Sunday, April 14, 2002		Nature

Sentinel of the salt-marsh
Nutan Shukla

THE redshank is often known as the ‘sentinel of the salt-marsh’. It is first to spot danger and raise the alarm. This bird’s alarm call is shrill. Unlike other calls, the alarm call warns not only other redshanks but also birds of other species; curlew, plovers, gulls soon follow with a chorus of concern. In the mass confusion, a predator is often distracted and may be deprived of its meal. In the wood, the jay is usually first to warn of approaching threat. A day-flying owl will evoke a hard rattling call from the jay which serves to encourage other birds in the tress to join in and, together, noisily to chase away the predator. In the garden, the robin gives a whispy call as a crow flies overhead. The sound may be untraceable but all the other birds nearby become agitated.

Alarm calls seem to fall into three types:those like that of the redshank are clear shrieks of despair which travel a fair distance across the marsh or estuary; those of jay are hard-edged and raucous and draw the attention of neighbours to the area of excitement; and those of the robin travel short distances and hard to pinpoint.

Alarm calls which are difficult to locate have been of particular interest to researchers. Study conducted on chaffinches revealed that the least locatable alarms is a call given by the male chaffinch to the mate and young at the nest when danger threatens. It is a very high, thin whistle which has the curious property of being extremely difficult for a human observer to locate. The reason is that this call is designed in such a way as to reduce the cues that might enable a potential predator to pin-point the calling bird. It is not completely non-locatable for it is not possible to produce such a sound, but it does minimise the cues available.

Not only that, but the call says meaningful things to birds of different species. Alarm calls are unusual in communicating across species boundaries. A chaffinch in the top of a tree might spot a sparrowhawk, give a hawk alarm call, which would transmit the presence of danger to a great tit, which would in turn signal to a robin and so on. Inexperienced birdwatchers often confuse the different calls. The ‘wheat’ call of the chaffinch, for example, is almost identical to the anxiety call of the chiffchaff, the willow warbler and the redstart.

The alarm calls of stonechats have a two-tiered defence system-one difficult to pin down like that of the chaffinch, but the other surprisingly easy to locate. Throughout the breeding season the stonechat has the characteristic call with two different sets of notes. One note is this, short whistled sound which has been dubbed the ‘whit’ note. The second consists of harsher ‘chuck’ notes which sound a little like two stones being struck together and which may have given rise to the name, stonechat. These two types of calls are related to the type of predator.

Stonechats face two different threats. One is from air in which birds of prey may attack the adults themselves and their nestlings. Terrestrial predators are a threat only to young in the nest. A sparrowhawk approaching povokes the ‘whit’ call from the parents, whereas a human or a dog are greeted with the ‘whit’ and ‘chat’ calls mixed together and delivered in a seemingly frantic string of loud notes.

Experiments have proved that ‘whit’ calls are warning calls to nestlings about the approaching danger which causes them to stay still and quiet. ‘Chat’ calls, accompanied, inevitably, by wing flicking exposing flashes of white from the conspicuous wing coverts is in fact part of the distraction display to lure the predator away from the nest.

Stonechats often have three breeding periods each summer and the rates of calling increases each time after hatching. It is suggested that increased begging activity with louder, noisier calling by the chicks would require more ‘whit’ calls from the parents to keep them quiet and not attract predators to the nest.

From the above facts it is clear that the ‘whit’ and ‘chat’ calls, together with visual distraction displays, have evolved as a means of protecting the stonechat’s offspring. The young stonechats were not alone; other birds in the heathland ecosystem have realised that listening to the stonechat’s calling might have benefits for them too. Stonechats spend a great deal of time on exposed perches from which they can spot danger from afar. Linnets, redpolls, yellowhammers, and reed buntings spend much of their time in the bushes, hidden from predators but also unable to see them coming. By associating with stonechats these birds have discovered that the stonechat alarm system is an effective early warning system. It also allows them more time for foraging with less time spent on scanning the skies for danger. In observations it has been found that meadow pipits in the company of stonechats spent half as much time in predator surveillance as unaccompanied ones. Stonechat alarm calls, like those of the chaffinch and many other birds, serve to warn, not only individuals of their own species but also those of other species.