1282 



HORMONAL REGULATION OF BEHAVIOR 



host building it. She watches intently and 

 for long periods during the nest-building. 

 She visits the nest regularly in the absence 

 of the owners before laying. Hann suggests 

 that the development of the eggs, and their 

 ovulation, in the cowbird are stimulated by 

 the sight of the potential host building a 

 nest, and that this accounts for the syn- 

 chronization of the laying of the cowbird's 

 and of the host's eggs (note that the para- 

 site's eggs must be laid at a time when a 

 host is prepared to incubate them). Accord- 

 ing to Hann's observations, the cowbird's 

 egg is laid some 4 to 5 days after she first 

 begins watching, so that his hypothesis 

 about stimulation of ovulation is plausible. 

 However, observation of the behavior of 

 cowbirds (Nice 1949), as well as histologic 

 studies of their ovaries (Davis, 1942a I, in- 

 dicate that the cowbird's eggs are laid in 

 clutches of 3 to 5 eggs, with a rest period of 

 some 5 to 8 days between clutches. This sug- 

 gests a possibility that the cowbird, when 

 such a clutch is growing, must find a nest, 

 and that it finds a series of host nests be- 

 cause it is about to lay the eggs, rather than 

 laying the eggs because it has found the 

 host nests. However, a series of studies by 

 Chance (1940) on the European cuckoo in- 

 dicates very strongly that a brood parasite 

 may actually be stimulated to lay eggs by 

 the availability of host nests. Chance in- 

 duced cuckoos to lay abnormally long series 

 of eggs (on the order of 20 to 25) by remov- 

 ing eggs from the nests of foster species 

 {i.e., potential hosts) so that they built new 

 nests and relaid. He thus managed the situ- 

 ation so that potential host nests were avail- 

 able to the cuckoo over a much longer pe- 

 riod of time than that during which the 

 cuckoo normally lays eggs, and during 

 which it normally has hosts available. By 

 this method, he induced cuckoos which nor- 

 mally lay 5 to 7 eggs to lay 20 to 25. 



The anis are a New AVorld subfamily of 

 cuckoo-like birds, closely related to the 

 parasitic cuckoos. Although not themselves, 

 for the most part, brood parasitic, their 

 breeding habits are peculiar in that some of 

 the species nest in communal nests, several 

 females laying in one nest. In some species 

 only a few of the females will incubate, even 

 though many more have laid the eggs. Davis 



(1940, 1942b) reported that these birds may 

 lay their eggs on the ground, even quite far 

 from the nest. Such eggs, of course, are not 

 incubated. He also found that ovulation 

 may be stimulated by the presence or ac- 

 tivity of other birds. In several flocks, he 

 noted that no egg-laying might take place 

 for a long time, and that a sudden burst of 

 egg-laying activity would occur after a new 

 female joined the flock. Davis suggested 

 that the breakdown of the normally rigid 

 relationships between nest-building, egg- 

 laying, and incubation, and the ability of 

 these birds to lay eggs in response to visual 

 and/or auditory stimulation by other birds, 

 regardless of whether they have built a nest 

 and regardless of whether they will incu- 

 bate, may be features of their reproductive 

 cycle which encourage the development of 

 brood parasitism. 



Effect of eggs in the nest on ovulation. In 

 nature each species of bird lays a charac- 

 teristic number of eggs in a clutch, the vari- 

 ation within species sometimes being ex- 

 tremely narrow. In some cases the number 

 of eggs laid is independent of the presence 

 of other eggs in the nest. In other cases the 

 number of eggs laid may be considerably 

 extended by removing eggs as they are laid, 

 the bird continuing to lay until the number 

 of eggs present in the nest is approximately 

 the normal clutch size. The term "determi- 

 nate layer" is commonly used for those spe- 

 cies in which the number of eggs laid is 

 rather rigidly determined by physiologic re- 

 lationships internal to the bird, whereas the 

 term "indeterminate layer" is used for those 

 species in which the size of the clutch may 

 vary according to the situation in the nest 

 (Cole, 1917; Laven, 1940b; Lack, 1947; 

 Davis, 1955) . Among the domestic birds, the 

 pigeon is a familiar example of a determi- 

 nate layer, whereas the domestic hen is, of 

 course, an indeterminate layer. Among wild 

 birds, too, there are variations from species 

 to species. For example, the lapwing 

 (Klomp, 1951), and some songbirds (Davis, 

 1955) seem to be determinate layers, at- 

 tempts to increase the size of the clutch by 

 removing eggs as laid having been unsuc- 

 cessful. On the other hand, female house 

 sparrows have been reported to lay up to 50 

 eggs in regular succession when the eggs 



