Foraging in landfills: Feeding behavior of the White Stork (Ciconia ciconia) and kleptoparasitism by Black Kites (Milvus migrans)

The decline of the White Stork (Ciconia ciconia) in the Iberian Peninsula was averted within the last 30 years, partially due to an increase in food availability in landfills. In this study, we compared 2 colonies located at different distances from a landfill in southern Portugal, with the aim of assessing differences in stork diets. We also compared our current results with data from the same colonies before the landfill was built. Additionally, we studied storks' exposure to kleptoparasitism by Black Kites (Milvus migrans) as a potential factor affecting their feeding behavior in landfills. We analyzed 182 pellets collected in 2005 and 2010 in non-landfill colonies and from landfills only in 2010. We carried out >47 h of focal observations of storks feeding at the landfill. Pellet contents are primarily insects (15 families from 5 orders) as well as unidentified and inorganic materials. Our analyses of variance revealed significant local, seasonal, and annual differences in Coleoptera, Orthoptera, Hymenoptera, and total number of insects. Our observations at the landfill showed that storks feed on large amounts of organic material, mostly fresh meat and fish, which means that observational data differs from pellet data. Black Kites kleptoparasitize White Storks mainly at the beginning and end of the morning limiting their food intake and increasing the amount of time storks spend at the landfill. We conclude that foraging in landfills has provoked an important shift in the White Stork's diet, partially replacing these birds' natural food items and exposing them to potential new threats such as the ingestion of inorganic material and exposure to kleptoparasitism by kites.