The above sections illustrate different types of evidence indicating that events at stopover sites can influence migratory bird populations. They show that food supplies at staging sites can be heavily depleted, slowing rates of fattening, which in turn can delay spring migration so much that it reduces breeding success, or prevents breeding altogether. Moreover, the intraspecific crowding, or the disturbance caused by predators or human hunters, can affect the behaviour and fattening rates of migrants, causing them to leave stopover sites prematurely and with lower fuel reserves than otherwise, with affects on their subsequent survival and breeding success. The most crucial question, however, is whether these effects on individuals are sufficient to reduce population sizes below what they would otherwise achieve. The mortality from botulism, although sometimes large scale, occurs mainly in late summer or autumn, so could be compensated to some extent by greater overwinter survival of unaffected birds.
In the Greater Snow Geese Chen caerulescens atlantica and Pink-footed Geese Anser brachyrhychus mentioned above, increased disturbance at a major spring stopover site led to reduced breeding output, and at least in Pink-footed Geese also to reduced survival. This would be expected to influence the subsequent population trend, and the Pink-footed Goose population has now ceased its long-term increase. In the Red Knot Calidris canutus rufa population that breeds in arctic Canada and winters in Tierra del Fuego, marked decline has also been tied to changed conditions at Delaware Bay, the last major spring staging site of knots en route to arctic breeding areas. Numbers staging there in spring fell from 51 000 to 27 000 individuals between 2000 and 2002. Decline coincided with collapse (through human overfishing) of the Horseshoe Crab Limulus polyphemus population, the eggs of which form the main food of knots at this site (Baker et al. 2004). From 1997 to 2002, increasing proportions of knots studied in the Bay failed to reach the usual departure mass of 180-200 g. Annual survival of adults fell by 37%, and the proportions of immature birds in wintering flocks fell by 47%. Of birds caught in the Bay, known survivors were heavier at initial capture than were birds not seen again. By 2004, the numbers of knots had fallen even further, along with those of Turnstone Arenaria interpres and Sanderling Calidris alba. However, a moratorium was introduced on crab-fishing during May-June; and enough crab eggs were produced that year to feed the reduced numbers of shorebirds, enabling them to fatten at the normal rate. Because Horseshoe Crabs do not breed until they are about 10 years old, the situation could remain precarious for several further years.
Recent problems at stopover sites may have influenced the population levels of some other species too. Widespread decline in the numbers of Lesser Scaups Aythya affinis in North America has been linked with females arriving on breeding areas in poorer condition than before (Anteau & Afton 2004). In breeding areas in Minnesota and Manitoba, mean body mass of females was about 8% lower in the years after 2000 than in the 1980s, and lipid reserves were 30% lower. Mineral reserves were also lower in the Manitoba females, and the mean body mass of males was 41% lower in Minnesota. All these downward trends were statistically significant. With lower body reserves, scaup are unable to breed, or must wait until they have replenished reserves on breeding areas, a delay which reduces breeding success.
Among White Storks Ciconia ciconia migrating between eastern Europe and Africa, annual survival and population change were found to correlate with autumn conditions on a major staging area in the eastern Sahel zone (Schaub et al, 2005). The birds remained in this region for up to two months during their southward migration, and again more briefly in spring. Annual variation in the rainfall-dependent condition of the vegetation in this region (assessed from satellite images) accounted for 88% of the annual variation in stork survival. Survival was high in years when primary production in the eastern Sahel was high. It led to synchrony in the annual survival of adult and juvenile storks, and their population changes, across a large part of eastern Europe over the 19-year study period. In contrast, vegetation conditions further south in Africa, where some of the storks spend the winter, made no significant contribution to variation in annual survival. Similarly, in British Barn Swallows Hirundo rustica, breeding numbers from year to year were correlated with the preceding rainfall in the western Sahel zone, at its driest during spring migration, and not with rainfall in the South African wintering areas (Robinson et al. 2003).
These various observations imply that conditions at stopover sites, including competition and predation/disturbance, can influence the migration speeds, reproduction and survival chances of individual migrants, and in extreme cases can affect their breeding numbers. Such processes can act in a density-dependent manner, at least among the individuals that are present together at particular sites. Where most individuals depend on the same small number of stopover sites (as in Pink-footed Goose Anser brachyrhychus and Red Knot Calidris canutus), and birds are competing largely at the same time in the same area, such processes could have density-dependent effects at the level of the entire population. It remains to be seen whether the same holds in species in which individuals are spread over a large number of stopping sites at all stages of the journey, each individual competing with whichever other individuals happen to be at the same sites at the same times, as in many passerines that migrate over land.
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