OBSERVATIONS ON THE INCUBATION BEHAVIOR

effectiveness of the bird's camouflaging plumage maintained. Lack (1932), however, found that less than half the female Nightjars he observed faced the sun. Few workers in North America seem to have considered orientation behavior in the Nighthawk; but Gross (1940: 214) observed that a Nighthawk he watched "usually faced the sunrise in the morning and the sunset in the evening." In the present study, this orientation was measured more precisely by means of a sun dial constructed by mounting a vertical rod in the center of a board marked with a circle scaled in 100 intervals. This was placed near the roof door from which observations were made. Most observations were made at hourly intervals. The direction of the sunlight was determined by the shadow of the rod of the sun dial and the position of the bird's axis was estimated in degrees by reference to the dial. These observations are summarized for one complete day in Figure 1. Remarkably precise agreement is shown between the bird's position and the angle of the sun. The bird maintained the sun at her back throughout the day. At only one observation was a conspicuous disagreement noted between the bird's long axis and the angle of the sun. This occurred at 9 a.m. during a partly cloudy period. By 10 a.m., when the temperature at roof level reached approximately 1100 F (430 C), the bird had not only moved