Compass orientation drives naïve pelagic seabirds to cross mountain ranges.

Wildlife migration is a spectacular phenomenon [1]. Studies using telemetry - tracking devices attached on free-living animals - have shown that large topographic barriers and obstacles, such as oceans and deserts, elicit extreme feats of migration [2]. Overcoming the challenges of these obstacles might require experience and skill that young individuals lack [2-5]. Further, younger, inexperienced animals might determine their migration routes using navigation strategies different from those of older animals [6-9], but our knowledge of how orientation mechanisms and experience drive migration strategy is limited. We have studied how experienced (adults) and inexperienced (first-time migrating fledglings) streaked shearwaters (Calonectris leucomelas) approach the challenge of migration using animal-borne tracking devices. The study birds migrate from a colony on the north of a large topographic barrier, Honshu Island, Japan. Shearwaters use a wind- and wave-based flight pattern-dynamic soaring-to extract energy for highly efficient travel over oceans [10]. We therefore expected that shearwaters migrating southward from the colony would make substantial detours to avoid any landmasses. We found that migrating adults followed one of two paths that detour around landmasses that hinder direct southerly migration. In contrast, inexperienced fledglings followed a straight course in a south-oriented direction that forced them to complete a trans-mountain journey, suggesting that the birds rely on an innate compass. Thus, we suggest that fledglings would eventually override the simple compass navigation, which appears to be the primary driver for their extreme migration, before being able to interact appropriately with the marine environment.