Marked declines in food supplies, mainly through depletion, have been measured at stopover sites during the migration season. In some studies, this was done by excluding birds from some places, and comparing the trends in prey populations inside and outside the exclosures. For example, during the shorebird passage (July-September) in Massachusetts, 7-90% declines in different invertebrate species were recorded, mainly due to shorebird predation (Schneider & Harrington 1981). Similarly, following spring migration over the Gulf of Mexico, some passerine migrants were found to depress woodland insect supplies rapidly by up to 67% at coastal stopover sites (Moore & Yong 1991). Depletion by migrants of some other types of foods, such as berry crops, can be observed each year by even the most casual of observers (for case studies see Parrish 2000 and Ottich & Dierschke 2003). There can be no doubt, therefore, that some birds can seriously deplete their food supplies at stopover sites, both in autumn and in spring. Some food supplies, such as the plant leaves eaten by geese in spring, undergo repeated cycles of depletion and re-growth, a system which still limits the numbers of individuals that can feed effectively at a site at one time (Prop 1991).
This type of information is not necessarily useful without corresponding measures of bird performance, such as feeding or fattening rates. This is because, although food stocks might be estimated accurately, we usually have no means of telling what proportion is available to the birds, or how much they could remove before being disadvantaged. Nevertheless, depletion measures are sometimes useful in showing how close birds are to a food limit. If it is found that birds remove almost all their favoured foods from a site, and that no apparent alternatives are available locally, then a big increase in the numbers of migrants able to fatten at that site is clearly not possible. At the other extreme, if food is replenished as it is removed (say by growth, reproduction, or immigration of food organisms), then depletion is probably not an issue.
Some researchers have taken a different approach and measured feeding rates, rather than food itself. For example, depletion of food supplies by Pied Flycatchers Ficedula hypoleuca in autumn was inferred from declining insect capture rates the longer a bird spent in a tree, and by increasing feeding rates with time since the last visit to a given tree (Bibby & Green 1980). Competition at stopover sites can come not only from conspecifics, but from other species taking the same foods, the resulting depletion lowering the feeding rates of several species at once (Moore & Yong 1991). Where a food supply is depleted by passage migrants, latecomers, arriving after most of the food has been eaten, could be penalised by reduced rates of feeding and fat accumulation. Bewick's Swans Cygnus columbianus bewickii were studied at a staging site in the White Sea, the last stop on spring migration before the Siberian breeding grounds (Nolet & Drent 1998). In this locality, the swans could obtain their main food, tubers of Fennel Pondweed Potamogeton pectinatus, by upending to reach the sea-bed, mainly during low tide. In the course of the staging period, the swans tended to forage at progressively lower water levels, indicating that they gradually depleted this food supply, and exploited increasingly deeper parts of the tuber bank as the days went by. This depletion reduced the swans' main foraging period from 6.0 hours per tide on 20 May to 3.3 hours per tide on 28 May. The authors calculated that this must have greatly reduced the refuelling rate during the staging period. Accordingly, swans arriving late stayed longer than those arriving early. It seemed important for the swans to arrive at the stopover site as soon after ice breakup as possible, for a month later, the tubers were greatly depleted, and any remaining began to sprout. The first swans to arrive could also leave the site first, and (in theory) reach the breeding grounds earliest, get the best territories and achieve the highest breeding success. The latest swans seen to leave this site would have arrived in the nesting areas too late to breed that year.
This study hinted at how competition for limited food supplies at a stopover site, used for no more than a few weeks each year, could have helped to regulate the swan population. The White Sea provides the only sizeable stopover site for swans on this part of the spring migration, so with severe food depletion there, the birds would be limited in how much they could respond to any improvements in conditions that might occur in breeding or wintering areas.
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