Garden birds in decline?
2 November 2009
There are natural ups and downs in garden bird populations throughout the year created natural events like the weather and changes in the amount of daylight.
Birds respond in different ways to different weather conditions.
Cold winters can kill a lot of birds through starvation. The smaller the bird, the more likely it is to be affected by prolonged winter cold. Robins, wrens and blue tits can be affected by cold snaps, but starlings and blackbirds remain unaffected.
Food availability in May and June can be crucial in deciding how many chicks survive to fledge.
Blue tits and blackbirds are a good example of birds affected by the weather. For a pair of blue tits to succeed in raising a brood, it needs a plentiful supply of moth caterpillars. The best caterpillar ‘crops’ are available in years when the spring is warm and sunny with low rainfall, and the tits’ breeding success is correspondingly high.
These conditions spell trouble for blackbirds though, which rely on earthworms to feed their chicks. Warm dry weather dries the soil and forces earthworms deep down, beyond the reach of blackbirds. On the other hand, wet weather ensures a plentiful supply of earthworms for blackbirds, but delays the caterpillars in the trees. Worse still it washes them off, starving blue tit broods.
A particularly poor breeding season can result in lower numbers of birds the following season, so you might see less in your garden following a bad season. Most songbirds take a couple of years to recover from a really bad breeding season.
I'm getting no small birds in my garden, only pigeons and magpies. Are they killing off the other birds?
This is most likely a coincidence caused by any number of reasons. Pigeons and magpies are more adaptable to the man-made changing conditions in towns than smaller birds. They are also large and dominant, so many other birds may prefer to keep out of their way rather than compete with them for food.
Although large numbers of pigeons can keep smaller birds off bird tables by their sheer size and number, they don't predate chicks or adults of other birds. Magpies are omnivorous, and despite taking eggs and chicks of other birds, only 10% of their diet (at most) involves birds.
Extensive research has failed to find evidence that magpies are the reason for the declines in bird populations. House sparrows and starlings are both declining in number, even though their nests in holes and other cavities are safe from magpie predation, while chaffinches and greenfinches, which build open cup nests vulnerable to predation, are doing well.
I'm in London and there are no sparrows at all. What's going on?
House sparrows are declining seriously across parts of the UK as a result of changes in the wider countryside, especially in faming methods. Around 60% of house sparrows have been lost since the mid-1970s. The declines have been particularly acute in centres of large cities including London, Edinburgh and Glasgow.
This is of particular concern since the house sparrow is one of very few species that actually thrives with close proximity to people, even in city centres. We're conducting a programme of research into sparrow decline.
The early indications are that, yet again, the cause is food availability at some parts of the year and availability of nesting sites. Since house sparrows are sedentary and rarely move far from their birthplace, it can take a long time for them to return to areas from where they've disappeared.
I used to have several swallow nests in my out-buildings, but now there are only a couple. Are they declining? Why are they in trouble?
There is anecdotal evidence to suggest that swallow numbers are declining in many areas across the UK.
It's believed that changes in their breeding grounds are responsible. As more farm buildings are converted and modernised, suitable nesting sites become scarcer. Swallows feed on flying insects, which are plentiful especially in wetland areas and pastures. As more grazing land is converted to arable land, there are fewer insects for them to eat and feed to their young.
As swallows do not go far from their nest to forage, nesting and feeding areas must always be close together. Dry weather in early summer may result in lack of mud for building and repairing their nests, and effectively prevents them from nesting, or forces them to move elsewhere.
We've always had a particular bird nesting in the garden, but they haven't bred this year. Are their numbers going down?
Not necessarily. Birds don't always use the same nest site every year. Reasons include:
- A territory may not be occupied every year.
- A nest site may be vacated if the birds have found an alternative nest they prefer within the same territory (eg. a neighbour has put up a new nest box), or trees or shrubs that were the key feeding source have been removed in your, or your neighbour’s garden. Without a food source a territory is useless.
- Birds may divide their territories in a different way, resulting in your garden getting less attention than in previous years.