We used to visit golden eagle nests several times a summer to watch the young grow up and leave the nest. These nests were across a narrow but very deep (1000 feet) canyon. The nests were in vertical cracks in the sheer canyon walls. We'd sit on the opposite rim rock and watch through a 32x spotting scope. My "bird watcher" friends told me that the nests were used generation after generation and that each year the birds added more nesting material. One day while we were watching, a rock climber came by for a chat and said that the previous winter he had climbed that wall and measured the nest we were watching. It was 40 feet tall.