112 BIRDS OF LA PLATA 
intrude, there is no reason here to regard the foster- 
parents’ affection as blind and stupid ; the similarity 
being close enough to baffle the keenest sagacity. 
Nor can the instinct here appear in the light of an 
outrage on the maternal affection; for the young 
M. rufoaxillaris possesses no advantage over its 
foster-brothers. It is not endowed with greater 
strength and voracity to monopolise the attentions 
of the foster-parent or to eject the real offspring ; 
but being in every particular precisely like them, 
it has only an equal chance of being preserved. To 
this wonderful parasitical instinct we may well apply 
Darwin’s words, when speaking of the architecture 
of the hive-bee: ‘“‘ Beyond this stage of perfection 
natural selection could not lead.” 
BAY-WINGED COW-BIRD 
Molothrus badius 
Dull grey, or mouse-colour, slightly tinged with olive; wings 
chestnut; tail blackish; bill and feet black; length 7.6 inches. 
Female similar. 
IN this species the sexes are alike ; the plumage of 
the body is grey-drab colour, with a black spot 
between the eye and beak; tail dark, the quills 
cinnamon-colour; beak and legs black. Azara, 
describing it under the name of Tordo pardo roxiso, 
says it is a rare bird, so that it has probably greatly 
