THE VOYAGE OF THE BEAGLE 



63 



its habits. Several may often be seen standing together on 

 the back of a cow or horse; and while perched on a hedge, 

 pluming themselves in the sun, they sometimes attempt to 

 sing, or rather to hiss; the noise being very peculiar, resem- 

 bling that of bubbles of air passing rapidly from a small 

 orifice under water, so as to produce an acute sound. Ac- 

 cording to Azara, this bird, like the cuckoo, deposits its eggs 

 in other birds' nests. I was several times told by the coun- 

 try people that there certainly is some bird having this 

 habit ; and my assistant in collecting, who is a very accurate 

 person, found a nest of the sparrow of this country (Zono- 

 trichia matutina), with one egg in it larger than the others, 

 and of a different colour and shape. In North America 

 there is another species of Molothrus (M. pecoris), which 

 has a similar cuckoo-like habit, and which is most closely 

 allied in every respect to the species from the Plata, even in 

 such trifling peculiarities as standing on the backs of cattle ; 

 it differs only in being a little smaller, and in its plumage 

 and eggs being of a slightly different shade of colour. This 

 close agreement in structure and habits, in representative 

 species coming from opposite quarters of a great conti- 

 nent, always strikes one as interesting, though of common 

 occurrence. 



Mr. Swainson has well remarked, 8 that with the excep- 

 tion of the Molothrus pecoris, to which must be added the 

 M. niger, the cuckoos are the only birds which can be called 

 truly parasitical ; namely, such as " fasten themselves, as it 

 were, on another living animal, whose animal heat brings 

 their young into life, whose food they live upon, and whose 

 death would cause theirs during the period of infancy." It 

 is remarkable that some of the species, but not all, both of 

 the Cuckoo and Molothrus, should agree in this one strange 

 habit of their parasitical propagation, whilst opposed to each 

 other in almost every other habit: the molothrus, like our 

 starling, is eminently sociable, and lives on the open plains 

 without art or disguise: the cuckoo, as every one knows, 

 Is a singularly shy bird ; it frequents the most retired thick- 

 ets, and feeds on fruit and caterpillars. In structure also 

 these two genera are widely removed from each other. 

 8 Magazine of Zoology and Botany, vol. i. p. 217. 



