1298 



HORMONAL REGULATION OF BEHAVIOR 



the time of incubation, and prolactin causes 

 change in the ventral apterium only after 

 pretreatment with estrogen. 



D. CARE OF THE YOUNG 



1, Types of Young and Methods of Feeding 

 Them 



Birds vary widely with respect to the de- 

 gree of development at hatching. Although 

 all sorts of intermediates occur, two types of 

 young are generally recognized among birds. 



Precocial young, which emerge from the 

 eggs after a relatively long incubation pe- 

 riod, are hatched covered with body feath- 

 ers, capable of locomotion within a few 

 hours, and capable of picking up food, or in 

 some cases even finding food without the 

 assistance of the parents. Precocial young 

 usually occur among families of water birds 

 or of birds living and feeding on the ground, 

 such as pheasants, cranes, sandpipers, 

 ducks, grebes, etc. (Mayaud, 1950). Meth- 

 ods of feeding and caring for precocial 

 young vary widely. For example, in some 

 species of the mound-building megapodes, 

 the young are completely independent, the 

 parents playing no role in rearing them 

 (Frith, 1956b). In other cases, such as the 

 pheasants and ducks (and domestic chick- 

 ens) , the young follow the parents, thus 

 being led to food, and may be excited to feed 

 by the behavior of the parents in the pres- 

 ence of food (Bent, 1923, 1925; Beebe, 

 1936). In still other families having pre- 

 cocial young, the young birds, although cov- 

 ered with down and capable of walking, 

 may be fed for some time after hatching by 

 food brought by the parents (terns, Hardy, 

 1957) , or by partially digested food regurgi- 

 tated by the parents [e.g., penguins, Bag- 

 shawe, 1938; gulls, Tinbergen, 1953). Stone- 

 house (1953) reported that the emperor 

 penguin feeds the young in part by regurgi- 

 tating ''strips of epithelium or similar tissue 

 invested with fat globules" presumably 

 from the crop wall. 



Altricial young are hatched naked of 

 feathers, usually blind until the eyes open 

 some days later, and incapable of locomo- 

 tion or of finding or selecting food, so that 

 they must be fed by the parents for a con- 

 siderable number of days or weeks (May- 



aud, 1950; Kendeigh, 1952). Altricial young 

 are fed by a variety of methods. Many in- 

 sect-eating birds carry food to the young in 

 their bills (Bent, 1942) . Other species may 

 regurgitate food which they have carried in 

 the throat (pelicans. Bent, 1922) or regurgi- 

 tate from the crop or stomach food that has 

 been partly digested (night heron, Allen and 

 Mangels, 1940). In still other species, the 

 regurgitated food may include substances 

 secreted or produced in the digestive tract of 

 the parents. Food regurgitated by the 

 mother hummingbird contains digestive se- 

 cretions mixed with ingested food (Mayaud, 

 1950) ; the young fulmar is fed partly on an 

 oil secreted by the proventriculus of the 

 parents (Fisher, 1952) ; food regurgitated to 

 young pigeons just after hatching consists 

 of epithelial cells desquamated into the 

 lumen of the crop (Beams and Meyer, 

 1931). 



Young birds are brooded by their parents, 

 who sit on the nests containing the young 

 and allow the young to huddle under them. 

 The parents may cover them for some days 

 after hatching, in a manner somewhat simi- 

 lar to the way in which they sit on eggs, 

 although differences can usually be ob- 

 served (Tinbergen, 1953). Parents of al- 

 tricial young may continue to brood them 

 for a number of days or weeks {e.g., Fisher, 

 1952), whereas precocial young may be 

 brooded for only a short time after hatching 

 or not at all (Kendeigh, 1952). 



It is apparent that the types of behavior 

 included in the expression "parental care" 

 vary widely among different groups of birds, 

 and therefore that analysis of the physio- 

 logic basis of this type of behavior in a few 

 domesticated species will do no more than 

 suggest the variety of mechanisms which 

 are possible. 



Roles of the parents. The roles of the par- 

 ents in caring for the young need not neces- 

 sarily be, in all respects, the same as their 

 respective roles in incubating the eggs, a 

 fact of considerable importance for its bear- 

 ing on the problem of the role of hormones 

 in the induction of behavior toward the 

 young. 



In the species in which the male and fe- 

 male both take part in incubation of the 

 eggs, brooding and feeding of the young are 



