South Africa Australia South America

spent the winter. A similar trend occurs among different wintering populations of Ringed Plovers Charadrius hiaticula and others (Evans & Davidson 1990). Oversummering young birds can certainly achieve high survival rates. For example, among Bristle-thighed Curlews Numenius tahitiensis that migrate from Alaska to the Hawaiian Islands, the average annual survival of birds after their first arrival in Hawaii exceeded 91% for the first year, 93% for the second year and 97% for the third year, compared with 85% in migrating adults (Marks & Redmond 1996). Clearly, the younger age classes suffered no survival penalty from staying in 'wintering' areas year-round, and may have benefited, but on the samples available, such slight differences did not achieve statistical significance.

The main question about deferred return to breeding areas is whether it occurs because young birds are unable to accumulate the fat reserves necessary for the return journey (owing to lower feeding rates or heavier parasite loads, McNeil et al. 1994), or whether deferred return is an inherent feature, evolved because young birds have no chance of breeding and benefit from avoiding the risks of a long return journey to breeding areas. These explanations are not mutually exclusive, and each may apply to different populations. Many studies have shown that young birds are less efficient foragers than older, more experienced ones, and often suffer from their subordinate status in competitive interactions. Young birds may, therefore, have difficulty in accumulating migratory fat, at least in time to breed (Hockey et al. 1998). The fact that over-summering in wintering areas seems commoner in populations that winter furthest from their breeding areas (on the southern continents) supports the evolutionary explanation; but the fact that some birds migrate only part way to their breeding areas (still travelling up to several thousand kilometres) supports the food-limitation hypothesis. First-year birds that do not return to breeding areas normally show little or no gonad development, and no spring moult or pre-migratory fat deposition, but remain in drab winter plumage (Chapter 12).

0 0

Post a comment