1874.] MOLOTHRI OF BUENOS AYRES. 163 



undergoes, and in the peculiar language and gestures of these four 

 species that is complex enough ; but the complexity will probably be 

 much increased when we become familiar with the instincts of the 

 other members of the genus. We may wait to hear something out 

 of the common in their nesting-habits, as confidently as we expect 

 to find pale green eggs in the nest of a Coccyzus or feathers in the 

 stomach of a Grebe. 



April 15, 1873. — This morning I started in quest of the Bay-wings. 

 As soon as I got near them (for they were in the usual place) I ob- 

 served one bird, that had somehow escaped detection the day before, 

 assuming the purple plumage. This bird I shot ; and after the flock 

 had resettled a short distance off, I crept close up to them under the 

 shelter of a hedge to observe them more narrowly. One of the 

 adults was closely attended by three young birds ; and they all, whilst 

 1 observed them, fluttered their wings and clamoured for food each 

 time the parent bird stirred on her perch. One of the three young- 

 birds was spotted with purple ; and this bird I brought down, together 

 with its foster-parent and one of its foster-brothers. These last two 

 specimens (for I could see no more) were more interesting than the 

 others I had obtained, as they had fewer purple feathers ; and it may 

 be seen in them how closely at first these birds resemble their foster- 

 brothers the young of If. badius. The hunger-cry of the youn°- 

 If. badius is quite different from that of the young If. bonariensis. 

 The cry of the latter is a shrill two-syllabled note, the last syllable 

 being lengthened out into a continuous squeal when the foster- 

 parent approaches to feed it. The hunger-cry of the young 

 If. badius is short, somewhat strident, tremulous, and unin- 

 flected. The resemblance of the young If. rufoaxillaris to its 

 foster-brothers in language and plumage is the more remarkable 

 when we reflect that the adult If. rufoaxillaris in all its habits, 

 gestures, and notes, as well as in its purple plumage, comes vastly 

 nearer to If. bonariensis than to If. badius. It seems impossible 

 for mimicry to go further than this. A slight difference in size is 

 quite imperceptable when the birds are flying about ; but in language 

 and plumage the keenest ornithologist would not detect a differ- 

 ence. But it may be questioned whether this is in reality a case of 

 an external resemblance of one species to another acquired by natural 

 selection for its better preservation. Is it not as reasonable to be- 

 lieve that the young of If. rufoaxillaris in the first stage of its 

 plumage exhibits the ancestral type (that of the progenitor of both 

 species), that it has not supplanted the unvarying and consequently 

 unimproved descendants (If. badius), simply because its elective 

 parasitical instinct has made its existence dependent on that species ? 

 Did the If. badius belong to some other group, Sturnella or Pseudo- 

 leistes for instance, it would not then be possible to doubt that the 

 resemblance of the young If. rufoaxillaris to its foster-brothers 

 resulted from mimicry ; but as the two species belong to the limited 

 group Ifolothrus the resemblance might be ascribed to community 

 of descent. 3 



VIII. Probably Molothrus badius always hatches its oivn eggs. 



