It is almost as if they were tired of being taken for granted.
Common house sparrows, symbols of European city life, have almost disappeared from some of the Continent's capitals, and nobody has the slightest idea why.
In central London, where cheeky "cockney sparrows" once flocked to eat crumbs from peoples' hands, there is not a sparrow to be seen today.
In Amsterdam, the little gray and brown birds no longer cock their heads by the sides of the city's canals. The grimy north German port of Hamburg, the urban habitat par excellence for man's closest avian companion, is now a sparrow-free zone.
They have faded from the landscape without anybody paying attention.
"You don't notice common species because they are around you all the time, and then suddenly they aren't there any more," says bird expert Rosie Cleary. Dubbed Britain's "sparrow czar," she has just taken charge of a nationwide survey of the sparrow population for the British Trust for Ornithology.
Sparrows are seen fondly in European eyes. Cockney East Enders in London took the diminutive, resilient "sparrer" to their hearts, and the French singer Edith Piaf, who grew up in the streets of Paris, was nicknamed "Kid Sparrow."
The mystery of the sparrow's disappearance from so many European cities is all the more baffling because the bird is still found in normal numbers in other metropolitan areas, such as Berlin.
"The most fascinating thing about the situation is that it is by no means uniform across Europe," says Denis Summers-Smith, a British ornithologist who has been studying sparrows for half a century and who first raised the alarm.
That would seem to assuage the fears of those who have suggested that the sparrow could be the canary of modern urban life, warning us of hidden dangers. But it does not help explain the phenomenon.
Britain, where the sparrow is now on the "red list" of species in danger, has seen the most dramatic decline in numbers. Where 12 million pairs nested 30 years ago, there are no more than seven million pairs today - a drop of 10 million birds. And central London has suffered the most.
In Kensington Gardens, 2,603 house sparrows were counted in 1925.
Last year just four males were left, and this summer they moved on.
"In big towns, the population went into freefall around 1990, and there is no evidence that it is stabilizing," says Dr. Summers-Smith.
Questions have been asked in Parliament. A national newspaper has offered a reward of $8,000 for anyone who can solve the mystery of the missing sparrow. The British Trust for Ornithology is calling on thousands of volunteers across the country to report on sparrows they see over the next 18 months, to try to clarify what is happening.
Across the Channel, the French Bird Protection League is organizing a similar survey in Paris this spring, after local bird-watchers reported puzzling sightings. In one park, they found the same number of sparrow nests as there were 10 years ago. In another, the population has dropped by two thirds.
In both places, says bird-lover Christian Gallinet, who worked on the study, "the couples that we fed had no more than one or two young, when normally they should have three or four.
"Something odd is going on, and nobody has any explanation for it yet," he adds.
There are plenty of theories, though.
Some experts have suggested that as cities have protected their suburbs, prohibiting building, developers have turned their attention to inner-city wastelands, robbing sparrows of the overgrown, weedy areas where they typically forage for seeds.
Others wonder whether modern buildings lack the nooks and crannies that sparrows like to nest in. But that idea takes a knock in Holland, where birdwatcher Guus van der Poel found as he counted sparrows that "some city centers are sparrow-free, and in areas where the houses were built before 1953 there is a good chance there will be no sparrows at all."
Suspicion has also fallen on Felis domesticus, the house cat.
One study found that as many as 25 percent of sparrows in an English village fall prey to prowling felines. But cats have been hunting birds since time began, and though food shortages may have forced sparrows to take greater risks, it is hard to believe that predators could have halved the British sparrow population in 30 years.
A key problem, a number of experts agree, appears to lie in a shortage of the tiny insects that fledgling sparrows need during the first three or four days of their lives, before they move on to a vegetation diet. Mr. van der Poel blames industrial agriculture, which he says has robbed the countryside of the variety of flowers and weeds that insects need.
Summers-Smith points the finger at another culprit - unleaded gasoline, though he acknowledges that the evidence is only circumstantial. The catastrophic fall in urban sparrow populations began in 1990, he points out, one year after unleaded gas was introduced in Britain.
Unleaded gas contains unstable toxic compounds that Summers-Smith believes might be killing off the invertebrates that baby sparrows depend on. The fact that sparrows in Paris are doing better than their London cousins, he suggests, may have something to do with the fact that diesel fuel is much cheaper, and thus much more commonly used, in France than in Britain.
But he says his ideas are "highly speculative and highly circumstantial. This problem requires academic research."
Belatedly, perhaps, ornithologists are now turning their attention to the lowly sparrow.
"When I first started studying the sparrow just after the (Second World) War, most of my colleagues didn't think it worthy of even being called a bird," Summers-Smith recalls. "Now it is a high-profile bird."