PARENTAL BEHAVIOR 



1283 



^vere removed daily (Pearl, 1912; Witschi, 

 1935), and a flicker from whose nest an egg 

 was removed daily, starting with the 2nd 

 egg, laid 72 eggs in 73 days (Phillips, 1887). 

 The wryneck has similarly been reported to 

 lay up to 48 eggs under the same conditions 

 (quoted by Pearl, 1912). Goodwin (1948) 

 reported that the golden pheasant may lay 

 clutches of up to 40 eggs if eggs are removed 

 as laid. 



The best experimental work on indetermi- 

 nate egg-laying has been done with gulls, 

 which normally lay 3 eggs, at intervals of 

 about 2 days (Goethe, 1937; Tinbergen, 

 1953). In the case of the black-headed gull, 

 the average time between the laying of the 

 1st and the 3rd eggs is about 84 hours {ZV2 

 days) (Weidmann, 1956). The experiments 

 of Weidmann on the black-headed gull may 

 be summarized as follows (see also Ytre- 

 berg. 1956): 



If the 1st egg is removed just after it is 

 laid, the birds will lay a 4th egg, so that 

 they end up with a 3-egg clutch. In these 

 cases the 4th egg is laid at a normal interval 

 after the 3rd. If successive eggs are removed 

 as they are laid, most birds will lay more 

 than 4 eggs, Weidmann having found birds 

 laying up to 7 eggs. If the 1st egg is left in 

 the nest, and subsequent eggs are removed 

 as they are laid, no birds lay more than 3 

 eggs. If both eggs are removed after the 2nd 

 <egg is laid, some of the birds will lay a 4th 

 legg, others will stop at the 3rd egg and in- 

 cubate it, although it is now the only egg 

 in the nest. If the birds are allowed to lay 

 3 eggs, and the whole clutch is removed 

 immediately after the laying of the 3rd egg, 

 all birds desert, none laying a 4th egg. 



In seeking to account for these results, it 

 is important to note that gulls begin incu- 

 bating with the laying of the 1st egg (Tin- 

 bergen, 1953; Weidmann, 1956). Weidmann 

 suggested that no additional eggs are laid 

 if the birds incubate, even though they are 

 incubating an incomplete clutch. Since many 

 birds will lay a 4th egg if the eggs are re- 

 moved after the laying of the 2nd egg 

 (when the birds have been incubating for 

 about 2 days) , but will lay no 4th egg if the 

 eggs are removed after the laying of the 3rd 

 egg (when the birds have been incubating 

 for about 4 days), Weidmann's suggestion 



is that, in these birds, about 4 days of 

 brooding will suppress the production of 

 further eggs. Paludan's (1951) results on 

 the herring gull are in general similar to 

 those of Weidmann. When birds were killed 

 for autopsy after the laying of the 1st egg, 

 a 4th, and sometimes a 5th follicle were 

 found maturing in the ovary ; but when the 

 l)ird was not killed until after it had been 

 incubating for 2 or 3 days these follicles 

 were found degenerating. 



If Weidmann is correct in suggesting that 

 participation in incubation provides stimu- 

 lation which suppresses the production of 

 further eggs, we might expect to find that 

 the addition of eggs to the nest before lay- 

 ing has started would suppress the laying 

 of some or all of the clutch, and that this 

 effect would be found only when the birds 

 start to incubate them before they lay. 

 There is some evidence that these assump- 

 tions are indeed true, although they have 

 not yet been thoroughly exi)lored. Poulsen 

 (1953b) found that, although he could not 

 induce domestic pigeons to increase the size 

 of the clutch beyond the normal 2 eggs by 

 removing the 1st egg immediately after it 

 was laid, he could suppress egg-laying alto- 

 gether in about 50 per cent of the birds by 

 placing 2 eggs in the finished, empty nest, 

 whereui:)on all the birds immediately began 

 to incubate. We have obtained similar re- 

 sults with the ring dove. Although barn 

 swallows, American magpies (Davis, 1955), 

 and tricolored redwinged blackbirds (Em- 

 len, 1941) do not lay more eggs as a result 

 of having the eggs removed as they are laid, 

 there is some evidence that the addition of 

 eggs near the beginning of egg-laying may 

 inhibit the production of eggs. It may be 

 pointed out that these birds normally do not 

 incubate until after the entire clutch is laid. 



It is apparent that stimuli provided by 

 the egg, in all probability through the act 

 of incubation, have, in many species, the 

 ability to change the pattern of pituitary 

 secretion in such a manner that the inhibi- 

 tion or suppression of the full maturation 

 of some of the follicles is brought about. We 

 shall reserve discussion of the nature of the 

 stimulus and of the effect for our analysis 

 of the physiologic basis of incubation be- 

 havior (see below, page 1295), 



