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The language of birds: 2. Calls (continued)

In the vocabulary of the chaffinch and others, there is one alarm call specifically to warn of aerial predators, and another alarm call used in other circumstances, for example, when a ground enemy appears - like a fox Vulpes vulpes. This distinction can be seen even with the domestic fowl in a farmyard. Imagine a scenario with a mother hen and recently hatched brood that she is tending protectively, leading them from one suitable feeding area to the next. As she walks, she clucks and these sounds encourage the chicks to follow her; when she finds food she gives a distinctive "kuk-kuk-kuk-kuk" in rapid sequence and her chicks come running to the precise spot. Now, should a hawk appear suddenly from nowhere, the mother hen at once gives a loud raucous scream and the chicks scatter and hide under or against the nearest shelter. Their response when their mother gives the ground predator alarm - a quite different "cut!-cut-cut-cut-kaaaah!" - is also quite different: they freeze exactly where they are and that way stand the best chance of survival. Thus it is that the domestic chicken applies in its farm habitat, to which man introduced it, the sound communication system its ancestors (red jungle fowl Gallus gallus) evolved in the rain forests of India and south-east Asia. The hawk has a commanding view so it is necessary to hide under something if possible, despite the fact that by running you may draw attention to yourself. The fox has only a horizontal perspective, and to best defeat him it is best to crouch stock still and hope to escape detection that way.

In almost any British garden this same distinction may be observed with robins. In response to a prowling cat the robin gives a sharp "tick tick" call; to warn of a passing crow Corvus corone a thin "seep" note is rather quietly given.

Listen
Robin Erithacus rubecula
'tic' call, recorded by William Pedley, 14 October 1973, Woodchester Park, Gloucestershire, England.

Listen
Robin Erithacus rubecula
'seee' call, recorded by Philip Rudkin, 25 May 1980, Shortwood, Oundle, Northamptonshire, England.

This "seep" note brings us to one vital and fascinating difference between song, which we said was attended to only by other individuals of the singer's species, and some (not all) calls which are understood and responded to by species other than that of the caller. In other words, wrens and others are unaffected by robin song, but understand and respond to robin alarm calls. This is, of course, mutual aid, but only "in effect", it is not "intentional". This between-species communication has probably come about because of convergent evolution; thus several species facing the same problem came up with the same answer - a call with particular physical characteristics - and then found they were all "speaking the same language"! If you make voice prints on transparent paper of the hawk-alarm calls of chaffinch, blackbird, great tit, blue tit Parus caeruleus and others and place them on top of one another, the match is very close indeed.

Sonogram - Blackbird hawk-alarm call (3K) Sonogram - Great Tit  hawk-alarm call (4K)
Blackbird
Great tit
Sonogram - Robin  hawk-alarm call (4K) Sonogram - Song Thrush hawk-alarm call (4K)
Robin
Song thrush

Calls of different passerine bird species given when a hawk flies over. These calls are difficult to locate.

Listen
Great Tit Parus major
'seee' calls, recorded by Richard Ranft, 10 January 1988, Brook Wood near Bromley, Kent, England.


One major reason why they need to be similar is that all the birds, while needing to give warning as already explained, nevertheless employ as unlocatable a sound as possible. The remarkable thing about this hawk-alarm call is that its physical' form is such that when you hear the sound it is difficult to detect from which direction it comes. And if you were to ask an acoustics engineer to come up with such a sound (preferably without telling him why you want to know!) it is exactly this character of sound he would recommend. The hawk-alarm call is (a) high-pitched, (b) almost pure in tone and (c) starts quietly, gets louder and then fades slowly, and these three characteristics make the call (and therefore the caller) difficult to locate.

In contrast, the most important characteristic of the mobbing call is that it is locatable. This is the call given by birds wishing to rally reinforcements to help drive off a bird menacing them. If you were a wren and you "wished" to work alongside another wren already harassing an owl, you would know at once in which direction the help was needed. The voice prints of the mobbing calls of the chaffinch, blackbird, robin, wren and mistle thrush are very similar and are utterly different from the highly unlocatable hawk-alarm calls. Mobbing calls are sharp sounds, almost like hand claps, their direction of origin comparatively easy to detect and like the hawk-alarm calls they too are understood by different species, as for example the "mayday" distress call is understood by mariners who speak different languages.

Sonogram - Blackbird mobbing calls (5K) Sonogram - Great Tit mobbing calls (8K)
Blackbird
Great tit
Sonogram - Robin mobbing calls (5K) Sonogram - Song Thrush mobbing calls (8K)
Robin
Song thrush

Calls from four different passerine bird species given while mobbing a predator. These calls are highly locatable.

Listen
Great Tit Parus major
mobbing calls, recorded by Victor Lewis, 6 June 1965, Gloucestershire, England.

It is also noticeable that the mobbing calls are louder than the hawk-alarm calls, for the bigger the catchment area from which you draw your "pals" the better for the mobber. But for the raiser of the hawk-alarm it would be advantageous if the carrying power of its call could embrace only his companions but not his enemy. In many instances, his companions will be closer to him than the enemy, and if the loudness of his call could be so arranged that at their distance they hear it but by the enemy's distance it had attenuated to nothing, then what an ideal situation!

The ability of birds of prey to pinpoint the hawk-alarm "seeet" call of the American robin Turdus migratorius has been the subject of a recent experiment. Captive red-tailed hawks Buteo jamaicensis were placed in turn in a cage in the middle of a wood, they were then played a recording of the hawk-alarm call from a hidden loudspeaker. On hearing the sound the birds responded at once but averaged 84 off target! They then rotated their heads as if searching for the right direction and ended up an average of 124 off target. This is enough to justify labelling the "seeet" call "ventriloqual". For comparison, the same birds were also played the highly locatable mobbing calls of the red-winged blackbird Agelaius phoenicus. Their directional response to this was an average of only 50 off beam.

The sharp "chack" warning call of the stonechat is another easily locatable sound and a recent suggestion is that this "chack" cry, plus a conspicuous wing-flicking action, functions as a distraction display. When a potential predator approaches the nest it will be lured away by these antics.

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