Fear in Birds. 95
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. When the falcon has disappeared
96 The Naturalist in La Plata.
in the sky, and the wave of terror attending its progress subsides behind it, the birds still continue wild and excited for some time, showing how deeply they have been moved; for, as a rule, fear is exceedingly transitory in its effects on animals.
I must, before concluding this part of my subject, mention another raptor, also a true falcon, but differing from the peregrine in being exclusively a marsh-hawk. In size it is nearly a third less than the male peregrine, which it resembles in its sharp wings and manner of flight, but its flight is much more rapid. The whole plumage, is uniformly of a dark grey colour. Unfortunately, though I have observed it not fewer than a hundred times, I have never been able to procure a specimen, nor do I find that it is like any American falcon already described; so that for the present it must remain nameless. Judging solely from the effect produced by the appearance of this hawk, it must be even more daring and destructive than its larger relation, the peregrine. It flies at a great height, and sometimes descends vertically and with extraordinary velocity, the wings producing a sound like a deep-toned horn. The sound is doubtless produced at will, and is certainly less advantageous to the hawk than to the birds it pursues. No doubt it can afford to despise the wing-power of its quarry; and I have sometimes thought that it takes a tyrannous delight in witnessing the consternation caused by its hollow trumpeting sound. This may be only a fancy, but some hawks do certainly take pleasure in pursuing and striking birds when not seeking prey. The
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peregrine has been observed, Baird says, capturing birds, only to kill and drop them. Many of the Felidae, we know, evince a similar habit; only these prolong their pleasure by practising a more refined and deliberate cruelty.