232 To the River Plate and Back 



birds were at that time common in Paraguay, but 

 scarce in the neighborhood of Buenos Aires. Times 

 have changed, and they are now quite common about 

 La Plata, but there is reason to think that they are 

 tropical birds, which for some cause are trying to adapt 

 themselves to the more temperate climate of the south, 

 for which nature has not quite prepared them, as they 

 lack plumage with which to resist the cold. Hudson 

 says that they often die of cold in the winter, in spite 

 of the fact that at that time of the year they have the 

 habit of congregating in flocks and roosting huddled to- 

 gether upon the branches in order to keep warm. They 

 are said to be somewhat foul in their habits, and to be 

 very prolific. The latter fact, and the fact that they 

 find with the advent of civilized man into the region a 

 greater supply of food than was formerly the case, 

 seems to account for their survival and their increase 

 in a part of the country which they have only recently 

 invaded. 



About the roots of the willow-grove there ran a small 

 brooklet, not more than a foot or two wide. What was 

 my surprise to discover that this tiny stream of water 

 was full of mussel-shells and of great fresh-water snails 

 belonging to the genus Ampullaria. I obtained speci- 

 mens of the latter, which my colleague, Dr. Arnold E. 

 Ortmann, since my return has determined to be Am- 

 pullaria canaliculata D'Orbigny. A number of these 

 shells were lying about on the greensward under the 

 trees, evidently recently having been robbed of their 

 content, consisting of the animal which tenanted them 

 in life. Empty shells of the same species were found 

 here and there under telegraph poles and along the 

 fences. The explanation of this fact is found in the 

 habits of the commonest hawk of the region, the Ever- 



