1284 



HORMONAL REGULATION OF BEHAVIOR 



C. INCUBATION 



The eggs of birds are fertilized internally, 

 and then laid within a few hours after fer- 

 tilization. The development of the embryos 

 within the eggs requires temperatures higher 

 than normal environmental temperatures, 

 and this temperature is, in almost all birds, 

 provided by the body of the parent or par- 

 ents. The parents provide warmth for the 

 eggs by sitting on them in such a way that 

 the eggs are brought into contact with a 

 featherless area or areas on the ventral side 

 of the body. The behavior of the bird in sit- 

 ting on its eggs is called "incubation" or 

 "incubation behavior," and it is the purpose 

 of this section to discuss the physiologic 

 bases, and the physiologic consequences, of 

 incubation behavior. 



1. Incubation Patterns 



As in the case of the other types of be- 

 havior with which we are dealing, there is 

 an extraordinary interspecific diversity in 

 the patterns of incubation behavior to be 

 found in nature. The roles of the parents, 

 the pattern of attentiveness to the egg, the 

 duration of incubation, etc., show the wid- 

 est variation. The types of incubation be- 

 havior found in nature have been summa- 

 rized by Kendeigh (1952), and by Skutch 

 (1957), and the following abbreviated sum- 

 mary is adapted from their accounts. 



In many species of birds, including most 

 song birds, the female alone does all of the 

 incubation. In most such cases, the incubat- 

 ing bird leaves the nest several or many 

 times each day in order to feed, although 

 there are birds, like some pheasants (Good- 

 win, 1948; Delacour, 1951 ) , which sit on the 

 eggs for the entire incubation period, with- 

 out taking any food. Female hornbills also 

 sit on the eggs continuously, but they are 

 fed by the male while doing so. There are 

 some species in which the male does all of 

 the incubation. In phalaropes, unlike most 

 sexually dimorphic birds, the female is more 

 l)rightly colored than the male, and the role 

 of the sexes in sexual and parental behavior 

 is reversed from the usual pattern: the fe- 

 male defends the territory and the male does 

 all of the incubation, brooding, and caring 

 for the young (Tinbergen, 1935). The male 



emperor penguin sits on the eggs continu- 

 ously for as long as two months, without 

 taking any food (Stonehouse, 1953; Rivo- 

 lier, 1956). 



There are many groujxs of birds in which 

 both parents share in the incubation activi- 

 ties. These include birds like gulls (Tin- 

 bergen, 1953) in which the male and female 

 change places several times during the day, 

 and birds like the Adelie penguin (Sladen, 

 1953) in which the males and females 

 change places at intervals of several days. 

 The male and female short-tailed shear- 

 water change places on the eggs every 11 to 

 14 days (Marshall and Serventy, 1956). In 

 pigeons and doves (Whitman, 1919) the fe- 

 male sits on the eggs from late afternoon 

 through the night to midmorning, while 

 the male sits for one long session (about 6 

 hours) during the day. 



£. Horinonal Regulation of Incubation 



It i.'^ obvious tliat the diversity of patterns 

 of incul)ation behavior found among the 

 various species of birds must depend on a 

 considerable diversity of physiologic mecha- 

 nisms. As usual, however, our information 

 about these mechanisms is limited to rela- 

 tively few species. 



Relation of onset of incubation to time 

 OF EGG-LAYING. Since the pattern of endo- 

 crine secretion changes so rapidly during the 

 ])eriod of egg-laying, we may, as a first step 

 in the analysis of the hormonal basis of in- 

 cubation behavior, examine the relationship 

 between egg-laying and the onset of incuba- 

 tion behavior in seasonal-breeding bii'ds. 

 This temporal relationship should suggest 

 the identity of the hormone or hormones 

 which underlie the onset of incubation be- 

 havior. 



The ornithological literature reveals wide 

 variation from species to species with re- 

 spect to the time when incubation starts. 

 Some birds, such as the snow bunting (Tin- 

 bergen, 1939b) and the cedar waxwing (Lea, 

 1942) do not begin sitting on the eggs until 

 after the last egg is laid. A much more com- 

 mon pattern is for incubation to start just 

 before the laying of the last egg (black- 

 capped chickadee, Odum, 1941 ; shrikes, 

 Miller, 1931 ; wliite-crowned sparrow, Blan- 

 chard, 1941; bullfinf^h. Nicolai, 1956; and 



