What Selective Pressures Have Driven the Evolution of Deferred Northward Migration by Juvenile Waders?

Abstract

Deferred northward migration by young waders has been ascribed to the of long-distance migration, but the nature of these risks has not been explored. We suggest that the low foraging proficiency of young waders is an important factor in the evolution of deferred migration by some small to mediumsized waders. Because of the disparity between adult and juvenile energy intake rates, juveniles would be prevented from migrating synchronously with adults if their migration involved one or more refuelling stops. The slower migration of juveniles would result in their late arrival at the breeding grounds, with a concomitantly high risk of breeding failure. The longer the migration and the higher the breeding latitude, the greater this risk, conforming with the conclusion of Summers et al. (1995) that young waders overwintering close to the breeding grounds are more likely to migrate north than are young waders which overwinter further south. We further suggest that the probability of successful breeding by late arriving long-distance migrants is so low that the risk of first-year birds dying on northward migration does not have to be high for the behaviour to be selected against and thus be lost from the population.