The impact of non–local birds on yellow–legged gulls (Larus michahellis) in the Bay of Biscay: a dump–based assessment

Abstract

Understanding how animals exploit non–natural feeding sources such as garbage dumps is necessary from many perspectives, including conservation, and population dynamics and management. Several large predatory gulls (Larus spp.) are among the species which most clearly benefit from using dumps. The yellow–legged gull (L. michahellis) is the most abundant gull in the southwestern Palaearctic, and its fast population increase until at least the 2000s was partly due large waste dumps becoming more numerous. The Bay of Biscay is an area that hosts resident local and also wintering non–local yellow–legged gulls. Using data collected over a period of eight years (bird counts, identification of colour–ringed individuals) at four dumps situated within a 60–km radius from the colonies of Gipuzkoa (southwestern Bay of Biscay), we aimed to answer: (1) the origin of gulls using dumps at the Bay of Biscay; (2) the impact of local and non–local gulls at these dumps; (3) the possible age–dependent use of these sites; and (4) the possible seasonal fluctuations in the use of dumps by gulls. Gulls in our area (study dumps) came from nearby colonies in Gipuzkoa, Atlantic Iberia, the Mediterranean region, and other areas such as Atlantic France and inland colonies (Navarra, Germany). Our study dumps seemed to be used mostly by local gulls.