18/4.] MOLOTHRI OF BUENOS AYRES. 159 



although the hird itself cannot understand its foster-parents, and 

 welcomes all things that, whether with good or evil design, come 

 near it, the unfitness is not so apparent as when the nest is in open 

 fields and plains. 



The young M. bonariensis differs from the true offspriugof its foster- 

 parents in its habit of quitting the nest as soon as it is able, trying 

 to follow the old bird, and placing itself in the most conspicuous 

 place it can find, such as the summit of a stalk or weed, and there 

 demanding food with frequent and importunate cries. Thus the 

 little Polioptila had acquired the habit of perching on the back of its 

 charge to feed it, because parent birds invariably perch above their 

 young to feed them ; and the young If. bonariensis prevented this by 

 always sitting on the summit of the stalk it perched on. The habit 

 is most fatal on the open and closely cropped pampas inhabited by 

 the Cachila (Anthus correnderd). In December, when the Cachila 

 rears its second brood, the Milvago chimango also has young, and 

 feeds them almost exclusively on the young of other, chiefly small, 

 birds. At this season the Chimango destroys great numbers of the 

 young of the Cachila and of Anitmbius rufigularis. Yet these birds 

 are beautifully adapted in structure, coloration, and habits to their 

 station. It thus happens that in districts where the Molothrus is 

 abundant, their eggs are found in a majority of the Cachilas' nests : 

 and yet to find here a young M. bonariensis out of the nest is a rare 

 thing ; for as soon as they are able to quit the nest and expose them- 

 selves they are all or nearly all carried away by the Chimango. 



V. Nidification of Molothrus badius. — A pair of Leiiateros 

 (Anumbius acuticaudatus) have been nearly all the winter building a 

 great nest on a locust-tree within sixty yards of the house. This nest 

 is about 27 inches deep and 16 or 18 in circumference, and appears 

 now nearly completed. To-day (September 28, 18/2) I saw a bay- 

 winged Molothrus on the nest ; it climbed about it, deliberately in- 

 specting every part, taking up and rearranging some sticks and 

 throwing others down. Whilst thus engaged, two Blackbirds (J/. 

 bonai-iensis), male and female, came to the tree; the female dropped 

 into the nest, and began also to examine it, peering curiously into the 

 entrance and quarrelling with the first bird. After a few minutes she 

 flew off followed by her consort. The Bay-wing continued its strange 

 futile work until the owners of the nest appeared, whereupon it hopped 

 leisurely to one side, sung for a few moments, and then flew away. 

 The similarity in the behaviour of the two birds struck me very 

 forcibly ; in the great interest they take in the nests of other birds, 

 especially in large covered nests, the two species are identical. But 

 when the breeding-season has come their habits begin to diverge : 

 then the M. bonariensis lays in nests of other species, abandon- 

 ing its eggs to their care ; whilst the Bay-wings usually seize on 

 the nests of other birds, and rear their own young. Yet, as they 

 do occasionally build a neat elaborate nest for themselves, the habit 

 of taking possession of nests of other birds is probably recently 

 acquired ; probably also its tendency is to eradicate the primitive 

 building-instinct. 



