Another curious instance is that of
the sociable hawk (Rostrhanrus sociabilis). This bird spends the summer
and breeds in marshes in La Plata, and birds pay no attention to it, for
it feeds exclusively on water-snails (Ampullaria). But when it visits
woods and plantations to roost, during migration, its appearance creates
as much alarm as that of a true buzzard, which it closely resembles.
Wood-birds, unaccustomed to see it, do not know its peculiar preying
habits, and how little they need fear its presence. I may also mention
that the birds of La Plata seem to fear the kite-like Elanus less than
other hawks, and I believe that its singular resemblance to the common
gull of the district in its size, snowy-white plumage and manner of
flight, has a deceptive effect on most species, and makes them so little
suspicious of it,
The wide-ranging peregrine falcon is a common species in La Plata,
although, oddly enough, not included in any notice of the avifauna of
that region before 1888. The consternation caused among birds by its
appearance is vastly greater than that produced by any of the raptors I
have mentioned: and it is unquestionably very much more destructive to
birds, since it preys exclusively on them, and, as a rule, merely picks
the flesh from the head and neck, and leaves the untouched body to its
jackal, the carrion-hawk. When the peregrine appears speeding through
the air in a straight line at a great height, the feathered world, as
far as one able to see, is thrown into the greatest commo-tion, all
birds, from the smallest up to species large as duck, ibis, and curlew,
rushing about in the air as if distracted.
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