Hid­den BRI­TAIN A

RE­VEALS A FAS­CI­NAT­ING WORLD OF WILDLIFE THAT WE OF­TEN OVER­LOOK.

BBC Wildlife Magazine - - Wild May - NICK BAKER NICK BAKER is a nat­u­ral­ist, au­thor and TV pre­sen­ter.

com­plex, eerie win­now­ing fills your ears. An in­tense, multi-har­monic pulse that seems so com­pletely out of place on Earth, it might have you scan­ning the sky for an im­pend­ing alien in­va­sion. Yet this acous­tic am­bi­ence places you pre­cisely in both habi­tat and time – the air over Bri­tain’s marshy areas will be res­onat­ing with the strange mu­sic from spring through to sum­mer.

The fact that the source is so dif­fi­cult to lo­cate only fuels its mys­tery. But look care­fully sky­wards and you might be lucky enough to spot the dis­tinc­tive, long-billed pro­file of one of our most se­cre­tive birds, the ‘com­mon’ snipe. It is do­ing what is of­ten called drum­ming.

It’s a big sound that car­ries up to 0.8km – the all-per­vad­ing noise is out of pro­por­tion to the wader’s diminu­tive size, which can make it dif­fi­cult to pin­point. The em­a­na­tion, how­ever, is not a ‘song’ in the usual sense of the word. No, the fab­u­lously odd drum­ming is a sound re­served for the skies – it can only be pro­duced in the air by me­chan­i­cal means. Sounds made in this way are known as sonations to dis­tin­guish them from the more usual types of bird vo­cal­i­sa­tions.

In com­mon with many other birds, the snipe does also have a true voice, mak­ing a var­ied reper­toire of croaks and grunts and a dis­tinc­tive ‘chip… per… chip… per… chip’ song. These ut­ter­ances, as in other species, are pro­duced by the sy­rinx: a so­phis­ti­cated col­lec­tion of mem­branes un­der mus­cu­lar con­trol in the bronchi.

Aerial ac­ro­bats

Should you man­age to lo­cate a snipe while it is drum­ming, you’ll see its ar­row-like body de­scribe an as­cend­ing arc be­fore it rapidly plum­mets at about 40–45 de­grees; it is dur­ing this pow­ered de­scent that the sound is pro­duced. The de­lay of the sound reach­ing your ears can make sync­ing the phys­i­cal ac­tions and sound a lit­tle dif­fi­cult. But if you have a ‘sniper’s’ wits about you, you might be able to get your binoc­u­lars on the bird, and in a re­ally good view you’ll no­tice the strange form of its tail.

As it de­scends, the male sticks out its two outer tail feath­ers at near 90° to its body, and when it reaches speeds of 50–85kph, they start to vi­brate. The faster the bird dives, the higher the pitch of the sound cre­ated.

Re­cent re­search has shown that aeroe­las­tic flut­ter pro­duces the noise, a phenom­ena that is a self-feed­ing vi­bra­tion, a func­tion of in­ter­ac­tion be­tween the wind speed and the struc­ture of the feath­ers.

The pair of spe­cialised sound­mak­ing feath­ers are stiff­ened, ex­cept for a zone of weak­ness along their length. This sets up a zone of flex­ion, which causes the trail­ing edge of each feather to flap back­wards and for­wards vi­o­lently as air passes over it. Think of it like a flag flap­ping in the wind but at a higher fre­quency and you have the mech­a­nism be­hind one of the most unique ter­ri­to­rial dis­play flights of any bird.

Drum­ming snipe cir­cle, then dive earth­wards with outer tail feath­ers at right an­gles.

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