STOPOVER ECOLOGY OF MIGRATING BIRDS - SOME UNSOLVED QUESTIONS

Stopover ecology, that is, the ecology of birds at resting sites visit ed between migratory flights, has received relatively Little attention in comparison with other fields of bird migration. In this review I d iscuss a few unsolved questions concerning stopover ecology: (1) Which are the selective forces acting on birds during migration? Several st udies have shown that time-minimization may be important. The importan ce and occurrence of minimizing energy consumption among migrants rema ins unknown. (2) What causes the initial body mass loss upon arrival a t a stopover site? Handling effects can not be ruled out, but several studies hint at ecologically interesting phenomena, such as food-compe tition and social interactions. (3) How fast can birds increase in mas s? Metabolic constraints Limit fat deposition rates in migrants, and m aximum fat deposition rates are higher in smaller birds than in larger ones. The interplay between the size of the metabolic machinery (refl ected in the basal metabolic rates) and fat deposition rates needs to be addressed. In the field of stopover ecology there is a general need for more theories predicting stopover behavior. More experiments are also wanted which can be used to test predictions and disentangle the often complex relationship among factors influencing birds on stopover .