1302 



HORMONAL REGULATION OF BEHAVIOR 



Hormonal correlates of broodiness in- 

 duced by the young. It will be recalled that 

 a high level of pituitary prolactin is asso- 

 ciated with the normal occurrence of in- 

 cubation behavior, and that when the pres- 

 ence of eggs causes the extension of incuba- 

 tion behavior past the normal period, the 

 prolactin level remains high. The situation 

 is strikingly different with respect to both 

 normally occurring broodiness toward the 

 young and broodiness experimentally in- 

 duced by the presence of young. The pro- 

 lactin content of the pituitary gland of hens 

 goes down sharply after hatching, and re- 

 mains down while the chicks are being 

 brooded (Saeki and Tanabe, 1954) . Further, 

 when hens are made broody by being kept 

 with young, the prolactin content of their 

 pituitary glands is not elevated (Burrows 

 and Byerly, 1938; Saeki and Tanabe, 1955). 

 The same is true of immature hens and ca- 

 pons. Both can be induced to become broody 

 by being kept with young, without their 

 pituitary glands showing increased prolactin 

 content (Saeki and Tanabe, 1955). 



Although the evidence discussed above 

 indicates that the presence of young does 

 not stimulate any detectable increase in 

 prolactin in the pituitary gland, there is 

 some evidence that the presence of young 

 inhibits or delays the onset of the next cycle 

 of nest-building and egg-laying, and there- 

 fore presumably inhibits the secretion of 

 gonad-stimulating hormones by the parent's 

 pituitary gland. In various species of wild 

 birds, a recrudescence of sexual behavior 

 can be noted after removal of the young 

 (Howard, 1940; Blanchard, 1941; ^Arm- 

 strong, 1947). Kobayashi (1953a) found 

 that removing the young of domestic pi- 

 geons would cause the laying of new eggs 

 about 6 or 7 days later, although the feed- 

 ing period is normally about 35 days long. 

 The. European wren under normal condi- 

 tions often produces a second brood during 

 a single breeding season. If the nest is de- 

 stroyed, the 1st egg of the new clutch will 

 be laid some 5 to 8 days later. However, if 

 the eggs are allowed to hatch normally, the 

 1st egg of the new brood will not be laid 

 until some 10 to 14 days later. Therefore, 

 the fact that the birds are caring for the 

 nestlings delays the beginning of the new 

 cycle (Armstrong, 1955). 



4- Physiologic Nonidentity of Incubation 

 Behavior and Broody Care of the 

 Young 



In much of the literature, including par- 

 ticularly the agricultural research literature, 

 the term "broodiness" is used indiscrimi- 

 nately to refer to incubation behavior, 

 brooding of the young, and caring for the 

 young in other ways. I have been rather 

 careful to use the term "incubation" or "in- 

 cubation behavior" when referring to be- 

 havior toward the egg, and to restrict the 

 term "broody" or "broodiness" to behavior 

 toward the young. I believe this distinction 

 is justified, and even necessitated, by a 

 number of lines of evidence. 



Distribution of incubation behavior and 

 of parental care of the young. Perhaps the 

 most striking evidence to be found in the 

 natural history of birds for the distinctive- 

 ness of incubation behavior and of parental 

 care of the young lies in the fact that in 

 many species of birds the female does all 

 of the incubating, whereas the male plays a 

 major role in feeding the young. Skutch 

 (1953b) points out that in many species of 

 songbirds the male discovers the nestlings 

 when the eggs hatch, in part as a result of 

 his behavior during the incubation period. 

 ]\Iales which do not incubate may stand 

 guard at the nest, they may bring food to 

 the female, they may visit the nest occa- 

 sionally, they may escort the female to and 

 from the nest when she leaves it to feed, 

 etc. In some species it seems fairly clear 

 that the behavior of feeding the nestlings 

 has much in common with that of feeding 

 the female while she is on the nest, as in 

 the Florida jay, in which the male brings 

 food to the female during the incubation 

 period and, after the eggs hatch, brings 

 food to the nest and may either give it to 

 the young, or to the female, who then gives 

 it to the young (Amadon, 1944b). 



At least in birds with altricial young a 

 distinction must be made between the pa- 

 rental behavior of feeding the young and 

 that of brooding them (i.e., sitting on them 

 so as to provide heat during their early 

 poikilothermic days). The usual situation 

 in songbird species in which the female in- 

 cubates and the male feeds the young is 

 that the female also does all of the brooding 

 of the young (Lack, 1946; Putnam, 1949; 



