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 sub- 
ject, 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 uni- 
formly of a dark grey colour. Unfortunately, 
though I have observed it not fewer than a hun- 
dred times, I have never been able to procure 
a specimen, nor do I find that it is lke 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 pro- 
ducing 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 wit- 
nessing 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 
