Natal Dispersal

In many bird species, when the ring recoveries of young birds in a subsequent breeding season are plotted in relation to the hatch site, the numbers of recoveries tend to be greatest in the vicinity of the hatch site, and decline progressively with increasing distance. Typically, the recoveries come from all sectors of the compass, indicating no directional preference at the population level. Such patterns are shown for several species in Figures 17.1 and 17.2; they reflect the settling patterns of individuals with respect to natal site, regardless of their movements in the interim. In each species, the density of recoveries declines in approximately exponential manner in concentric circles out from the natal site. Such dartboard settling patterns do not result merely from an inability of some migrants to re-find their home areas for they also occur in sedentary species. Factors such as landscape structure, habitat and nest-site availability, and prior occupancy by other individuals, can all influence how close to its natal site a bird is likely to settle.

While this type of settlement pattern has been found in almost all species of birds that have been studied, whether resident or migrant, some species disperse over much longer distances than others (e.g. Newton 1979, Paradis et al. 1998, Wernham et al. 2002). In general, larger species tend to breed further from their natal sites than do small ones, as might be expected, but this relationship is rather loose because of the other factors that influence dispersal distances.

Figure 17.1 Locations of Sparrowhawks Accipiter nisus ringed as chicks and recovered in a later breeding season, shown in relation to natal site (centre). Recoveries came from all sectors of the compass and declined in density with increasing distance from the hatching site. From Newton (1979).

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