Charles Darwin would have been thrilled by the performance of our female osprey. Last weekend, on the nest beside a Perthshire loch to which she has been returning every spring for 20 years, this grand old lady and ace fish-catcher laid her 50th egg.

It is something of a record in the osprey world: no female bird has ever been observed in this country continuing to produce young at such an advanced age. She is not only a fine testament to longevity, she is the ultimate feathered symbol of monogamy, and the belief to which Darwin clung - often in the face of contradictory evidence - that female birds will almost always stick to one mate. If you are constantly on the wing, then the stability of a single relationship and a steady male provider is a better guarantee of survival than a succession of sexual adventures.

Darwin did not get everything right. Being a great Victorian as well as a great scientist, he tended to overlook evidence of female misbehaviour; even, on one occasion, turning a blind eye to the disgraceful lechery of a neighbour's goose. But his general view that 90 per cent of birds prefer a monogamous relationship seems to be borne out, and the remarkable story of the survival of the osprey population in Britain is living proof of it.

At the end of the First World War, these magnificent birds of prey were extinct in Britain. The first pair ventured back in the 1950s, and they are now, if not common, at least widespread in Scotland, Wales and the North of England. A 50-year turnaround is not bad going.

Their success story is based on a code of ethics that any Conservative minister - though not, of course, every Liberal Democrat leader - would be proud of. Unlike Homo sapiens, which seems increasingly to regard a single, steady relationship as the poor, boring relation to a continuous dating game with a succession of partners, these birds are rarely deflected from the straight and narrow. Their long migration every year, from Scotland to Africa and back, is a hazardous operation, so when the female finally returns, to the nest that she and her mate have so painstakingly built the previous year, she cannot be absolutely certain that her partner has also made it back.

She therefore mates with the first male she encounters. This year, to the acute embarrassment of the Scottish Wildlife Trust observers, who chart every movement in the ospreys' nest at the Loch of the Lowes, the female osprey, soon after her arrival, had a fling with a male bird who turned out to be one of her own previous offspring. Flighty female in two-in-a-nest incest shock! But before this shameless liaison could degenerate into dysfunctionality, the male bird came back, saw off his Oedipal rival, and took over his marital duties. The result - two legitimate eggs, with another on the way.

Thus social, if not sexual, monogamy, seems to be the key to survival. And in the osprey world, this is ruthlessly applied. Last year, at the Loch Garten nest, farther north, a male bird whose female partner had not only mated with an errant osprey, but had produced eggs, reurned rather late to discover this palpable evidence of infidelity. Nothing was said, not a feather seemed ruffled. But as soon as the female had gone fishing, the male simply scraped the alien eggs out of the nest. A week or so later a new clutch appeared. The family line remained pure. And with only 180 breeding pairs in Scotland, this kind of behaviour means that the gene pool remains surprisingly intact.

It is always dangerous to draw conclusions from nature's lessons. A social order based on the elimination of all illegitimate offspring, an anti-immigration policy excluding outsiders, and a system of racial propagation that comes uncomfortably close to eugenics is hardly the best advertisement for a modern democracy; our social drift has taken us in the opposite direction. As G.K.Chesterton once observed: “The ideal of monogamy hasn't so much been tried and found wanting, as found difficult and left untried.” Or, as a character in Nora Ephron's novel Heartburn, snaps: “You want monogamy? Marry a swan!”

But in terms of family relationships I commend the osprey as a model to follow. And for once the male bird, far from being the dissolute philanderer, the useless parasite, or the hen-pecked husband, plays a model role: sober, reliable, stern and vigilant. He it is who selects the mate in the first place, builds the nest, produces the fish, and even sits on the eggs while his mate stretches her talons. He is a remarkable combination of upstanding 19th-century fatherhood and late 20th-century New Man. Were there a kitchen in that prickly nest, he would be doing the washing-up. Not so much survival of the fittest as the triumph of domesticity.

Next year is the bicentenary of Darwin's birth, and there will be no lack of reassessments of the great man and his profound influence on modern thinking. A useful starting point might be to demolish that phrase “survival of the fittest”, which was, in any case, not coined by Darwin himself, and substitute the “natural selection” that he preferred. What this meant in practice, in his view, was the success of a particular species in producing healthy offspring and perpetuating a breeding stock.

What the osprey demonstrates is that, whatever indiscretions may be committed in the course of a relationship, a stable family background is ultimately the best guarantee that the species will prosper. It works for ospreys. It probably works for humans too.

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