We can, no doubt, learn best by observing the behaviour of nestlings and young birds; nevertheless, I find much even in the confirmed habits of adults to strengthen me in the belief that fear of particular enemies is in nearly all cases--for I will not say all--the result of experience and tradition.
Hawks are the most open, violent, and persistent enemies birds have; and it is really wonderful to see how well the persecuted kinds appear to know the power for mischief possessed by different raptorial species, and how exactly the amount of alarm exhibited is in proportion to the extent of the danger to be apprehended. Some raptors never attack birds, others only occasionally; still others prey only on the young and feeble; and, speaking of La Plata district, where I have observed hawks, from the milvago chimango--chiefly a carrion-eater--to the destructive peregrine falcon, there is a very great variety of predatory habits, and all degrees of courage to be found; yet all these raptors are treated differently by species liable to be preyed on, and have just as much respect paid them as their strength and daring entitles them to, and no more, So much discrimination must seem almost incredible to those who are not very familiar with the manners of wild birds; I do not think it could exist if the fear shown resulted from instinct or inherited habit. There would be no end to the blunders of such an instinct as that; and in regions where hawks are extremely abundant most of the birds would be in a constant state of trepidation. On the pampas the appearance of the comparatively harmless chimango excites not the least alarm among small birds, yet at a distance it closely resembles a henharrier, and it also readily attacks young, sick, and wounded birds; all others know how little they have to fear from it. When it appears unexpectedly, sweeping over a hedge or grove with a rapid flight, it is sometimes mistaken for a more dangerous species; there is then a little flutter of alarm, some birds springing into the air, but in two or three seconds of time they discover their mistake, and settle down quietly again, taking no further notice of the despised carrion-eater. On the other hand, I have frequently mistaken a harrier (Circus cinereus, in the brown state of plumage) for a chimango, and have only discovered my mistake by seeing the commotion among the small birds. The harrier I have mentioned, also the C. macropterus, feed partly on small birds, which they flush from the ground and strike down with their claws. When the harrier appears moving along with a loitering flight near the surface, it is everywhere attended by a little whirlwind of alarm, small birds screaming or chirping excitedly and diving into the grass or bushes; but the alarm does not spread far, and subsides as soon as the hawk has passed on its way. Buzzards (Buteo and Urubitinga) are much more feared, and create a more widespread alarm, and they ars certainly more destructive to birds than harriers. 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.