Incidence of beak deformities in austral thrushes (Turdus falcklandii) increases with urbanization and flocking behavior.

Beak deformation, known as avian keratin disorder (AKD), can impair feeding and preening of birds, reducing their survival. This disorder is apparently caused by Poecivirus infection, although to date, the viral origin has been corroborated in only a few North American bird species. Considering that fruit-eating birds can track spatiotemporal variations in fruit abundance and that AKD may have a viral origin, the incidence of this disease can be expected to increase with flocking by birds. Therefore, we evaluated if austral thrushes (Turdus falcklandii) were attracted to urban areas when exotic plants offered fruits and if flocking of thrushes in urban areas increased the spread of AKD in this species in a Patagonian town. We fitted GPS loggers on some individuals with normal beaks in rural areas and found that they visit the town in fall. Through point count censuses, we recorded greater abundances of thrushes with normal and deformed beaks during fall-winter in urban sites than in rural sites. However, the abundance of birds with AKD declined more (78-87%) than that of individuals with normal beaks (44-52%) during the transition from fall-winter to spring-summer. In urban zones in our study area, fruits of exotic species ripen during fall, attracting austral thrushes from rural sites. Nevertheless, such an attraction for food resources may be an ecological trap for this species, as the increase in incidence of AKD in urban areas may drastically reduce the survival of birds during the most unfavorable period of the year.