Fidelity To Stopover Sites

Most migrants pause for refuelling up to several times during their journeys. Individuals of some species have been identified in successive years at the same staging sites which they visit for at most a few days or weeks at a time before moving on. Moreover, because some species take different routes on their outward and return migrations, individuals may use different stopping sites at the two seasons. This leads to the remarkable implication that some migratory birds remember the specific locations of several sites scattered over two continents, which they visit successively each year on a circuit that is repeated annually throughout their lives. To some extent, the landscape itself and its associated habitat areas are likely to impose patterns of recurrence, regardless of any inherent tendencies in the birds themselves. Species with an infinite number of potential stopping places on their migration routes are perhaps less likely to show strong fidelity to particular sites than species that have only a small number of possible sites.

Most passerines migrate in a broad front over more or less continuously suitable habitat, offering a wide choice of potential stopping places. In addition, the turnover of birds at stopover sites is high, as individuals normally remain only for short periods, often less than one day, so the chances of recording particular individuals are low. Not surprisingly, therefore, most studies in which passerine migrants were trapped year after year at the same stopping sites have provided little or no evidence of individual year-to-year site-fidelity; others gave small, but highly variable, proportions of retraps, exceeding 10% only in extreme cases (Moreau 1969, Nisbet 1969, Winker et al. 1991, Winker et al. 1992b, Cantos & Telleria 1994, Merom et al. 2000, Dowsett-Lemaire & Dowsett 1987). Some of the low return rates were perhaps no greater than expected by chance, if birds had paused at random in suitable habitat encountered en route, but this possibility cannot be tested statistically. Recurrence might be higher at habitat patches situated on the edges of seas or deserts that offer the last chance to feed before a crossing, or the first chance to feed after the crossing.

Another problem with existing data on stopover fidelity in passerines is that they derive from sites where large numbers of birds were caught each year, and the chance of catching any one individual, even if present, was low. Moreover, in some studies migrants could not always be separated from summering or wintering birds of the same species, so that the samples did not necessarily consist entirely of birds on migration. However, one study in which attempts were made to address these problems, and in which trapping was done in a consistent manner from year to year, was undertaken at a reed-bed site in south Portugal over five years (Catry et al. 2004b). In the five species and populations known to include only passage migrants at this site, the proportions of individuals retrapped there in a subsequent year varied between 0 and 1.2% of those ringed over a five-year period (Table 17.8). Two other populations that showed higher recovery rates could have included summer visitors (Reed Warbler Acrocephalus scirpaceus) and winter visitors (Bluethroat Luscinia luscinia). After attempts to correct for low recapture probabilities and annual mortality, maximum possible return rates in the five migrant populations were estimated at 0-12.9% of surviving birds (Table 17.8). Hence, fidelity to this stopover site (considered as good habitat) was generally low.

A similar study on Pied Flycatchers Ficedula hypoleuca was made in central Spain (away from breeding areas), where mist nets were operated in a 1-ha garden throughout the migration seasons of 1983 and 1984. To judge from fattening rates, this garden provided good habitat, but of 122 Pied Flycatchers caught in 1983, only one was retrapped there the following year (Veiga 1986). The maximum possible proportion of surviving birds estimated to have returned to the site was 10.6%, a figure within the range found for other passerines. These rates were much lower than those reported from breeding and wintering sites. However, in Eurasian Reed Warblers Acrocephalus scirpaceus, for birds caught two

Table 17.8 Overall between-year re-capture rates for different species and populations (short-winged and long-winged) caught in the autumn migration season mainly at Santa André, southwest Portugal

Total Number (%) Estimated maximum ringed re-captured return ratea

Migrants known to be on passage

Table 17.8 Overall between-year re-capture rates for different species and populations (short-winged and long-winged) caught in the autumn migration season mainly at Santa André, southwest Portugal

Sedge Warbler Acrocephalus schoenobaenus

0 0

Post a comment