Foraging behaviour of sympatric razorbills and puffins

Many marine predators coexist at colonies, creating a zone where there could be sig- nificant inter- and intraspecific competition. To minimize the potential for direct competition, under the principle of competitive exclusion, sympatric predators may differ in their foraging be - haviour at the colony. At Skomer, Wales, razorbills Alca torda and puffins Fratercula arctica both breed at the same time of year, forage on sand eels Ammodytes sp. and their populations are sta- ble or declining, meaning that they may be close to carrying capacity and experiencing competi- tion. To examine how they differ in their foraging behaviour, time-depth-temperature recorders were attached to the legs of chick-rearing individuals of both species. Puffins have lower wing- loading and lower total oxygen stores than razorbills and are therefore expected to invest more time in flying and less time in diving than razorbills. Mean (±1 SE) dive depth was 11.8 ± 0.45 m for puffins and 8.2 ± 0.21 m for razorbills, while mean dive duration was 40 ± 0.45 s for puffins and 24 ± 0.21 s for razorbills. Both species spent most of their dive time making shallow, V-shaped dives during daylight hours. In contrast to our prediction, foraging behaviour was very similar between the 2 species, although puffins tended to spend more time both diving and flying. The higher diving and flying rates of puffins may be associated with multiple prey loading, as puffins tend to bring back smaller (and therefore more) prey items than do razorbills.