Traveling or stopping of migrating birds in relation to wind: an illustration for the osprey

Although it is often assumed that birds strongly prefer tailwinds for their migratory flights, we predict that a strategy of no wind selectivity (traveling independently of winds) may be more favorable than wind selectivity (traveling on tailwind occasions but stopping to rest under headwind occasions) for birds with low energy costs of travel relative to rest and for birds that cannot use stopover time for efficient fuel deposition. We test this prediction by analyzing the daily traveling or stopping as recorded by satellite tracking of five ospreys Pandion haliaetus, a species often using energy-saving thermal soaring, during their migration between northern Europe and Africa. Besides wind, precipitation is another weather factor included in the analyses because thermal soaring migrants are expected to stop and rest in rainy weather. In logistic regression analyses, taking into account the effects of latitude, behavior on previous day, season, date, and individual for discriminating between traveling and stopping days, we found a lack of influence of winds, suggesting that the ospreys travel or stop without regard to wind. This lack of wind selectivity under light and moderate winds is in agreement with our prediction. We expect a low degree of wind selectivity and thus regular flights under headwinds also among other types of birds that cannot use stopping time for efficient foraging and fuel deposition. We also found an unexpected lack of influence of precipitation, possibly because of relatively few instances with rainfall in combination with poor geographic precision for estimates of this weather variable