Alarm and mobbing calls
Imagine a flock of waders on a mud flat. At any one point in time, several birds will have their heads up while the rest are probing the ooze for food. One of these spots a peregrine falcon, gives its alarm call, and in an instant all have their heads up and are ready to take appropriate action.
In the vocabulary of some birds, 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. In almost any British garden this distinction may be observed with robins. In response to a prowling cat the robin gives a sharp “tic tic” call; to warn of a passing crow a thin “seee” note is rather quietly given.
Robin 'tic' alarm call
“Tic” call of a robin, recorded in Woodchester Park, Gloucestershire, England, by William Pedley.
The 'seee' hawk-alarm call
This “seee” note brings us to one vital and fascinating difference between song - which is attended to only by other individuals of the singer's species - and some 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”.
Robin 'seee' alarm call
The “seee” alarm call of a robin, recorded in Northamptonshire, England, by Philip Rudkin.
One major reason why they need to be similar is that all the birds, while needing to give warning, 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.
The hawk-alarm call is:
- almost pure in tone
- starts quietly, gets louder and then fades slowly.
These three characteristics make the call (and therefore the caller) difficult to locate.
Great tit 'seee' alarm call
The “seee” alarm calls of a great tit, recorded in Kent, England, by Richard Ranft.
In contrast with the hawk-alarm call, 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.
Mobbing calls are sharp sounds, almost like hand claps, and their direction of origin is comparatively easy to detect. 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.
An experiment with red-tailed hawks
Captive red-tailed hawks were placed in turn in a cage in the middle of a wood. They were then played a recording of the “seeet” hawk alarm call of the American robin 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. Their directional response to this was an average of only 50° off beam.