What is the point of writing fiction, some people ask?

In what way can it possibly change the world? Well, Jonathan Franzen's latest work is one answer to those who doubt the value of anything made-up. Among America's most feted writers, Franzen has turned his attention from scrutiny of the middle classes and put his hard-earned literary reputation to work on a much more difficult, intractable and upsetting subject: migratory birds and their mindless slaughter.

Franzen is like a flamethrower in his fury about what is happening to the world's songbirds. Where his novels leave the reader feeling they have been put through an emotional mangle, this ornithological crusade multiplies that sensation a hundredfold.

The novelist's subject is the plight of songbirds as they travel across Africa and Europe and fall into the hands of hunters in the Mediterranean who brazenly defy the law for a few mouthfuls of these tiny, exquisite creatures. That a cooked oriole – a Bedouin favourite – offers two bites of meat gives an idea of how many birds must perish in order to fill a single dish. Photos of these meals could be mistaken for gnocchi, there are so many little mounds under the sauce.

The July edition of National Geographic carries Franzen's latest salvo on this most painful story, along with pictures that would make a high-street butcher weep. For while it is one thing to rear and kill animals for meat, it is altogether different to destroy some of the world's most beautiful, wild and supposedly protected creatures. Yet, illegal though it is, millions of these birds every year are netted, or glued to trees, shot, snared or drowned in brandy, simply to satisfy the greed of a parcel of venal foodies who refuse to accept that their traditional and barbarically captured game is now off limits.

To be followed by a documentary aired this autumn, Franzen's article takes the reader across the Mediterranean, where the worst outrages are perpetrated. From Egypt, Italy, Cyprus, Sicily, Spain and beyond, he tracks a level of cruelty and carnage that for the first time makes his writing almost unreadable. For these birds – robins, ortolans, chiffchaffs, ducks, and their kin – crossing the Mediterranean is as dangerous as stepping onto no-man's land in the great war's trenches.

As Franzen makes clear, hunters in countries like Albania may be poor, but they are not starving. Only that would excuse such behaviour. But nobody he points at in this article is famished. Some, in fact, are doctors. The killing of these birds is the result not of necessity but of an engrained cultural appetite and an insistence on exercising old habits.

Some might see Franzen's leap into environmentalism as a sideways step, but you could equally argue that the observational skill a novelist uses to evoke contemporary society and get under its skin is precisely the kind of watchful talent that would explore such practices. Novelists with a passion for wildlife are common, from the likes of Peter Matthiessen, who tracked the snow leopard to magical and lyrical effect, or Margaret Atwood, herself something of an environmental campaigner, to biologist turned novelist Barbara Kingsolver, Norman MacCaig, whose poems about animals, like toads and hens, are among his finest, or Kathleen Jamie, whose poetic eye has recently been used to trace the natural world of Scotland and beyond.

What makes Franzen's crusade so immediate is that you don't need to be a nature lover to be repulsed by what's happening. Birdsong is one of the most natural, important and uplifting sounds in our lives. The eeriness of a birdless place sends a primitive shiver down our spines, because birds have been the backdrop to our lives since we lived in caves. When they go quiet, something is wrong. They are a bellwether of environmental health, the mineshaft canaries whose death denotes danger. The silent spring Rachel Carson warned of is a phrase laden with dread, because when birds have disappeared, we know we are not far behind.