Influence of food availability on interactions between Larus cachinnans and L. audouinii

Abstract

Predatory and (or) kleptoparasitic interactions are greatly facilitated in mixed-species gulleries, particularly when one of the species is larger than the others. Larus audouinii, the Audouin's gull, is a threatened species breeding in sympatry with the larger L. cachinnans, the yellow-legged gull, throughout the Mediterranean. The yellow-legged gull has often been cited as the main threat to the Audouin's gull. On the Chafarinas Islands, the second largest breeding place for Audouin's gulls in the world, both gull species depend largely on commercial fisheries for food. We analyze the influence of food availability, assessed through fishery activity, on the frequency and intensity of interaction pressure by yellow-legged gulls upon Audouin's gulls during the breeding season. We studied five different types of interaction: (1) flyovers of yellow-legged gulls; (2) ground intrusions; (3) egg predation; (4) chick predation, and (5) aerial kleptoparasitism. Moreover, intensity of interaction pressure was assessed using logistic regression analysis to build a model of the"relationship between Audouin's gulls' response to yellow-legged gull flyovers as a dependent variable and fishing fleet activity as an independent variable. All interactions except aerial kleptoparasitism were significantly more frequent during days without sardine fishery activity. However, chick predation is significantly higher only during the period when yellow-legged gull has fledglings. Overall, the results of the estimated logistic model indicate a positive association between depleted food and the response by Audouin's gulls to aerial intrusions.