1272 



HORMONAL REGULATION OF BEHAVIOR 



observations, which vary widely with re- 

 spect to the continuity of observation, num- 

 ber of birds observed, distribution of ob- 

 servations during the day and during the 

 cycle, etc. There is nevertheless a strong 

 impression that, in many species of birds, 

 the physiologic conditions encouraging cop- 

 ulation {i.e., sexual receptivity of the fe- 

 male) are the same as those inducing nest- 

 building. 



Nest-building not correlated with copula- 

 tion. There are some significant exceptions 

 to this general impression. The female rook 

 usually does not permit copulation until 

 after the nest has been built by the male 

 (Marshall and Coombs, 1957) . In the Euro- 

 pean coot, another species in which the nest 

 is built by the male, copulation is also de- 

 layed until after the main shell of the nest 

 is built (Kornowski, 1957) . In these cases it 

 may be suggested that the nest-building 

 activity of the male may play some role in 

 stimulating those physiologic changes in the 

 female which induce sexual receptivity (see 

 below, p. 1275. 



Nest-building during incubation. Cases in 

 which nest-building continues into the incu- 

 bation period are for the most part of two 

 general types, (a) There are some species 

 in which the main part of the nest is com- 

 pleted before any eggs are laid, and in which 

 the lining of the nest (with a material differ- 

 ent from that used for the main construc- 

 tion) may continue during incubation (Cape 

 weaver, Skead, 1947; graceful warbler, Sim- 

 mons, 1954; bank swallow, Petersen, 1955). 

 This suggests that, in such species, the se- 

 lection of the different materials may have 

 different hormonal bases, a suggestion for 

 which there is some experimental evidence 

 (see below, p. 1274). (b) Many species of 

 gulls, terns, and shorebirds continue to build 

 up the nest during the incubation period by 

 virtue of a tendency to pick up nesting 

 materials and drop or incorporate them in 

 the nest whenever the birds' need to sit on 

 the eggs is frustrated, or in conflict with 

 some other behavioral tendency. Such nest- 

 building has been called "displacement nest- 

 building" (Tinbergen, 1952). It may occur, 

 for example, when the bird is sitting on eggs 

 abnormal in shape, size, or number (Moyni- 

 han, 1955; Baggerman, Baerends, Heikens 



and ]\Iook, 1956), or when the members of 

 the pair relieve each other at the nest {e.g., 

 Cuthbert, 1954; Ytreberg, 1956). Baerends 

 (1959) found that such displacement nest- 

 building by a sitting bird also occurs when 

 the temperature of the eggs departs too much 

 from an optimal level. 



3. Nest-building and Gonadal Cycles 



So far, we have established a probable 

 temporal relationship between nest-building 

 and copulation, at least in those species in 

 which the female participates in nest-build- 

 ing. As a further step in the analysis of the 

 cyclic basis of nest-building behavior, we 

 may now look into the relationship between 

 the timing of nest-building activity and of 

 the maximal activity of the ovarian follicle. 

 In the absence of very much direct evidence 

 on this point, let us adopt the somewhat 

 roundabout procedure of considering, first, 

 the timing of follicle growth, and then the 

 timing of nest-building activity, in the cycle. 



Relation of follicle growth to time of egg- 

 laying (see chapter by van Tienhoven). The 

 ovary of a bird at the egg-laying stage looks 

 like a cluster of ova of varying size, the larg- 

 est being the one that is nearest to being ovu- 

 lated. If these ova are measured at the au- 

 topsy of a laying bird, the measurements 

 form a graded series, which can be arranged 

 in order, from most mature to least mature. 

 If the interval between successive eggs is 

 known for the species, and if the time of last 

 ovulation is known for the individual, it is a 

 simple matter to calculate the age (in days 

 before ovulation) of each of the larger ova. 

 The size of the ova can then be plotted as a 

 function of pre-ovulatory age. In addition, 

 the growth rate at each day before ovulation 

 can be calculated by comparing the sizes 

 of successive ova, the growth rate between 

 day a and day b being a function of the in- 

 crease in size of the ovum between day a 

 and day b, in relation to the absolute size 

 on day a (Romanoff, 1943). 



In a wide variety of species of birds which 

 have been studied, there is a sharp increase 

 in follicle growth rate starting from 4 to 11 

 days pre-ovulation, depending on the spe- 

 cies (Romanoff and Romanoff, 1949). Ro- 

 manoff (1943) has shown that the actual 



