PARENTAL BEHAVIOR 



1277 



cty of egg-laying patterns, and therefore of 

 relevant endocrine situations, as we noted 

 in the case of nest-building. 



Size of clutch. Birds typically lay a clutch 

 consisting of a definite number of eggs, and 

 then incubate the eggs until they hatch. 

 Some species of birds produce only one such 

 clutch per year; others may breed twice, or 

 even three times a year, but with the breed- 

 ing always restricted to a definite part of 

 the year. In the temperate zones breeding 

 is always in the spring and summer; in the 

 tropics some species may breed during the 

 wet season, others during the dry season, 

 and in a few cases breeding may be all year 

 round. 



The number of eggs constituting a clutch 

 varies from species to species within wide 

 limits. Some birds, such as the large pen- 

 guins, most auks and murres, petrels, and 

 some others lay only 1 egg. Some birds, such 

 as most species of pigeons and doves, char- 

 acteristically lay 2 eggs. Most gulls lay and 

 incubate 3-egg clutches. Most songbirds 

 lay from 4 to 7 eggs. Large clutches, rather 

 variable in size, are laid by ducks and 

 geese, and gallinaceous birds such as par- 

 tridges, pheasants, etc., may lay up to 20 

 eggs in a clutch. The domestic chicken is, 

 of course, derived by selection from ances- 

 tral birds of the latter type (Mayaud, 

 1950). 



Laying pattern. In those species in which 

 the clutch consists of more than one egg, the 

 interval between eggs is subject to wide in- 

 terspecific variation (Mayaud, 1950). Data 

 on the exact time of egg-laying are not as 

 generally available for wild birds, of course, 

 as they are for domestic birds, but certain 

 wild birds, such as some species of ducks, 

 appear to have a pattern like that of the 

 domestic hen — they lay eggs at intervals of 

 20 to 24 hours. Most pigeons lay their 2 

 eggs about 40 hours apart (Whitman, 1919) . 

 Although the most common pattern appears 

 to be for the birds to lay their eggs on suc- 

 cessive days, there are species in which the 

 interval is much longer such as the black- 

 headed gull, in which the interval is about 

 42 hours (Weidmann, 1956), some boobies, 

 in which the interval may be 6 or 7 days 

 (Mayaud, 1950) , and many others. 



Brood parasitism. An unusually interest- 



ing jihenomenon which poses several un- 

 usual endocrinologic problems is the occur- 

 rence of brood jiarasitism in several families 

 of birds. Parasitic birds do not build nests 

 of their own, nor do they incubate their 

 eggs. Instead, the female lays her eggs in 

 the nests of other (''host") species, and the 

 hosts incubate the eggs and rear the young. 

 This type of breeding habit appears to have 

 evolved independently in several different 

 families of birds. Of the 200 species of the 

 order Cuculiformes, some 80 are to some 

 degree jiarasitic (Makatsch, 1937) , includ- 

 ing all 40 of the species of cuckoos living in 

 the old world (Southern, 1954). Parasitism 

 has also evolved among the cowbirds, a sub- 

 family of blackbirds living in the new world 

 (Friedmann, 1929), and in the honey guides 

 of Africa (Friedmann, 1955). In addition, 

 individuals of many other species of several 

 families, especially ducks, quail, and pheas- 

 ants, may breed parasitically more or less 

 frequently (Weller, 1959). 



i. Hormonal Relations in Ovulation and 

 Egg-laying 



This is not the place for a detailed dis- 

 cussion of the hormonal basis of ovulation 

 and egg-laying, since these matters are ex- 

 tensively discussed in the chapter by van 

 Tienhoven. However, a brief summary will 

 serve as an introduction to certain prob- 

 lems concerning the regulation of egg-laying 

 behavior. 



The ovarian follicle grows under the in- 

 fluence of a gonadotrophic hormone from 

 the pituitary gland, which is presumably 

 similar to mammalian follicle-stimulating 

 hormone (FSH). The growing follicle se- 

 cretes estrogenic hormone, which in turn 

 stimulates growth of the oviduct. When the 

 follicle has reached ovulatory size, an ovu- 

 lation-inducing hormone, presumably simi- 

 lar to luteinizing hormone (LH), induces 

 the release of the egg. Traps <1955) sug- 

 gests that progesterone (or a progestin) 

 from the ovary induces the secretion of the 

 ovulation-inducing hormone by the pitui- 

 tary gland (Rothchild and Fraps, 1949). 

 Progesterone has been found in the blood 

 plasma of laying hens (Fraps, Hooker and 

 Forbes, 1948; Layne, Common, Maw and 

 Fraps, 1957; Lytle and Lorenz, 1958), non- 



