WHAT SELECTIVE PRESSURES HAVE DRIVEN THE EVOLUTION OF DEFERRED NORTHWARD MIGRATION BY JUVENILE WADERS

Deferred northward migration by young waders has been ascribed to the ''risks of long-distance migration'', but the nature of these risks ha s not been explored. We suggest that the low foraging proficiency of y oung waders is an important factor in the evolution of deferred migrat ion by some small to medium-sized waders. Because of the disparity bet ween adult and juvenile energy intake rates, juveniles would be preven ted from migrating synchronously with adults if their migration involv ed one or more refuelling stops. The slower migration of juveniles wou ld result in their late arrival at the breeding grounds,,vith a concom itantly high risk of breeding failure. The longer the migration and th e higher the breeding latitude, the greater this risk, conforming with the conclusion of Summers et al. (1995) that young waders overwinteri ng close to the breeding grounds are more likely to migrate north than are young waders which overwinter further south. We further suggest t hat the probability of successful breeding by late arriving long-dista nce migrants is so low that the risk of first-year birds dying on nort hward migration does not have to be high for the behaviour to be selec ted against and thus be lost from the population.