OVULATION IN BIRDS AND MAMMALS 219 



refuse to do so. Indeed, the assumption of the egg-laying 

 attitude is, to a limited degree, a voluntary activity (if the word 

 voluntary may be applied to animal activities), for it depends 

 upon the dove's disposition, her whole past history, and her 

 whole attitude toward the present situation. 



Just as certain environmental conditions stimulate egg-laying, 

 so there are certain environmental conditions which inhibit it. 

 Some of the facts already given illustrate such inhibition. A 

 clear case of it is seen in species of birds of which the female 

 tends to lay eggs continuously until they reach a certain num- 

 ber, the sight of which (or the touch?) inhibits further laying. 

 Egg collectors have often caused such a bird to lay an abnor- 

 mally large number of eggs, by leaving only one "nest egg" 

 in the nest, removing every additional egg as soon as it appears. 

 Perhaps the most remarkable instance recorded is that of a 

 flicker (Colaptus auratus) experimented on by Mr. Charles L. 

 Phillips, of Taunton, Mass. "On May 6, 1883, he found a 

 cavity in a large willow tree containing two eggs; he took one, 

 leaving the other as a 'nest egg,' and continued to do so day 

 after day until the female flicker had laid seventy-one eggs in 

 seventy-three days." (Davie, 1898.) 



Columbidae do not prolong their laying in this manner. In 

 them, the number of eggs in a set is predetermined in the ovary, 

 and is never more than two. But it is clear that all through the 

 incubation period the presence of eggs in the nest prevents the 

 ripening of the new 'set of ova, which will ripen as soon as the 

 bird reassumes the laying attitude. That the sight of eggs in 

 the nest can inhibit the assumption of the mating attitude, is 

 shown by the following scene, which is familiar to every keeper 

 of pigeons or doves : The hen dove is sitting on her eggs ; her 

 mate comes to her with bowing coo and other signs of intense 

 excitement ; she is gradually aroused, rises, shows a mating 

 attitude, and starts to leave the nest ; then she catches sight 

 of the eggs, and as she looks at them her attitude changes toward 

 that of brooding ; she stands hesitant for several seconds, drawn 

 one way by the stimulus from the mate and the other way by 

 the stimulus from the eggs ; gradually the brooding attitude 

 becomes stronger, the mating attitude disappears, she goes 

 back and' settles on the eggs. But if now the eggs be removed 

 from the nest, the hen dove loses her brooding attitude, and it 



