Alice’s dissertation research and portions of her post-doc tested multiple alternative hypotheses to explain why some, but not all, tropical birds (especially those dependent upon fruits) regularly engage in predictable, seasonal migrations from higher-elevation breeding sites to lower-elevations during the non-breeding season.
This body of research has resulted in numerous publications and one of the most comprehensive set of studies of the ultimate factors shaping migration behavior in wild birds. Much of that work was conducted along a protected gradient of wet forest from La Selva Biological Station in the lowlands, up through Braulio Carrillo National Park and several adjoining private reserves. Alice conducted a great deal of the breeding-season research at Rara Avis reserve, an incredibly beautiful and incredibly wet forest at about 750 m elevation.
A few of the key take-home messages include the following: (a) simple variation in food availability does not drive downhill migration in this system; (b) reductions in foraging time due to heavy rain likely DO drive downhill movements, and which species or individuals migrate depends on traits that influence energetic needs; (c) while migration may increase the chances of surviving during the non-breeding season, it has carry-over effects to the breeding season with migrants having lower chances of reproducing.
There have been many associated papers and side-projects related to this topic, including work that takes the plants’ point-of-view, studying phenological strategies and fruit removal rates in this seed dispersal mutualism.