THE EUROPEAN CUCKOO 265 



As for the young cuckoos which actually hatch, 43 per cent, die before 

 they are twenty days old. 



It is hardly surprising, therefore, that only a small number of species 

 have travelled successfully along such difficult and hazardous paths and 

 that we find brood-parasitism is relatively a rare phenomenon among 

 birds. Nevertheless it has arisen independently in several unrelated 

 families, and is found among American starlings or hang-nests 

 (Icteridae), African weaver-birds (Ploceidae), honey-guides (Indicatori- 

 dae, which are closely related to woodpeckers), ducks (Anatidae) and 

 cuckoos (CucuHdae). Within the last family the habit has probably been 

 developed several times over as it is highly uidikely that all the parasitic 

 cuckoos known to-day are descended from a single parasitic ancestor. 

 Some knowledge of these related forms is useful for a proper under- 

 standing of the European cuckoo. 



Brood-parasitism has probably originated in several different ways. 

 An important factor in the development of the habit must be the im- 

 pulse, manifest in a number of birds, to use or usurp the nests of other 

 species. In Britain it is a commonplace occurrence for sparrows to 

 drive out martins and swallows, and to raise their young in the vacated 

 nests. Stock-doves will make use of old magpies' nests and starlings 

 breed in cavities excavated by woodpeckers. Such examples could be 

 multiplied almost indefinitely. 



Transitional stages between this casual seizing of other birds' nests 

 and the total loss of nest-building instincts can be followed in some of 

 the cow-birds [Molothnis). The bay- winged cow-bird {Molothrus badius) 

 occasionally builds her own nest and broods her own young. More 

 frequently she usurps the nests of other birds, which she repairs or alters 

 and ejects or builds over any eggs which may already be present. 

 Quite often several females take a fancy to the same nest and lay in it- 

 one female only, however, incubates the multiple clutch. A close 

 relative, the screaming cow-bird (M. rufo-axillaris) , has progressed 

 considerably farther in the same direction and is an obligate brood 

 parasite. The female neither builds a nest, nor takes any interest in the 

 welfare of her young. She lays her eggs either singly or in twos or threes 

 almost exclusively in the nests of her close relative, the bay- winged 

 cow-bird. In fact she has become dependent on the latter species for 

 survival and is only found within the same geographical area. In 

 addition to the acquired habit of usurping nests, the cow-birds manifest 

 a progressive weakening of the protective territorial instinct of the males. 



