Shearwaters sometimes take long homing detours when denied natural outward journey information

The cognitive processes (learning and processing of information) underpinning long-distance navigation of birds are poorly understood. Here, we utilised the homing motivation of the Manx shearwater to investigate navigational decision making in a wild bird by displacing them 294km to the far side of a large island (the Island of Ireland). Since shearwaters are reluctant to fly over land, the island blocked the direct route home, forcing a navigational decision. Further still, on the far side of the obstacle, we chose a release site where use of local knowledge could facilitate a 20% improvement in route efficiency if shearwaters were able to anticipate and avoid a large inlet giving the appearance of open water in the home direction. We found that no shearwater took the most efficient initial route home, but instead oriented in the home direction (even once the obstacle became visible). Upon reaching the obstacle, four shearwaters subsequently circumnavigated the land mass via the long route, travelling a further 900km as a result. Hence, despite readily orienting homewards immediately after displacement, shearwaters seem unaware of the scale of the obstacle formed by a large land mass despite this being a prominent feature of their regular foraging environment.

Methods
These are the raw GPS files detailing the time, latitude and longitude of fixes taken evey ~5 mins. The data are cut to include only GPS fixes taken between releasee of the birds and their arrival at the colony at Copeland Bird Observatory, Northern Ireland.