Probably most bird-watchers aim to identify birds' songs correctly; many fewer, however, study birds' alarm calls which are perhaps more important than song in the survival of a bird species. Yet opportunities for hearing alarms calls occur almost daily and are common even in town gardens because of the large population of domestic cats. When small birds sight a hawk overhead they usually give a high-pitched call and flee into cover. These calls are difficult to localise and, at least for those I have heard, very similar in form. But when a potential enemy is on the ground, or perched, the calls are distinct and easy to locate and other birds are attracted quickly to the scene. The newcomers either provide moral support or else they join the general clamour of alarm, as a mobbing reaction. An obvious example would be a a group of blue and great tits, chaffinches blackbirds and a robin protesting at a tawny owl as it emerges from its evening roost on an ivy-covered forest tree. Such a performance may be observed I almost any wood and at most times of the year.
A group of blue tits mob and eject an owl from a bush, 29 Sept 2006
I can understand that birds' alarm calls do not attract the attention of the wildlife sound recordist in the same way as bird-song. Yet birds which are concentrating their energies on mobbing a predator are often approached more easily than those singing on their territorial boundaries. Also, as alarm notes, in the main, are of lower frequency than those of song, recordings do not get over-modulated so readily. Some biologists, largely to save time, study birds alarm by noting reactions to stuffed predators; of course, under those circumstances the alarm response usually fades rapidly. In addition, should this method be used during the breeding season then disturbance of the birds must be kept to the minimum for ethical reasons.
There is no doubt the season influences the intensity and type of alarm call produced. Not surprisingly, responses are most marked during the breeding season when birds have nests and young to protect. One unusual call, which can hardly be confused with any other, is given by the widely distributed willow warbler and sounds rather like the screaming of a small mammal in distress; in all probability it is only uttered by birds in breeding condition. The willow warbler approaches the predator along a branch while opening and closing its wings and making a shrill, impassioned cry; I have heard this call in the presence of a kestrel, a cuckoo and a grass snake. As far as I know, calls of this quality are not given by other bird species in this country expect brooding starlings. Of course, the common anxiety note of a willow warbler is the high pitched call "hu-eet, hu-eet". A very similar call is produced by another leaf-warbler, the chiffchaff. Yet chiffchaffs make no call resembling the screaming form of willow warbler alarm just described, and the remaining British leaf-warbler, the wood warbler, does not make this alarm call. This warbler's alarm, made when a buzzard, for instance, alights on a tree near its nest, is a repeated and high-frequency "pee-uu" and is given by both sexes. This call is of interest as male wood warblers introduce similar somewhat notes, at intervals, into their territorial songs; in the case of the willow warbler and chiffchaff, however, no notes of the anxiety type are incorporated in their song phrases.
Harsher calls of alarm are produced by the sedge and reed warblers; both are common summer visitors which frequent marshes and reedbeds. Sedge warblers are very vocal when free-flying young are threatened, making a hard "tacc, tacc" which has a peculiar wooden quality; the notes may be uttered separately or run together in short bursts. Reed warblers, under similar circumstances, make churring rasps. Obviously, I have never heard these alarm rasps when reed warblers have appeared to be menaced by a cuckoo which is, after all, a nest-parasite of the species. Instead, as a cuckoo has flown over the reed beds, I have noted that each male reed-warbler is stimulated to sing sequences of loud and excited song. It may well be, however, that a cuckoo which had perched in the middle of a reed warbler's territory would evoke quite a different response.
Two migrant warblers which breed commonly in undergrowth by the verge of woods are blackcap and garden warbler, and both have characteristic and loud alarm calls. Blackcaps repeat their "tac tac" notes at speed; should a stoat pry near a pair's nest in low brambles the resulting sound is as though several typists are working together with diligence. But garden warbler's alarm is the louder; if young are in the nest the birds has a vigorous clucking call which may be alternated with churrs of varying quality.
More open country with scattered bushes is the habitat favoured by the whitethroat. This common warbler can be remarkably demonstrative even if people walk towards herbage where its fledglings are hidden. Usually the bird calls its harsh churrs of alarm from dense cover, then it revels itself to display the striking white throat; a pair of these dark cheeked warblers will alarm readily, especially if nestlings have just flown. The effect is like a combination of blackcap calls and churrs of whitethroat of a higher than usual pitch.
Most of us are familiar with the scolding of blue tits as a cat saunters across the lawn. Probably not so familiar is the explosive hiss given by an incubating female id she is disturbed in her nest-hole; doubtless this snake-like hiss is effective in scaring off many intending intruders. An even louder hiss is uttered by a sitting great tit; besides this, both male and female great tits have a variety of other notes of aggression and alarm, some of which resemble those produces by chaffinches. The marsh tit, with its black cap, is more likely to be found nesting in woodland than on marshland. Its alarm call is a characteristic "pit-chu" which is quite distinct from the nasal "chay, chay" notes of willow tit, a tit looking much like the marsh tit. These penetrating "chay" calls can be hard at quite a distance; once I watched a willow tit which was almost spitting at a farm cat which was lurking in alder and willow scrub. I had the impression that the willow tit's persistent alarm had an effect in deterring the cat from further exploration of the area. It would be difficult, though, to imagine that the long-tailed tit's alarm, of a thin, rasping quality, could have sufficient power to discourage a determined cat.
But one bird which will deter a hunting cat is a mistle thrush. When young are in the nest an adult becomes most aggressive, swooping and diving at a prowling cat and making hard, chattering rasps of annoyance; should magpies, crows of jackdaws fly low over the nesting region they are harassed and worried until they veer away. Of course, the mistle thrush is a relatively large and powerful bird and quite capable of lunging at, and striking, any predator which approaches its nestlings. Like all thrushes, mistle thrushes are chats, a group of birds of which there are many British species, both resident and migratory.
Examples of other common chats found in woodland are the robin, redstart and nightingale. Robins have a repeated "tic, tic, tic" as an alarm, while the handsome redstart gives a warbler-like "hu-eet" which is interspersed with hard "tack, tack" notes. The alarm of nightingale, during the reproductive season, can be perplexing, especially as the notes are usually given from dense undergrowth. Calls similar to the "hu-eet" of redstart are made, but mixed with repeated croaks, like those of frogs.
Those chat species which nest on heath or moorland are brightly coloured, certainly the males in spring, and have a common basic type of alarm. Hillside scree in summer is incomplete without its calling wheatear as, in the same way, are coastal gorse strips devoid of whinchat or stonechat. Unlike nightingale, these chats are often quite conspicuous when alarming; the high-frequency "weet" calls are admixed with sharp "tac tac" notes and once these sounds have been heard it seems obvious how the stonechat, at least, acquired its name. There is, naturally, a difference in quality between the notes of these species; like alarm generally, the calls serve to warn the young to hide and to distract a possible predator.
One small bird which produces calls of surprising amplitude is the wren; indeed its churring and grating noises can dominate the sounds of a wood on a midsummer's afternoon, when a parent is tending demanding juveniles. Chaffinches also contribute a large proportion of woodland calls at this time. The cock chaffinch gives a "hu-eet" type of call while it is breeding, otherwise the well-known "spink" is the usual note. Another finch to be heard in the woods, both conifer and broad-leaf, is the bullfinch; its musical pipings make an alarm reacting which is most attractive to the human ear. In mature oak woods the charming pied flycatcher is not uncommon. Should a squirrel leap towards the tree bearing its nest-cavity, high-pitche4d and persistent "weet, weet" calls are heard, often starting with almost dramatic suddenness. In contrast, the alarm notes of the spotted flycatcher are distinct; here, grating rasps are mixed with har5d "tac, tac" sounds.
One feature essential to alarm calls is that they must be audible above a certain level of extraneous noise and for this reason some alarm is relatively easy to record. Predators do not attack only on windless days; to have functional value birds' alarm must be of an intensity which can be recognised above the background roar of a summer storm. Similarly birds breeding by water must have calls which may be identified even though a river is in flood. Grey wagtails, for instance, on their nesting territories by moor and mountain stream, are able to alert their young to danger by their "zee-seet" calls in spite of torrents following heavy rain. Alarm and mobbing situations may result in the building up of considerable nervous tension in the birds concerned. Hence, some mobbing calls continue long after the predator has retreated; the alarm of blackbirds and wrens, in particular, may be kept up in this manner. The stronger the reaction, the longer is the time taken for its dissipation.
I suspect that most wildlife recordists, even if they concentrate on bird-song recordings, aspire to a modest collection of alarm calls.
Clearly, the variation in the character of such calls must stimulate the storing of the sounds for different species under different circumstances and conditions. Like territorial song, birds' alarm responses can fascinate any enquiring naturalist and many questions are raised as to the significance of the sounds heard. One aspect of the subject is consideration of how the small bird, using the same vocal apparatus, is able to switch from the production of song to alarm calls of varying pitch and amplitude, depending on the position and activity of the enemy it has seen. Other aspects to draw the attention of the observer involve the relationship of the bird and its behaviour to its surroundings; bird-song should be recalled as part of a particular environment.