Travelling to breed

Traditionally, investigation of the dynamics of avian migration has been heavily biased towards the autumn return trip to the wintering quarters. Since the migratory prelude to breeding has direct fitness consequences, the European Science Foundation recently redressed the balance and sponsored a workshop on spring travels. We here survey the findings elucidating the complications arising during migration directed towards the breeding quarters. The evidence that early nesting confers advantage is overwhelming, hence demands of reproduction pose a constraint on both time and energy resources during spring. Both during migration and upon arrival there must therefore be strong selection in favour of saving time. Experimental results (e.g. using supplementary feeding) show that the date of laying is generally proximately constrained by the inability of the female to find sufficient nutrients. A key question thus concerns the implementation of the ‘capital’ strategy for breeding, the female accumulating nutrient stores along the way to bridge periods of shortage upon arrival on the breeding grounds. Eight studies on waterfowl (geese and eiders) and shorebirds (turnstones and knots) nesting in the arctic combine tracking of individuals (satellite telemetry) with direct observation (marked birds) and reconstruction of the origin of nutrients laid down in the eggs or in the form of body stores of the female parent (stable isotope signatures of tissues compared to potential food sources). The consensus emerges that in most cases a mixed strategy prevails, with nutrients garnered locally supplementing ‘imported’ body stores. The species concerned face a shortage of feeding time during incubation and suffer an energy deficit. Successful breeding thus depends on adequate fat depots that form part of the ‘capital’ the parents bring with them. Some headway has been made in predicting the extent of dependence on body stores for breeding in relation to body mass and length of migration from rates of fat deposition during stopover and fat consumption during flight for waterfowl. This work poses a challenge to refine field studies accordingly. The pressure to save time en route highlights the need to effectively exploit rich food resources. Several case studies underline the crucial significance of a very limited set of stopover sites, each with a narrow time window of optimal harvest. The influence of man at such sites often verges on the disastrous, and ongoing climate change may unhinge the finely tuned timing long-distance migrants depend on. There is a real need to extend this work to provide the scientific basis to underpin adequate conservation actions.