Science baffled by decline in sparrow numbers

THE rapid decline in house sparrow numbers in Britain and some other European countries has prompted concern and a flurry of research projects.

In August 2002 the species was put on the British Red Data list as a bird of conservation concern. Between 1977 and 2000, numbers in Britain declined by 65% overall, with total extinction in some urban areas.

The British data is better than the figures available in Ireland but there is both scientific and anecdotal evidence that house sparrows have declined in this country too. There is better scientific data in the pipeline.

On the anecdotal front, house sparrows disappeared from my garden about seven or eight years ago. At the time, I didn’t know about the international trends and put it down to the fact my neighbour, who was in the animal feeds business, moved to a neighbouring county. I reckoned the sparrows left because there was no longer any spilt grain lying in his yard.

But in the middle of last winter a female house sparrow appeared at my bird table. She was very shy and nervous, which is not typical house sparrow behaviour, but after a few days she was joined by a male and they slipped in to grab a beak full of food when there were no other birds around.

Then, a few months later, the bird table was invaded by growing numbers of immature sparrows which were full of confidence. It seemed obvious the pair I had fed through the winter had reared several successful broods. The birds are still around.

House sparrows are flocking birds and colonial nesters. Due to something that biologists call the Allee effect — when numbers of a colonial bird drop below a certain critical threshold — the population becomes unviable.

The classic example is the extinction of the passenger pigeon in the US in the 19th century.

But passenger pigeons were migratory and house sparrows are one of the most sedentary of all birds, hardly ever travelling more than two kilometres from where they were hatched. This makes sparrows particularly vulnerable to the Allee effect because depleted colonies are unlikely to be augmented by passing strangers.

But something must have caused the initial population decline, even if it was accelerated by the Allee effect. Intensive research into this is ongoing.

However, some facts are beginning to emerge that seem significant, at least to me.

House sparrows are predominantly seed and grain eaters. But for the first three days of their life they need a very high-protein diet if they are to survive. This protein is normally supplied by the parents in the form of insects and other invertebrates (one study suggests spiders are particularly important).

There is mounting evidence that the primary reason house sparrows are declining is that they’re not able to find enough protein-rich invertebrates during the breeding season. There doesn’t seem to be any good scientific research linking the decline of the bird with a decline in insect numbers but that’s because nobody is counting the insects.

But it seems likely the use of pesticides, in parks and gardens, is the reason for the decline in house sparrows. Think before you spray. [e-mail]