Sense of Hearing in Birds and Insects

Abstract

I do not know whether ornithologists are acquainted with the peculiar manner in which curlews frequently obtain their food on sandy flats which have been left bare by the tide. The birds force their long bills into the wet sand as far as the nostrils, and then again withdraw it, leaving a small hole, which, when probed, is found to be only just large enough to have taken in the bill. The animal, therefore, can only have made a single prolonged push without adding any lateral or exploring movements of the bill, as birds which feed in mud may be observed to do. Now it cannot be supposed that curlews adopt this mode of feeding without obtaining from it some degree of profit. Neither can it be supposed that they make their thrusts into the sand at random; for, their bills being so pointed and slender, the birds would usually require to make a vast number of ineffectual thrusts before they happened to hit upon a worm or other edible object. The question therefore is, How do the birds know the precise spots where their victims lie buried in the sand? That this knowledge is not derived by sight I am quite sure, for I have repeatedly observed innumerable curlew marks of the kind described occurring on tracts of sand which, in virtue of their high level, presented a perfectly smooth and uniform surface. I can therefore only suppose that the birds are guided in their probings by their sense of hearing. Doubtless it is difficult to believe that this sense is so delicate and precise as to enable the curlew to perceive so exceedingly slight a sound as that which must be caused by the movement, say, of a small worm at a distance of ten or twelve inches from the surface of the sand, and at the same time to localise the exact spot beneath the surface from which so slight a sound proceeds. I cannot see, however, that any other explanation is open, and perhaps the one now offered may not seem so incredible if we remember the case of the thrush. No one, I think, can observe this bird feeding and doubt that it finds its worms and grubs almost exclusively by the sense of hearing. And if the distance which it runs between successive pauses for listening represents—as we cannot but suppose it must—the diameter of the circle within which this bird is able to hear the movements of a worm, I think that the hypothesis I have just advanced with regard to the curlew ceases to be improbable.