Richard Collins: How storks know when the grass is cut

It's not just humans who enjoy a freshly mowed lawn
Richard Collins: How storks know when the grass is cut

Storks normally return to Spain early February after wintering in warm Africa. AP Photo

Tennison wrote: ‘Into the jaws of Death, into the mouth of Hell, rode the six hundred’, in The Examiner (not our Irish one) in December 1854. Light cavalry, 114 of them Irish, had charged the Russian guns head-on in a monumental cock-up during the Battle of Balaclava two months earlier.

Afterwards, hordes of scavenging birds descended on the field. Units were ordered to shoot them to protect the wounded. Ravens and hooded crows know the bonanzas battlefields have to offer, but vultures are the supreme carrion gourmets. Confrontations between armies, even in the 19th Century, were rare events. So how did the vultures discover so quickly that one had taken place?

When grass is newly cut, storks arrive from far and wide to gorge themselves on mice and frogs whose cover has been blown. But what tells the storks that food is on offer kilometres away? A team, led by Martin Wikelski of the Max Planck Institute, have managed to answer this question.

Richard Collins: 'As film buffs know, vultures circle high over wounded cowboys, knowing that lunch will soon be served.'
Richard Collins: 'As film buffs know, vultures circle high over wounded cowboys, knowing that lunch will soon be served.'

Seventy storks, foraging near Lake Constance, were monitored from the air. Once grass-cutting commenced, the birds headed immediately for the mowing location. Travel distances of up to 16.6km were recorded. Some birds crossed from the far side of the lake. Distances were such that the birds couldn’t detect mowing by sight or sound, nor could any known form of animal communication explain the behaviour. When freshly mown grass, cut elsewhere, was strewn on a field, the storks duly arrived. It was clear, therefore, that the grass itself attracted them.

The researchers discovered that only storks downwind of mowing travelled to a site. Those upwind of it failed to do so. ‘The further off-centreline of the wind direction the storks were, the slower they approached the test area’, they report. Mammals tend to be led by their noses when it comes to locating food but birds, with a few exceptions, have a poor sense of smell. However it seemed clear that the storks were detecting the wind-borne fragrance of cut grass.

To prove that this really was the case, the researchers prepared a cocktail of artificial substances which emitted a scent resembling that of new-mown vegetation. When an experimental site was sprayed with the mixture, the storks arrived on cue.

Do vultures, likewise, use smell to locate carrion? They could not have done so at Balaclava; vultures on this side of the Atlantic lack a sense of smell.

They have, however, very keen eyesight. As film buffs know, these undertakers of the bird world, circle high over wounded cowboys, knowing that lunch will soon be served. When gannets dive on a shoal of fish, their gleaming white bodies stand out against the dark sea, inviting others to join the party. Vultures, likewise, spot their peers wheeling in the distant sky. They gave us the term ‘vulture funds’.

New-world vultures, such as condors, are different. Experiments show they have an acute sense of smell. It was once thought that they were close relatives of the Old World ones, but genetic studies show that they aren’t. In fact, their immediate relatives are the storks.

  • Martin Wikelski et al. Smell of green leaf volatiles attracts white storks to freshly cut meadows. Scientific Reports. 2021.

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