Remote-controlled, unmanned aerial vehicles (UAVs) can be used to collect information in difficult-to-access places while potentially minimizing human disturbance. These devices have been widely used in a variety of ecological and behavioral studies in recent years, but additional study is needed to assess the magnitude of disturbance they cause to birds. We examined the responses of Great Egrets (Ardea alba) to a UAV in a breeding colony in Louisiana in 2016 where isolated patches of common reed (Phragmites australis) were used as nest sites by multiple breeding pairs. We examined the flush responses and flight initiation distances (FIDs) of nesting adults to the direct vertical approach of a UAV. Incubating adults were more likely to flush from nests and flushed at greater distances when approached by a UAV than adults with nestlings, suggesting that adult assessment of risk was based on the greater reproductive value of nestlings. We observed fewer flush responses and calculated lower set-back distances using a UAV to approach nesting Great Egrets (~50 m) than set-back distances calculated using traditional methods of approach (e.g., walking or boating; 87-251 m). We found that FIDs were shorter when more adults were present in nesting patches, suggesting that the perception of predation risk may be based in part on the reactions of other birds. Our results suggest that UAVs may be a useful alternative for monitoring colonial-nesting waterbirds. However, our analyses were based exclusively on behavioral observations. Additional studies of the physiological responses of birds to the approach of UAVs are needed to better understand the stress responses of birds to these devices. RESUMEN.Uso de distancia de iniciaci on de vuelo para evaluar la respuesta de Ardea alba en colonias de anidaci on a acercamientos por veh ıculos a ereos no tripulados Los veh ıculos a ereos no tripulados controlados remotamente (UAV) pueden ser usados para colectar informaci on en lugares de dif ıcil acceso minimizando potencialmente los disturbios por humanos. Estos veh ıculos han sido ampliamente utilizados recientemente en una variedad de estudios en ecolog ıa y comportamiento, pero estudios adicionales son requeridos para determinar la magnitud del disturbio que pueden causar en las aves. Examinamos la respuesta de Ardea alba a UAV en una colonia de reproducci on en Luisiana en 2016 donde parches aislados de Phragmites australis eran usados como sitios de anidaci on por m ultiples parejas reproductivas. Examinamos la respuesta de huida y las distancias de iniciaci on del vuelo (FID) de adultos en anidaci on al acercamiento directo vertical de un UAV. Los adultos en incubaci on tuvieron una mayor probabilidad de huir de los nidos y volar a mayores distancias cuando un UAV se acercaba, que los adultos con pichones, sugiriendo que la evaluaci on del riesgo por los adultos esta basado en el mayor valor reproductivo de los pichones. Observamos menos respuestas de huida de los nidos y estimamos menores distancias de retroceso utilizando...