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HORMONAL REGULATION OF BEHAVIOR 



the same age) were even smaller. This indi- 

 cates that the 3rd egg laid has been incu- 

 bated more effectively, presumably because 

 effective incubation of the last egg of the 

 clutch begins immediately, whereas the ear- 

 lier eggs are not incubated with complete 

 effectiveness until some days after they are 

 laid. Holstein (quoted by Swanberg, 1950) 

 found that the eggs of a European goshawk 

 which he observed to begin incubating with 

 the 1st egg, all hatched within 48 hours, al- 

 though 9 days had elapsed between the lay- 

 ing of the 1st and last egg of the clutch. 



The problem arises whether the relative 

 ineffectiveness of observed incubation early 

 in egg-laying is based on inadequate behav- 

 ioral response of the birds to the eggs, or on 

 inadequate temperature exchange between 

 egg and adequately responding bird. I have 

 already indicated that incubation behavior 

 occurring early in egg-laying may be quan- 

 titatively very slight compared with that 

 during the incubation period. There is, how- 

 ever, some evidence that, even when birds 

 do sit on the eggs early during the egg-lay- 

 ing period, they may not actually transmit 

 heat to them (Ryves, 1943a, b). Swanberg 

 (1950) reported that, in several species, 

 birds sitting on the eggs early during the 

 egg-laying period do not actually warm the 

 eggs, even though they may be sitting so 

 tightly that it is rather difficult to frighten 

 them away. Arnold (quoted by Swanberg, 

 1950) observed that blue jays sitting on the 

 eggs early during the egg-laying period had 

 the feathers of the ventral body surface be- 

 tween the body and the eggs, rather than 

 being erected as in a normally incubating 

 bird, so that the naked ventral skin could 

 be applied to the eggs. It is apparent that 

 the important problem of the basis for the 

 ineffectiveness of incubation during the egg- 

 laying period is far from solved, but that 

 the observational data at hand provide an 

 adequate background for defining the prob- 

 lem for experimental attack. 



We may summarize by saying that incu- 

 bation behavior develops gradually during 

 the egg-laying period, appearing at different 

 rates and at different times in different spe- 

 cies of birds. In spite of the interspecific 

 differences, there is clearly some relation- 

 ship between the hormonal changes associ- 



ated with egg-laying, and the onset of in- 

 cubation behavior. 



Onset of incubation behavior in the 

 MALE. It is not unexpected that the onset of 

 incubation behavior in female birds is re- 

 lated to the production of eggs, and pre- 

 sumably is regulated, in part, by the hor- 

 monal changes associated with ovulation 

 and egg-laying. But what of those cases in 

 which the male takes part in incubation? Is 

 the onset of readiness to incubate in his case 

 related to the time of production of eggs by 

 the female? The situation is somewhat vari- 

 able from species to species: in some cases 

 the male begins to sit with the 1st egg (night 

 heron, Noble, Wurm, and Schmidt, 1938; 

 turnstone, Bergman, 1946; Kentish plover, 

 Rittinghaus, 1956; etc.) ; in other cases the 

 female begins to sit during the egg-laying 

 period, with the male not incubating until 

 some time later (common tern. Palmer, 

 1941; lapwing, Laven, 1941). In species in 

 whicii the male does all of the incubating, 

 it is variously reported that the male begins 

 to incubate from the first egg (jacana, Hoff- 

 mann, 1949), or that he may not begin to 

 incubate until some of the eggs have already 

 been laid (northern phalarope, Tinbergen, 

 1935). These field observations are subject 

 to the reservations which we noted above: 

 "incubation" is poorly defined, and quanti- 

 tative data about the development of the 

 behavior arc usually lacking. 



Noble, Wurm and Schmidt (1938) re- 

 ported that male night herons will sit on 

 eggs that are placed in the nest before the 

 female has laid, and that the male does all 

 of the incubating during the first few days. 

 Poulsen (1953) found that both male and 

 female pigeons would sit on eggs offered to 

 them after the nest had been built, but be- 

 fore any eggs had been laid, and we have 

 verified this in the ring dove. Lehrman, 

 Brody and Wortis (1961) found that when 

 a pair of ring doves is placed in a breeding 

 cage with a nest bowl and nesting material, 

 readiness to incubate develops gradually 

 during the 6 to 9 day period before eggs are 

 laid, and that this readiness develops earlier 

 in males than in females. 



It is obvious that available data on the 

 development of incubation behavior in male 

 birds are quite inadequate. However, they 



