The most critical, and dangerous, time of the avian life cycle probably is the migratory period when not only does a migrating bird have to decide on the routes to take to and from the wintering grounds but also when to rest, where and for how long. To understood the migrants decision we studied migratory European Bee-eaters (Merops apiaster) along the Arava Valley, Israel, during their spring migration stopover immediately after crossing the combined ecological barrier of the Sahel, Sahara, and Sinai deserts. We evaluated the effects of body mass and age on the decision to stopover as a function of the distance from the northern edge of the deserts northwards up to the Dead Sea basin. We propose a new "drop-out hypothesis" wherein the weaker individuals dropout earlier from a migratory flock that is moving northwards from an ecological barrier. Most of the birds that drop out at the stopover sites closest to the ecological barrier are juveniles, on their first migration from Africa and that lack experience of the northbound flight. The proportion of adults increases as the flock moves northwards. The evolutionary and conservation implications of this study stress the importance of conservation of not only a single stopover site along the migratory route but that of several points staggered out such that they will allow individuals with varying degrees of body condition to advance in their desired direction with minimum stress.
- Merops apiaster