Many people noticed that the hawks appeared to be displaying aggression towards the windows last Friday. The hawks were perching on the railing above the nest and raising their wings in an aggressive posture.As soon as we heard about these behaviors we contacted the building managers at the site where the hawks are nesting. We discovered that the original tenants that occupied the offices next to the fire escape had moved out. The tenants had been wonderful stewards of the birds and had been very protective of the birds. Apparently a cleaning crew was working in the offices last Friday and this most likely was what was causing disturbance. During nesting season, birds of prey are typically disturbed by activities that are either new or which increase in intensity. Activities that are ongoing from before nesting season tend to not cause problems. At this site, the hawks were used to the comings and goings of the original office residents, but they sensed something new and different when the cleaning crew entered the offices. The change might seem slight to use, but in fact hawks can differentiate between different humans. For years the peregrines at the Fremont Bridge ignired a homeless person who camped each night right below their nest, but they would grow agitated if other unfamiliar people entered the area when the chicks were learning to fly. We have corrected the problem at the red-tail nest---the windows leading into the office surrounding the nest are now covered. We appreciate the ongoing cooperation of the building managers---they have been great to work with from the get-go!