Frequency-dependent morph differences in kleptoparasitic chase rate in the polymorphic arctic skua Stercorarius parasiticus

The relationship between colouration and kleptoparasitic efficiency (success rate and chase rate) was studied in the polymorphic arctic skua, Stercorarius parasiticus, at a Mediterranean migration site. A shift in the ratio of morphs through the spring, presumably caused by differential migration between populations that differ in morph frequency (the higher the latitude, the more frequent is the pale morph), allowed the comparison of the relative performances of pale and dark birds under different scenarios of morph frequency. Chase rate showed a clear frequency-dependent effect in chases by solitary skuas, birds of the rare morph always conducting more chases than expected. This trend was explained by chases of yellow-legged gulls, Larus michahellis, a relatively large and aggressive species that was difficult to chase (i.e. chases resulted in low success rate) but provided potentially higher reward (i.e. more food) than any other host. These differences could result from the perceived cost-benefit balance of chases, as skuas from the rare morph could find an advantage in chasing this host species by being less rapidly recognised. This is in agreement with the apostatic selection hypothesis, which says that the rare morph in a predator or parasite population is favoured through its unfamiliarity to potential prey or hosts (the surprise effect). Differences in success rate could have provided further support to this interpretation, but sample size was not sufficient for a proper comparison at the host-species level.