairs and productivity of 15 pairs of falcons in this region of southern Patagonia. In five of these pairs, both the adults were phenotypically F. peregrinus; in two pairs, both were pallid; and in ten pairs, the adults were mixed: a normal adult peregrine mated with a pallid individual. Phenotypic expression of the pallid trait from productivity data indicates that the genetic basis for pallidism is recessive to the normal genotype. My analysis of gene frequencies, based on this assumption, demonstrates a high frequency of heterozygosity among the local breeding population, which is characteristic of a clinal variation maintained by either low gene flow or selection for the pallid genotype. The Pallid Falcon (Falco kreyenborgi) has been described as the rarest and the least known of the large falcons (Johnson 1972). Until recently, information about the Pallid Falcon had been limited to only a few recorded sightings and five collected specimens, three of which are of uncertain geographic origin (Hickey 1969). An historical review of the bird was given by Ellis et al. (1981). Owing mostly to its unusual coloration, kreyenborgi's proper taxonomic placement has been the cause of considerable speculation (Kleinschmidt 1929, Peters 1931, Kovacs 1962-1963). Stresemann and Amadon (1963) showed that the Pallid Falcon's general morphological characteristics indicated a close association with F. peregrinus cassini, the South American subspecies of Peregrine Falcon. They concluded kreyenborgi might be a member of the Peregrine group. I conducted a study in order to investigate the distribution and breeding behavior of F. kreyenborgi with the intention of obtaining information necessary to determine its taxonomic placement in relation to F. p. cassini. STUDY AREA AND METHODS Between October 1980 and April 1982 I spent 16 months in southern Chile and Argentina (52-54OS, 68-73W). During the 1980-1981 breeding season, I surveyed (primarily on foot and from automobile) large areas of the eastern foothills of the southern Andes in what Kuschel (1960) termed the Magellanic moorland and the lower Magellanic forest. In the austral fall (March-April) of 1981, with the cooperation of the Chilean Servicio Agricola y Ganadera, I established falcon trapping sites. During the 1981-1982 breeding season, I surveyed the eastern and relatively xeric Patagonian Steppe, including approximately 380 km of coastline. The data presented herein pertain exclusively to observations made during the 1981-1982 nesting season.