Poleward non-breeding migration of a breeding population: challenging the traditional perspective of avian migration

Migration is a fundamental ecological phenomenon, which in the Northern Hemisphere involves directed movements between northern breeding grounds and southern non-breeding grounds. Migratory movements in the opposite direction to the prevailing seasonal routes had been seldom studied and are considered to be an exceptional behavior of animals. Here we present novel insights on avian movement and challenge the traditional perspective of avian migration, by revealing an exceptional movement pattern. We combine analyses of a large dataset based on high-frequency GPS animal tracking data, deployed on the breeding population of the Judean long-legged buzzardButeo rufinus(LLB), along with analyses of remotely sensed data (of a satellite-based greenness index NDVI - normalized difference vegetation index) to reveal the migratory patterns of this population. This species had never been tagged before and its annual movements were previously unknown. We show that the Judean LLB population is migratory rather than sedentary, and that this population performs a poleward non-breeding migration from their southern breeding grounds in Israel to their northern non-breeding grounds in Syria, Turkey and Russia. We suggest that this unusual phenomenon is likely related to food availability in their breeding and non-breeding grounds. We provide support to the hypothesis that LLBs occupy rich environments, track specific ecological productivity levels along latitude gradients, and maintain consistently lower NDVI levels across their annual cycle compared to NDVI values available in their respective home range. These low NDVI levels are linked to the foraging ecology of LLBs, which forage in relatively open areas characterized by low vegetation. To the best of our knowledge, we present here a most extreme departure from the seasonally appropriate migratory direction of most avian species, by showing a significant poleward non-breeding pattern and by providing a possible suggestion for this behavior.