One large cottonwood tree, used by the parent-owls as a landing place whenever they were forced from the nest, was situated approximately 110 feet to the north and a five-story building was located 80 feet farther to the north. Numerous smaller trees line the street to the east and there are some on the lawns around the Museum. Also, there are about two acres of trees 225 feet west of the nest-site where the parent-owls took refuge when forced from the cottonwood tree. The nest, if it can be called a nest, was no more than a few bare branches of the Virginia creeper, which covers the side of the building, together with some excrement which the owls tended to push to the periphery of the nest. For most of the time the three eggs in 1945 lay directly on the metal which covered the ledge, because there was no definite floor to the nest. The single egg in 1946 lay on the cement shelf between the pillars and the wall of the building. This laxity in nest building by Great Horned Owls apparently is not uncommon (see Bent, 1938:300). PERIOD OF INCUBATION Incubation of the eggs probably began, in 1945, on February 5, the day the first egg was laid. It has usually been assumed that, in birds of prey, incubation begins when the first egg is laid. The last of the three eggs was laid February 7. In 1946, the single egg was being incubated on February 4. Since another egg had been laid two or three days before this--a broken egg was found beneath the nest and there were remnants of the egg in the nest--incubation may have started as early as February 1 or February 2. In comparing these dates of initial incubation with other recorded dates of nesting, only those from places at, or near, the latitude of Lawrence, Kansas, in the central United States, should be expected to be approximately the same since the times of egg-laying and incubation are progressively later in the year as approach is made toward the polar region. Baumgartner (1938:279) has previously pointed this out.