Site Attachment

Attachment of young birds to natal sites

The young of migratory birds must presumably learn the location of their natal area before they leave it, for only then could they return there after a long migration. This assumption has been confirmed experimentally in various species. When eggs or young were transferred from one area to another, tens or hundreds of kilometres away, the resulting adults were usually found breeding near their foster home rather than near their original home. The age of the birds at transference emerged as crucial, because attachment to a locality occurred at a specific stage of development: in passerines in the post-fledging period, between leaving the nest and leaving the area (Lohrl 1959, Berndt & Winkel 1979, Sokolov et al. 1994).

In one experiment, young Collared Flycatchers Ficedula albicollis were hand-reared at one site, and released at different ages at another, situated about 90 km to the south and previously unoccupied by this species (Lohrl 1959). Individuals released before or early in post-juvenile moult returned next spring to the release site, whereas those released late in moult or after moult did not return there. A short period of freedom during the first fortnight or so after leaving the nest appeared sufficient to fix the locality on these birds. Further research showed that fixation to a potential breeding locality occurred between 45 and 55 days of age in both Collared Flycatchers and European Pied Flycatchers Ficedula hypoleuca (Berndt & Winkel 1979). The young of these species are raised in cavities, so would have no opportunity to view the outside world until they left the nest.

In another experiment, young Chaffinches Fringilla coelebs released in a new area at less than 30 days old returned to the area in a subsequent year, while those older than 40 days returned after migration mainly to their natal place (Sokolov 1997). Evidently, site imprinting in Chaffinches occurred between ages 30 and 40 days, again mainly in the post-fledging period, before the young dispersed. However, if Chaffinches were hand-reared without sight of the outside world and then released at 50 days of age, some still returned to the release site in a later year. Hence, if birds were denied the chance to learn the site at the usual age, they could do so later. If they learnt at the usual age, subsequent experience did not alter the preference established then. Age at site attachment for various other small passerines lay in the range 30-55 days, depending on species and rate of development (Sokolov 2000b).

Attachment to a locality does not mean that we can expect all surviving young to breed there subsequently. They may return there initially in spring but, on finding breeding sites already occupied, they may have to move elsewhere. Alternatively, they may not return with exact precision to the natal area, but to somewhere nearby, perhaps to a place they located during their post-fledging dispersal before they left on migration (see above).

Other experiments have involved pelagic seabirds, in which individuals range over large areas of ocean before they return to their natal colonies up to several years later. Owing to the building of a military airbase, 3124 Laysan Albatross Diomedea immutabilis fledglings were transferred from Midway Atoll in the Pacific to other colonies up to 400 km away (Fisher 1971). Most of the survivors returned to Midway Island years later, as they reached breeding age. They had been raised in open nests, and by the time they were moved, they had already become attached to their natal area, and were unaffected by the move. If young were relocated long before they reached fledging age, some of the transported chicks later returned to the release site rather than to the natal site. The sensitive stage, when site attachment occurred in this species, fell in the last fourth of the nestling period.

Even in burrow-nesting Short-tailed Shearwaters Puffinus tenuirostris, the young developed an attachment to the natal colony long before they could fly (Serventy et al. 1989). Attachment occurred in the nestling period, as revealed by translocation experiments in which eggs and chicks of different ages were moved between different islands (Serventy et al. 1989). It presumably occurred at night when chicks sat outside their burrows, for only then could they see the outside world. After several years at sea, with annual migrations covering 30 000 km, returning birds distinguished their own colony from other colonies only 1 km and 3 km away.

The tendency of young birds to breed near where they were raised has contributed to the success of conservation programmes in which certain bird species have been reintroduced to areas from which they were earlier eliminated. Even where young raised in the release area moved away after becoming independent, most of the survivors returned there to breed, leading to the establishment of a new local population. This has occurred in a wide range of species, including waterfowl, raptors and seabirds (Newton 1979, Kress & Nettleship 1988, Cade 2000). Again, the implication was that the young became fixated to the locality during their early life.

Attachment of young birds to wintering sites

The attachment of young migrants to specific wintering localities seems to occur soon after their arrival there. Of 111 wintering Palm Warblers Dendroica palmarum that were caught soon after arriving on the Bahamas, 34 were released where they were caught, and the rest were transported and released at sites 9.7 and 22.5 km away. The proportion recaught at the capture site was about the same in all three groups, showing that displaced birds had returned to the capture site. Moreover, birds translocated at early dates in their stay (28 November-2 December) showed no difference in return rates from birds moved at later dates, suggesting that in this species site fixation had occurred fairly soon after arrival there (Stewart & Connor 1980).

Similarly, juvenile and adult Dunlins Calidris alpina that had arrived at a wintering site mainly in September-October were displaced 133 km from the site at different dates in November-December (Baccetti et al. 1999). From the proportions that returned from birds displaced at different dates, the juveniles seemed to have become attached to the site during November, within 1-2 months after arriving there. By December, return rates were the same as those of adults, which showed no seasonal pattern, presumably because they had known the site from previous years (for similar findings on Sanderling Calidris alba see Table 9.1; Myers et al. 1988).

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