When it comes to assessing the wellbeing of a particular species, there's an old saying in conservationist circles; numbers are less important than trends.

With this in mind, it's worth having a second look at the latest figures compiled and published by the Royal Society for the Protection of Birds (RSPB) as part of its Big Garden Birdwatch 2011 survey. Certainly, ornithologists are likely to take heart from the findings of the latest survey, which saw thousands of volunteers monitor the winged visitors to gardens and other green spaces the length and breadth of Britain.

Indeed, sightings of many of the UK's smallest birds were up significantly on the preceding year. Long-tailed tit sightings were up by a third, for exampled, while sightings of coal tits were seen to increase by 25 per cent. Additionally, house sparrows and starlings once again emerged as the most commonly-spotted of all of Britain's birds, with their numbers up by ten and 25 per cent respectively over the same period.

However, with the conservationists' maxim of looking at trends rather than numbers in mind, a different picture emerges. It is one that has got experts and amateur twitchers alike understandably worried, and nowhere is this more the case than in London.

Put simply, while both sparrows and starlings can be found in vast numbers in all corners of the capital - not for nothing have they long been known as 'Cockney sparrows' - the RSPB's data shows that numbers have been declining over the past five years. Alarmingly, in fact, the figures point to a drop of around 50 per cent over the past 15-to-20 years in both sparrow and starling populations in the capital.

Quite why this is the case, nobody is quite sure, with even Tim Webb, the RSPB's London Communications Manager unable to pinpoint one specific reason, though, in correspondence with the Earth Times, he explained that there are "multiple factors driving the decline of house sparrows".

''Our research has highlighted a couple of issues, the biggest one being a lack of natural food in our managed environments (gardens, parks and other public spaces). Chicks were starving to death in the nests and controlled feeding experiments found increased survival rates where mealworms were provided. So, we know that protein (ie insects) is an important part of the early diet of house sparrows,'' he said.

''Post mortems of sparrows, conducted in partnership with the Zoological Society of London, have failed to find any mystery disease. They did find the dead birds were malnourished and were suffering from some ailments but these were diet related, linking us back to a lack of suitable food.''

While the findings of several recent studies suggest that urban predators such as cats only have a limited impact on small bird populations, the RSPB is still open to the theory that car emissions and electromagnetic waves from mobile phone masts. Additionally, it is also believed that, with urban habitats being lost to development, colonies of birds are becoming disjointed, making it harder for sparrows and other small birds to interact and interbreed, though research into this theory is still ongoing

Fortunately, while the reasons for the trend may be varied and hard to understand, what is clear is that individual Londoners have the power to make a significant difference. Research has found that increasing seed and insect availability for sparrows can help boost their numbers, meaning both households and local authorities across the city are being urged to plant shrubs, hedges and trees, and to let grass grow long and run to seed. Additionally, keeping dead plants in-situ over the winter months rather than uprooting them will also lead to an increase in insect numbers, thereby providing these small birds with the protein they need to survive the harshest months of the year.

Notably, not only will such an approach to green space management benefit London's sparrows, but, Mr Webb explains, making gardens more bird-friendly will also benefit a range of other urban wildlife, attracting the likes of bees, ladybirds, bats, butterflies and even small mammals such as dormice and hedgehogs to back yards and gardens.

''What you're doing is increasing the variety of habitats you have in a single location, increasing nesting opportunities and food availability,'' he says. And as if that isn't motivation enough, the more insects and birds there are, the fewer pests such as aphids, snails and slugs there are likely to be, spelling good news for both the capital's green-fingered enthusiasts and urban ornithologists alike.