Grasshopper warbler king of songbirds

MAY 6 was International Dawn Chorus Day. People struggled out of bed at cockcrow to hear the birds.

The session I attended was exhilarating; about 20 species sang or called.

On a cool still morning the chorus was especially loud, a reminder of the enormous energy birds put into singing and the extraordinary power of their songs. If a wren were scaled up to the size of an aircraft and its song amplified accordingly, the sound would be as deafening to our ears as that of the plane’s engines.

This raises an intriguing question; what effect does prolonged exposure to such noise have on a bird’s hearing? The little ears are so close to the source of the sound that their delicate mechanism must surely be damaged.

Bats have a similar problem; the navigation pulses they transmit are deafening. Thankfully, the frequencies are too high for us to hear, so we’re not troubled by them.

The bat’s ears, however, are disabled just before a pulse is emitted and switched on again in time to receive the returning echo. Birds have evolved a similar mechanism; when a songbird opens its bill wide to sing, the tension in the eardrums is released; the singer becomes temporarily deaf.

We humans have no such defences and our hearing, alas, wears out as we grow older. Prolonged noise damages our ears, something the wired-up younger generation should remember. Forty years ago, I walked with my father on the banks of the Shannon overlooking Limerick’s Westfields Marsh. Towards dusk on spring evenings a skulking mouse-like bird, the grasshopper warbler, would begin singing. I could hear it but my father couldn’t; the pitch was too high and the sound level too low for his old ears. Now that I’ve reached the age he was then, I can’t hear it either.

As the name implies, the song of this little bird resembles the sound of a cricket rubbing its hind legs together or the steady stream of mechanical clicks a fisherman’s reel makes as the line runs out. The song can be prolonged; in 1959, a bird was recorded in Luxembourg singing continually for 110 minutes.

The grasshopper warbler, like those other nocturnal singers the corncrake and nightingale, shuns the limelight, relying almost entirely on sound to secure a territory and attract a mate. Corncrakes and bitterns have developed very loud calls. A corncrake may be audible 3km away. A bittern’s ‘boom’, under the right conditions, can be heard 5km from the source. The nightingale too is a powerful singer; you wouldn’t get much sleep if there was one in your garden.

The grasshopper warbler, however, is self-effacing on all fronts, being neither gaudily attired nor loud-mouthed. Its song may be a form of aural camouflage; the bird is trying to pass itself off as an insect. Warbler expert Stephen Rumsey considers it to be ‘the most inconspicuous species in the Western Palearctic’.

People who hear the song can’t locate the singer. Its frequency is chosen so that predators can’t do so either. A sound arrives at each ear at slightly different strengths and times. We locate the source from this ‘head shadow’ effect, which works well if the pitch is high. The other directional give-away is a slight distortion between the sounds arriving at each ear. This is effective at low frequencies. The grasshopper warbler’s song, at 5 to 8kHz, is too low for head shadow and too high for frequency distortion to be of much use to its enemies.

You need patience to see this bird. It seldom breaks cover and, when it does so, flies low and soon drops out of sight.

To catch these warblers, ringers must set their mist nets very close to the ground. Of 17,000 grasshopper warblers ringed in the British and Irish scheme, only 31 came to light subsequently, the lowest ring recovery rate of any European bird.

The Rye Ringing Group trapped 4,500 over a 12-year period but only four of the birds were ever found again.

Ringing, however, helped establish that grasshopper warblers from Western Europe spend the winter in Morocco and Senegal.