Positive interactions among species are widespread in nature. In a foraging context, these interactions may consist of local enhancement, when an individual uses cues from individuals of another species to locate food, or facilitation when food is made more readily available by the individuals of other species. Facilitation may evolve as mutualism when it is reciprocally beneficial, as commensalism when the facilitator is neither benefited nor harmed or as parasitism when the host is used as a resource and harmed. Feeding associations have been reported in which birds take advantage of foraging opportunities by associating with mammals. Associations between piscivorous birds and foraging otters (Carnivora, Mustelidae) have been occasionally described, but seem rare and are still poorly understood. We report here on a previously undescribed feeding association between piscivorous birds and Eurasian otters Lutra lutra. In Spain, common kingfisher Alcedo atthis and grey heron Ardea cinerea were observed closely following foraging otters and benefited from feeding opportunities provided by these. Behavioural observations of otters in central Spain (28.4 hrs; 19 days) revealed that an association with kingfishers occurred in 33% of otter foraging events (n=92). Simultaneous observations in northern Spain (14.2 hrs; 16 days) showed an association between otters and kingfishers or grey herons in 41.6% and 11.7% of otter foraging events (n=77), respectively. The association probability between kingfishers and otters increased significantly when otters foraged closer to the shore and on small fish rather than other prey (crayfish or large fish). Birds fed on prey remains left by the feeding otters, on small fish captured by otters when these were satiated and playing, or on prey displaced by otters. Our observations are consistent with facilitation and commensalism: piscivorous birds gained feeding opportunities provided by the otters, with no apparent costs or benefits to the latter. Similar feeding associations have been described between other species of otters and piscivorous birds (kingfishers, herons, egrets, storks) in Asia, South America and Southern Africa, but had not yet been described in Europe. The occurrence of piscivorous bird-otter associations in different species and regions suggest that this commensalism may be often overlooked but widespread. We have shown that the association can be frequent and is context-dependent, with benefits for associating birds depending on otters´ behaviour and targeted prey.

Resumen del trabajo presentado al XXIV Congreso Español y VII Ibérico de Ornitología, celebrado en Cadiz del 13 al 17 de noviembre de 2019.

Peer reviewed