1280 



HORMONAL REGULATION OF BEHAVIOR 



siderable space. Vaiigicn ( 1948) found that 

 single female serins in individual cages 

 would not lay eggs, but that eggs would be 

 laid within a few days if a male was placed 

 in a cage with the female. In the case of the 

 shell parakeet, Vaugien (1951) showed that 

 the sounds made by other birds influenced 

 the growth of the oviduct. When female 

 parakeets were kept isolated in small dark 

 boxes of various sizes, they laid no eggs. 

 When the box was placed inside an aviary 

 containing a breeding pair (so that the ex- 

 perimental bird could hear, but not see the 

 breeding birds) about half of the experi- 

 mental birds laid eggs within 12 days. When 

 the remaining birds were sacrificed for au- 

 topsy some 3 weeks later, they had enlarged 

 oviducts, with the largest ova averaging 9 

 mm. in diameter. Controls kept out of hear- 

 ing of breeding birds laid no eggs and, on 

 autopsy, were found to have ova no larger 

 than 1.5 mm. in diameter. Ficken, van 

 Tienhoven, Ficken and Sibley (1959) veri- 

 fied this effect on ovarian activity of sounds 

 made by other individuals in parakeet 

 flocks, and also reported that testis devel- 

 opment was stimulated. Marshall (1952, 

 1954) described the mating behavior of 

 bower birds, in which the male has a special 

 display ground, where he builds a bower 

 and displays to the female. This display 

 stimulates the female to go off and build her 

 nest and rear her young alone. 



It is clear from the above data that stim- 

 uli provided by the courting male induce the 

 secretion of gonadotrophic hormones by the 

 female, and that this is possibly a source of 

 the synchronization of the sexual cycles of 

 male and female birds during the breeding 

 season. 



(c) The presence of an appropriate nest- 

 ing site and the availability of nesting ma- 

 terial seem to be important factors in the 

 conditions stimulating normal gonadotro- 

 phin secretion during the breeding season 

 in birds. Lack (1933) observed three col- 

 onies of arctic terns at three different loca- 

 tions near a lake. The first location was per- 

 manently dry ; the second was water-logged 

 for a short period in the spring, because of 

 melting snows; the third was water-logged 

 for a longer period, until a marshy area 

 dried up. Although birds were present from 



the beginning of the season in all three of 

 these locations, the birds in the first colony 

 laid their eggs earliest, those in the third 

 colony latest. Similar observations were 

 made by Linsdale (1938), who found that 

 the yellow-headed blackbird, which builds 

 its nest only over water, will abandon the 

 nest in midbuilding if the water dries up 

 while the nest is being built, and will then 

 build a new nest elsewhere, this involving a 

 delay in ovulation. However, if the eggs are 

 laid first, so that incubation is in progress 

 when the water dries up, the birds stay on. 

 It can thus be stated that the presence of 

 ai)propriate nesting conditions facilitates 

 ovulation, and thus presumably the secre- 

 tion of gonadoti'oi)hic hormones by the pi- 

 tuitary gland. 



The induction of ovulation by the avail- 

 ability of nesting material has been shown 

 experimentally in several species of birds. 

 Like some other tropical species (Roberts, 

 1937; Bullough, 1951) , the red-billed weaver 

 finch of central Africa breeds at irregular 

 times, always following rainfall. Marshall 

 and Disney (1957) showed that the stimu- 

 lating factor following the rainfall is actu- 

 ally the availability of nesting material. 

 During the dry season, when no reproduc- 

 tion was taking place in free-living birds of 

 the species, Marshall and Disney kept 

 groups of male and female red-billed weav- 

 ers in four outdoor cages variously provided 

 with combinations of the following: insect 

 food, artificial "rain" from a sprinkler, dry 

 grass of the type normally used by the birds 

 as nesting material, and green grass of the 

 same type. Birds having green nesting grass 

 available built nests, regardless of whether 

 "rain" was falling, and regardless of 

 whether insect food was available. Further- 

 more, the only females to lay eggs during 

 the experimental period were those in the 

 cages in which the males were building 

 nests. Clearly, manipulation of nesting ma- 

 terial by the males induced in the females 

 hormonal changes leading to ovulation. 

 Marshall and Disney also noted that the 

 bills of the females kept with such males 

 assumed breeding color earlier than did 

 other females. This change in color is, of 

 course, under hormonal control (Witschi, 

 1938). 



