PARENTAL BEHAVIOR 



1287 



justify the impression that readiness to in- 

 cubate by the male (in those species in 

 which the male participates in incubation) 

 is more or less synchronized with the lay- 

 ing of eggs by the female, although, under- 

 standably, not as closely synchronized as is 

 the case with the incubation behavior of the 

 female. 



The incubation patch, (l) Structure and 

 function. When a bird sits on its eggs, it 

 erects the contour feathers on the lower ven- 

 tral body surface, exposing an unfeathered 

 area of skin which in the nonsitting bird is 

 normally covered by the contour feathers 

 lying flat along the body surface (Simmons, 

 1955b; Barth, 1955). This naked area, called 

 the incubation patch, is applied to the sur- 

 face of the eggs, and is the source of the 

 heat which is exchanged between the body 

 of the parent and the egg. Bailey (1952) 

 has provided a detailed description of the 

 incubation patch, which is not merely a 

 nonfeathered area, but one in which there is 

 a characteristic increase in dermal and sub- 

 dermal vascularity, accompanied by edema 

 and by a thickening of the smooth muscle 

 layer of the dermal blood vessels (Petersen, 

 1955). The vascularity and edema are read- 

 ily apparent to the naked eye: the skin of 

 the incubation patch appears distinctly 

 reddish, thickened, and is sometimes thrown 

 into loose folds. 



The area covered by the incubation patch 

 corresponds to the ventral apterium. (The 

 contour feathers of birds grow in definite 

 tracts called pterylae; between these pter- 

 ylae are areas called apteria, in which grow 

 only down feathers, or in some cases no 

 feathers at all.) The distribution and shape 

 of the incubation patches in the various 

 orders of birds correspond with the distribu- 

 tion and shape of their respective ventral 

 apteria. Thus songbirds typically have a 

 single ventral patch. Gulls and shorebirds 

 have paired patches lateral to the midline 

 of the body, sometimes accompanied by a 

 small median patch. No incubation patches 

 are found in the pelican-like birds, or in the 

 ducks and their relatives. 



The behavior of incubating birds sug- 

 gests that contact between the ventral ap- 

 terium and the eggs plays an important role. 

 When starting to sit the bird makes side-to- 



side settling movements which suggest to 

 the observer that it is adjusting the contact 

 between the eggs and ventral body surface. 

 The contour feathers remain erected until 

 these settling movements subside, when the 

 feathers are relaxed around the sides of the 

 eggs. When Weller (1958) removed one of 

 the eggs of a nighthawk, the bird, on re- 

 turning to its eggs, poked with its bill at the 

 incubation patch where the egg should have 

 been, and failed to settle for a considerable 

 time. Gulls have three incubation patches, 

 and the eggs are moved in the nest by the 

 settling motions of the bird and by poking 

 with its bill until each egg is in contact with 

 one of the incubation patches. By observing 

 the under-surface of incubating black- 

 headed gulls from a trench dug under the 

 nest. Beer (1961) found that the settling 

 movements continue until each egg is firmly 

 in contact with one of the patches. 



{£) Incidence. In almost all cases, the oc- 

 currence of incubation patches in the male 

 or in the female of a given species corre- 

 sponds to the occurrence of incubation be- 

 havior. In most species of songbirds only 

 the female incubates, and in almost all cases 

 the incubation patch is found only in the fe- 

 male (Miller, 1931; Price, 1936; Davis, 

 1941; Dixon, 1949; Putnam, 1949; Brack- 

 bill, 1958; and others). In species in which 

 only the male incubates, such as the tina- 

 mous (Pearson and Pearson 1955), phala- 

 ropes, and jacanas (Bailey, 1952) an in- 

 cubation patch is seen only in the male. In 

 many groups of birds, both the male and 

 the female participate in incubation, and 

 in these cases, both sexes have an incubation 

 patch. This includes woodpeckers (Howell, 

 1952), petrels (FLsher, 1952), gulls (John- 

 ston, 1956), and a number of other families 

 (Bailey, 1952). 



The coincidence between tiie occurrence 

 of the incubation patch and of incubation 

 behavior can be followed out in considerable 

 detail. According to Holstein (quoted by 

 Wingstrand, 1943), the male European 

 goshawk has a poorly developed incubation 

 patch, and takes part to only a slight extent 

 in incubation. In the Clark nutcracker, in 

 which the male and female both incubate 

 the eggs (Mewaldt, 1956), unlike most 

 members of its family in which only the fe- 



