(Photo: Paul Chouy, AP)

Aphrodite often had one perched on her finger. Jesus gave them a special shout-out. Chairman Mao included them as part of his Four Pests Campaign in 1958 and, in so doing, helped lay the groundwork for China’s Great Famine. House sparrows have been with us for a long time — some estimates suggest they’ve lived alongside humans for 10,000 years. But will these ubiquitous birds be with us much longer?

Urban sparrow populations have been in decline since the late 1920s, when automobiles began replacing horse-and-buggies as the transportation of choice and thus deprived sparrows of the chance to dip into horses’ feed. In the 1960s, rural populations began to drop as well; pesticides, silos and changing land use patterns were all thought to be part of that story, as they were for other farmland species whose populations went into decline. However, neither of these compared to the precipitous decline that began in urban areas in the early 1990s.

Research on the decline has been particularly active in the U.K., where greater London was believed to have lost 70% of its sparrows between 1994 and 2001. This mirrors declines in Bristol, Edinburgh and Belfast. In 2002, the house sparrow was placed on the U.K.’s Red Data list of species of conservation concern.

The story, though, isn’t confined to the British Isles. Cities across Europe (Dublin, Prague and Moscow) have witnessed similar drops. Researchers in India have also started ringing alarm bells. Less dramatic but steady declines have been documented in Canada and New England.

The precise reason for the decline is something of a riddle. Explanations range from increased predation by magpies, sparrowhawks and house cats; to a lack of nesting sites (holes and crevices) in modern buildings; to pollutants in unleaded fuel and electromagnetic radiation from cellphones.

Researchers believe that solving the riddle is important precisely because of sparrows’ long association with humans: Sparrows, rather like the miner’s canary, can act as a barometer of the health of human environments. And in a world where those environments are increasingly urban ones, understanding what makes cities so toxic for a species that has long called them home is an urgent task.

More statistics on the sparrow population decline:

• 60%: Estimated percentage of the British sparrow population living in cities or suburbs.

• 66%: The amount by which the U.K.’s sparrow population as a whole dropped between 1966 and 2012. (source: State of the U.K.’s Birds 2012 report)

• 50%, 60%: Approximate declines of sparrow populations in Hamburg and Prague, respectively, in the last three decades.

• 60%: The estimated percentage of the human population that will live in urban areas by 2030.

• 900-1800 MHz: Frequency of electromagnetic waves coming from cell phone towers in the Indian state of Kerala. This very low frequency was enough to penetrate both sparrow eggs and the unhatched chicks’ skulls, researchers believed, partially explaining why eggs in the region failed to hatch