Refuge abandonment in a formerly harvested waterbird and the consequent formation of multi-species bird colonies

The intense human persecution of wildlife in the past forced many animal populations to look for refuge in human-inaccessible habitats. With the decline of direct persecution, and the changes in the attitude of modern urban societies towards wildlife during the last few decades, an ecological process of abandonment of refuge habitats has become pervasive, and many species make habitat choices deemed surprising presently, such as colonizing sites that are more accessible to humans. We tested this process in the yellow-legged gull Larus michahellis by analysing long-term time series (1976–2022) of breeding pairs in seven colonies in southern Europe. Overall, yellow-legged gull numbers showed a strong pattern of decrease during the study period, due to the recent closure of open-air garbage dumps. However, while the number of pairs in colonies located in human-inaccessible sites (islets with cliffs) declined rapidly, new colonization together with a monotonic linear population growth was detected in human-accessible sites, coinciding in time with the departure from refuges. Moreover, we found that the new small yellow-legged gull colonies acted as cores for the formation of multispecies waterbird colonies likely by heterospecific attraction, that provides an added unexpected conservation value to yellow-legged gulls traditionally considered a pest species.