Social organization and home ranges of the hybrid partridge (Alectoris graeca saxatilis x Alectoris rufa rufa): seasonal and individual variations. Gibier Faune Sauvage, Vol. 8, mars: 1-30.
Seasonal and individual changes in the social organization and home range sizes of natural hybrids between Rock partridge ( Alectoris graeca saxatilis) and Red-legged partridge (Alectoris rufa rufa) were studied in the Southern French Alps by following 40 birds equipped with radio tags for periods varying between two months and two years, between September 1984 and May 1989. The partridges' social behaviour changes throughout the annual cycle. They are predominantly living in groups from late summer, soon after breeding, to early spring when groups will disperse. Between September and March, mean group size is 4.5 individuals. The partridges' tendency to gregariousness is apparently at its highest in winter when 90% of them are living in groups. From September to December, females are more gregarious than males. Groups may be composed of juveniles and adults, and of birds of both sexes. Family groups form fairly stable units contrary to groups made up of nonincubating or unsucessful nesting birds.
Partridges have a tendency to separate themselves from their wintering group prior to pairing. Breeding pairs maintain a fairly strict spacing pattern during spring (egg-laying period), but tolerate the presence of lone males within their territory. About 20 % of the birds are unpaired in spring: these are erratic subadult males, widowed males or males which became single after a divorce. In early summer the proportion of solitary birds reaches a peak (53 %) because of clutch incubation. During this period broods appear, but also groups consisting of birds that lost their nests or of nonnesting birds.
Seasonal home range areas vary from 2 to 190 ha and home range lengths (determined by the longest movement covered during one season) vary between 0.3 and 25 km according to the individuals. Home range size varies from season to season. Home range area thus decreases between early spring and spring. Then it increases in the late summer period to diminish markedly in winter; home range length also diminishes between early spring and spring then increases again in sum-mer. These seasonal variations are apparently induced both by the breeding cycle and by the food needs in relation to changes in the weather.
Further to these general trends, there is a marked difference in home range size between individual birds. It seems that from April to August this variability is neither sex- nor age related, but merely linked to the partridge's " breeding-life history" (whether the bird is paired, even for a short time, or not; bird having experienced nesting success, or not). From September till March, the differences in spatial behaviour between individual birds are seemingly not directly linked to sex, but to ethological factors, and/or to ecological factors like snow depth and duration of snow cover. Thus, some birds leave their breeding site without the occurrence of any apparent change in weather and settle at several kilometres from there to overwinter for 5-6 months. In March they move back to their former breeding range (a migration-like behaviour). Other birds leave their breeding sites only after heavy snow fall to reach areas that will rapidly be snow-free : some birds will stay there until March, but others move back to their breeding sites as soon as the snow has melted. Others still are wintering close to their breeding site.
Breeding birds use annual home ranges covering on average an area of 221 ha and measuring 6.7 km in length. Home range annual areas of individual birds vary little (from 194 to 338 ha). In contrast, there is a 1- to 12-fold difference in home range lengths according to the individual. Because of the long distances moved by the birds, all favour-able habitats situated within a radius of at least 6-7 km from the breeding area should be managed, if the goal is to maintain a nucleus of breeders.