Cue-conflict experiments between magnetic and visual cues in dunlin Calidris alpina and curlew sandpiper Calidris ferruginea

Abstract

Despite our extensive knowledge on various aspects of their lives, there has been limited investigation into the hierarchical relationships among different compass systems in shorebirds. The aim of this study was to investigate the relationship between magnetic and celestial compasses in two species of shorebirds, the curlew sandpiper (Calidris ferruginea; pre-breeding migration) and the dunlin (Calidris alpina; post-breeding migration) using cue-conflict experiments. Birds were captured in a Mediterranean stopover site, after which their magnetic orientation was determined under simulated overcast conditions at sunset using modified Emlen funnels fitted with infrared video cameras. Birds that demonstrated a well-defined directional preference were then exposed over two sunsets to conflicting directional information between the local geomagnetic field and the ±90° shifted band of maximum polarisation. These individuals were tested again for magnetic orientation at sunset in the same conditions as previous test, to determine whether their directional choices had changed after the cue-conflict. Our results showed that individuals from both species did not recalibrate their magnetic compass from visual cues after the cue-conflict, even though at least dunlins did not appear to completely disregard the information derived from celestial cues. This study is one of the few experimental studies on the migratory orientation of Charadriiformes and on the hierarchical relationships between the different compasses used by these birds during their extensive migratory movements. Migrating birds are able to use different compass mechanisms based on geomagnetic or celestial cues, and it seems reasonable to hypothesise that birds calibrate their various compasses to maintain the correct direction especially when the directional information does not agree. The hierarchy among different compasses has been studied largely on night migrating passerines, but it is still poorly understood. We investigated the hierarchy among geomagnetic and celestial cues (band of maximum polarisation) in two species of Charadriiformes by means of cue-conflict experiments. Our result showed that the geomagnetic cues have a dominant role in the orientation mechanisms of the studied species, even though the information derived from celestial cues did not appear to be completely disregarded.