Now, two members of the original team, Ángel Felicísimo (Universidad de Extremadura) and Jesús Muñoz (Real Jardín Botánico, CSIC) have joined a birds' specialist, Jacob González-Solís (Universitat of Barcelona) to demonstrate that those wind highways follow a very precise route and, moreover, they are only accessible during very precise periods of time.
In the present paper, published in the open-access journal PLoS ONE, they used birds equipped with geolocators to identify the routes they follow in their migratory flyways, something in which González-Solís has been working for years. Cory's shearwaters (Calonectris diomedea), which breed in the Canary Islands and winter offshore South Africa and Namibia were the ideal candidates for two reasons. Firstly, because far from following the great circle route to cover the approximately 8000 km that separate the Canary Islands of South Africa, they follow a detour of 3000 km to almost reaching Brazil before turning east to Africa. How can it be profitable for a 800 g bird to make such immense detour? Secondly, they fly using a very peculiar technique: surfing the air that waves push in front of them. Given that ocean waves are produced by wind, nothing better than using the data generated by the satellite that measures wind's characteristics –the QuikSCAT, already used by Felicísimo and Muñoz in their previous research– to assess if shearwaters used the wind highways between Canary Islands and southern Africa.
Analyzing daily oceanic winds measured by QuikSCAT and comparing them with the birds' location, the authors found that the previously unexplained flyway followed by the Cory's shearwaters is exactly the most efficient during the migration time. Shorter pathways would represent higher energetic costs by flying against prevailing winds.
Minimum cost corridors according to SeaWinds in the migration period of Cory's shearwaters (left) and density of their trajectories (right).
Source: PLos ONE
Source: PLos ONE
Another result –this unexpected– is that the trip cannot be done in any time of the year, as there is an invisible temporal “gate” north and close to the Equator that is closed for months in the form of calms or contrary winds. Only when favorable winds start do Cory's shearwaters initiate their great joint southward journey.
In this research, the authors used new techniques specifically developed for these spatio-temporal analyses, which allow to mathematically demonstrate for the first time how winds constrain the main migratory flyways both spatially and temporally. These results open new avenues of research in fields like pathogens spread or when prophylactic treatments are more effective, species dispersal and migration, how species evolve after colonizing new areas, and even which species can be the ancestor of others.
Source paper: Felicísimo, Á.M., Muñoz, J., & González-Solis, J. (2008) Ocean surface winds drive dynamics of transoceanic aerial movements. PLoS ONE, 3, e2928.