212 BULLETIN 17 6, UNITED STATES NATIONAL MUSEUM 



mortality of the birds that nest on roofs is much less than among 

 those that choose nesting sites on the ground. In the latter place 

 the birds are constantly exposed to the ravages of predatory animals 

 including the cat. In the country hundreds of young birds meet 

 with a tragic end without any of us being the wiser, while in the 

 city such cases are more likely to be brought to our attention. On 

 the city roof there is freedom from natural enemies. In the study 

 of a large number of nighthawk families I have noted relatively 

 few young that left the nesting roof prematurely, and it has been 

 a common experience to see them return to the roof many times 

 after the initial flight. One bird, the account of which is related 

 in a subsequent division of this paper returned to the roof every 

 day until it went south on its migration at the age of 52 days. 

 Many of the birds that nest on the roofs are never disturbed by 

 human beings during the entire nesting period, and under such, 

 circumstances the young do not leave the security of the roof until 

 they are able to fly well. It is obvious that the young of parents 

 that nest on roofs are the ones most likely to survive, and they in 

 turn will nest in similar situations the following seasons. It seems 

 reasonable to suppose that after the habit is established natural 

 selection and heredity play an important part in the general 

 departure from the old to the new nesting environment. 



No attempt is made by the nighthawk to construct a nest, and 

 no materials are added to those already present on the nesting site 

 chosen by the birds. The eggs may be in a slight depression, but 

 no material is excavated or removed by the bird except that inci- 

 dentally shoved aside by the incubating bird. 



Eggs. — Normally two eggs are laid, and there is seldom a depar- 

 ture from this number. In a letter dated January 20, 1936, F. W. 

 Rapp, of Vicksburg, Mich., writes that he found a nest of the night- 

 hawk containing three eggs on May 22, 1889. 



The eggs are elliptical-ovate or elliptical-oval, one end being 

 slightly smaller than the other. The shell is strong, closed grained 

 and moderately glossy. The ground color varies from pale creamy 

 white to shades of cream olive-buff and olive-gray. The eggs are 

 marked and speckled with shades of slate, black, drab, smoke and 

 lilac gray, and tawny-olive, and some of the eggs have shades of 

 pearly gray, lavender, and plumbeous. In some eggs the markings 

 are fine and uniform in size, almost obscuring the ground color; in 

 others they are less numerous but larger and more prominent. There 

 is an endless variation in the details of the markings and colors, 

 but the eggs in general exhibit a coloring that blends effectively with 

 their surroundings. 



The average measurements of 81 eggs in the United States 

 National Museum are long diameter 29.97, short diameter 21.84 



