1356 



HORMONAL REGULATION OF BEHAVIOR 



was extremely difficult to keep the embryo 

 alive over the 7th day. It is not known ex- 

 actly how the behavior of the parent during 

 this period makes it so superior to the best 

 efforts of a human being using an artificial 

 incubator. In the domestic hen, there are 

 several critical periods, during which most 

 deaths of embryos occur in artificial incu- 

 bators (Romanoff, 1949). New (1957) found 

 that eggs recjuired regular turning lietween 

 the 4th and 7th day of incubation in order 

 to hatch. Eggs turned only on those 4 days 

 had the same degree of hatchability as eggs 

 turned throughout incubation (21 of 35 eggs 

 so treated hatched, compared with 24 out 

 of 35 eggs turned throughout incubation). 

 If the eggs are turned only between the 8th 

 and 11th day of incubation, the number of 

 eggs which hatch (6 of 35) is the same as in 

 the case of eggs which are not turned at all 

 during incubation. Clearly the behavior of 

 the parents in regularly turning the eggs is 

 an essential prerequisite for the successful 

 development of the embryos (Westerskov, 

 1956) . 



Newly hatched altricial birds, such as the 

 house wren (Kendeigh and Baldwin, 1928; 

 Baldwin and Kendeigh, 1932) are poikilo- 

 thermic at hatching, with the capacity for 

 temperature regulation developing gradu- 

 ally, starting at 3 days post-hatching, and 

 continuing up to about 9 to 12 days of age. 

 The field sparrow becomes homoiothermic 

 at about 7 to 10 days of age (Dawson and 

 Evans, 1957). Precocial birds, such as the 

 western gull (Bartholomew and Dawson, 

 1952) are in part homoiothermic at hatch- 

 ing; there are some indications that these 

 birds may begin to develop the ability for 

 temperature regulation before hatching. The 

 parents of altricial birds characteristically 

 brood the young much more attentively, 

 and for a longer period, than do the parents 

 of precocial birds (Kendeigh, 1952). Bar- 

 tholomew and Dawson (1954) studied the 

 development of temperature regulation in 

 young browm pelicans and great blue her- 

 ons, both altricial species, and in western 

 gulls, a precocial species, all nesting on the 

 same hot, dry island in the Gulf of Cali- 

 fornia. When they deprived the birds of the 

 care of their parents, they found that the 

 young gulls had a consistently greater ca- 



pacity for temperature regulation than did 

 either the pelicans or the herons. In the 

 words of these authors, "the successful nest- 

 ing of these three species in the same area 

 at the same time despite their differences 

 in capacities for temperature regulation, 

 emphasizes the importance of behavior as 

 a supplement for physiological mechanisms 

 in birds." 



The feeding behavior of parent birds of 

 different species is differentiated in ways 

 which correspond to the different ways in 

 which their young take food. For example, 

 young passerine birds, such as the Euro- 

 pean blackbird, at first "beg" for food by 

 lifting the head vertically and opening the 

 gape wide, simultaneously uttering a char- 

 acteristic sound; the parents of such spe- 

 cies feed the young by dropping food into 

 the open mouth (Tinbergen and Kuenen, 

 1939). Parent gulls, on the other hand. 

 regurgitate fish which they hold in the bill 

 in front of the young, which peck at the 

 bill, stimulated by various aspects of its 

 color and shape (Tinbergen and Perdeck, 

 1950; Collias and Collias, 1957). Chicks of 

 some such species cannot pick up the food 

 if it is dropped on the ground, but must get 

 it from the bill of the parent (Hardy, 1957) . 

 We could compile, from the ornithological 

 literature, a considerable list of the varying 

 details of the manner in which parent birds 

 feed their young, and the correspondences 

 with the forms of behavior of the young. 



The development of feeding behavior in 

 young birds seems, in some species at least, 

 to be partly influenced by the behavior of 

 the parents toward the begging young. Rand 

 (1942) reared four loggerhead shrikes by 

 hand to the age of 21 days, when the first 

 signs of pecking at food (instead of simply 

 receiving it from the "parent") appeared. 

 Two of the birds were subsequently kept in 

 a cage with food always present, and hand- 

 feeding was stopped as soon as possible. 

 The remaining two birds had no food in 

 their cage, and were fed exclusively by hand. 

 The first group of two birds never begged 

 after 45 days of age, whereas the second 

 pair of birds (fed by hand) was still beg- 

 ging from their keeper at the age of 7^2 

 months, although they could pick up food 

 from the floor. Rand (1941) found similar 



