Inusual mortandad de pichones y juveniles de Garza Bruja Nycticorax nycticorax (Ardeidae) en una nueva colonia nidificante en el centro de Argentina

We report a case of mortality in juvenile Nycticorax nycticorax in a new colony present at an urban reserve adjacent to the Parana River (Santa Fe, Argentina). From August to November 2016, 138 nests were monitored. In August, there were few nestlings and juvenile deaths. In September, more than 800 nestlings and juveniles were found dead on the ground, as well as eggs abandoned by their parents. There was no evidence of unusual mortality in other egret species present in the area. We collected a sample of six individuals for laboratory analysis and necropsies. Individuals had very low body condition and almost empty stomachs, distended gallbladders intestines without solid material, predominantly filled with biliary liquid (fasting sign), and albumin and other serum proteins were found at very low levels, indicating starvation as the cause of death. Probably, this mortality was due to parental abandonment in ages at which the young birds still depend on external food provisioning. We considered the interaction of multiple causes, such as human intervention, habitat conditions, climatic factors, and population density. The magnitude of the reported waterfowl die-off is unprecedented in the Middle Parana river valley.