266 FLEAS, FLUKES AND CUCKOOS 



It is probably a combination of these two factors which is responsible 

 for the origins of brood parasitism in this genus. 



Among cuckoos we find there is the same tendency. The majority of 

 the American species build their own rather simple nests in the forks of 

 trees or on the ground. One or two species, however, such as the yellow- 

 billed cuckoo {Coccyzus americanus) — which is a rare vagrant in Britain, 

 recorded in this country on about fourteen occasions — frequently usurp 

 other birds' nests in which they lay and incubate their own eggs. 

 Again others, such as the ani {Crotophagus ani) lay in mutual or com- 

 munal nests. Obviously, in the cuckoo family as well as in the cow- 

 birds, nest-seizing is an important step in the development of the para- 

 sitic habit, although in the case of many cuckoos there is no slackening 

 of the territorial instinct — at least on the part of the female bird. 



Every countryman knows that a china tgg placed in a hen's nest 

 will encourage her to lay. It is quite possible that the parasitic habit in 

 certain birds originally arose from an exaggeration or perversion of this 

 psychological response. Thus, a female with an tgg in her oviduct, on 

 catching sight of an incomplete clutch, would be seized with an un- 

 controllable urge to lay then and there, even in another bird's nest. 

 Among ducks and geese, which build open nests on the ground, it is not 

 uncommon for two females of the same or different species to lay in one 

 nest — the original owner incubating and rearing the brood. At Myvatn 

 in Iceland, where up to twelve species of duck breed in an identical 

 habitat and in a similar style, there is much "adventitious parasitism" 

 of this type. It is particularly marked in the case of the long-tailed 

 duck. One South American duck {Heteronetta atricapilla) is an obligate 

 brood parasite, which lays her eggs principally in nests of other ducks 

 but also in those of various ground nesting species such as gulls, waders 

 and coots. It is quite likely that visual stimulation plays a big part in 

 determining her actions. When the brooding drive is uppermost some 

 gulls will attempt to "incubate" golf balls, tins or even suitably shaped 

 cakes if they are placed in their nests. It is, therefore, surprising that 

 brood parasitism is not found in this type of colonial bird. Certain 

 species of weaver-birds which are also colonial nesters have in fact ex- 

 ploited this situation. A less numerous species mingles with the main 

 colony and surreptitiously introduces its eggs into suitable nests. 



Notwithstanding the very strong brooding instinct shown by many 

 birds, this drive can be easily lost if it is not maintained by natural 

 selection. For instance a fact with which most countrymen are familiar 



