348 BULLETIN 17 9, UNITED STATES NATIONAL MUSEUM 



day or very early in the morning or toward evening. Nests can 

 be found under these circumstances by a systematic search of likely 

 liabitats and so flushing the bird from the nest. 



No evidence of the use of a natural depression was noted either at 

 Evanston or at Ithaca; all were dug by the female. According to 

 Sutton (1927) and my own observations of O. a. strigata in western 

 Oregon, this excavation is dug with both beak and feet. The nest 

 is constructed usually at the edge or partially under a grass tuft or 

 clod, which, in the case of the prairie horned lark, lies most frequently 

 on the west, northwest, or north, possibly because the cold and violent 

 winds of the early nesting season come from this direction. The 

 body of the nest consists of coarse stems and leaves with a finer lining 

 within. The time spent in nest construction varies from two to 

 four days. 



The majority of the nests of the prairie horned lark showed a 

 variable amount of clods, pebbles, or similar items laid about the 

 margin usually on the side away from the protective tuft or clod. 

 These so-called "pavings" were always composed of the material most 

 easily obtained regardless of its permanency. It is suggested that 

 the purpose of "pavings," if there is a purpose, arises from the 

 method of nest construction and from the desire of the larks to have 

 a bare-ground nest approach. 



Eggs. — The ^gg has a background of gray with an occasional 

 greenish tinge, which background is almost completely concealed with 

 a fine speckling of cinnamon-brown. The cinnamon-brown often 

 forms a denser ring about the larger end. The average size was found 

 to be 2.25 cm. by 1.55 cm. The eggs of natural second sets seemed to 

 be a trifle larger than first sets of the same individual. The number 

 of eggs per set varied from two to five; the average was about four, 

 the sets of fewer numbers occurred early, those of larger number, 

 later. [Author's note: The measurements of 50 eggs average 21.6 

 by 15.7 millimeters ; the eggs showing the four extremes measure 24.6 

 by 15.5, 23.1 by 17.3, 18.3 by 15.0, and 21.6 by 14.5 millimeters.] 



Incubation. — The incubation period was determined to be 11 days. 

 Only the female incubates. 



The male shows little or no solicitude during the incubation pe- 

 riod. The female has a highly developed series of automatic in- 

 stincts of solicitude, which are modified by time of day, condition of 

 weather, and frequency of disturbance. The most highly developed 

 and probably the most recently acquired of these has been given 

 the name nest concealment hy abandonment., or casual abandonment. 

 The female leaves the nest, in this reaction, when an intruder is at 

 a long distance, and flies quietly away, low against the ground, and 

 does not show other solicitude for a very considerable period. The 



