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Bird decline |
Birds in decline?
For some reason, we are not getting many birds of a particular species, though others seem to be doing OK. Are these particular birds in trouble?Not necessarily. Bird populations do not remain absolutely constant all the time, but there are ups and downs naturally between years, created by weather conditions and other natural events. Since different birds may not respond to given weather in the same way, only some species may decline in numbers, while others are unaffected. 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. As a result, a cold winter may drop the numbers of robins, wrens and blue tits, but leave starlings and blackbirds less affected. The food each bird needs can be affected differently by weather. 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 how birds are affected by weather. For a blue tit to succeed in raising its brood, it will need 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 will, however, spell trouble for the blackbird, which relies on earthworms to feed its chicks. Warm dry weather will dry the soil and force earthworms deep down, beyond the reach of blackbirds. On the other hand, wet weather will ensure a plentiful supply of earthworms for the blackbird, but will delay the caterpillars in the trees or worse still, wash them off, starving the blue tit broods. A particularly poor breeding season can result in lower numbers of birds the following season. Most songbirds will take a couple of years to recover from a really bad breeding season. I am getting no small birds in the garden any longer, only pigeons and magpies. Are these killing off the other birds?No, this is most likely a co-incidence caused by any number of reasons. Pigeons and magpies seem to be far more adaptable to the man-made changing conditions in towns than many smaller birds. They are also large and dominant birds, and 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 are entirely plant eaters and would not predate either chicks or adults of other species. Magpies are omnivorous birds, and despite taking eggs and chicks of other birds, at most only 10% of their diet involves birds. Extensive research has failed to find evidence that magpies are the reason for the observed 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 am in London. There are no sparrows at all. What is going on?House sparrows are declining seriously across parts of the UK as a result of changes in the wider countryside, especially the farming systems. Around 60% of sparrows have been lost since 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. The RSPB is conducting a programme of research to look 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 extremely sedentary birds, it can take a long time for them to return to an area from where they have disappeared. I used to have several swallow nests in the out-buildings, but now there are only a couple. Are they declining everywhere? Why are they in trouble?There have been problems with censusing swallows over the years, and so the population trends are unclear. However, there is anecdotal evidence to suggest that swallow numbers are declining in many areas across the UK, while there are very few reports of increasing numbers. It is believed that changes taking place in their breeding grounds are responsible for the perceived declines. As more farm buildings are converted and modernised, suitable nesting sites are often in short supply. Swallows feed on flying insects, which are plentiful especially in wetland areas and pastures. As more grazing land is converted to arable, the food supply is diminishing. As swallows do not go far from their nest to forage, nesting and feeding areas must always be close together. Dry weather in the early summer may result in lack of suitable mud for building and repairing nests, and effectively prevent the birds from nesting, or force them to move elsewhere. We have always had a particular bird nesting in the garden, but they don’t this year. Are their numbers going down?Not necessarily. Birds do not always use the same nest site, and there can be many reasons for this. Unless the population is at a saturation point, each territory may not be occupied in a given year. A nest site may also 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 have been removed in your or your neighbour’s garden, which had been the key feeding sites - 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. Last modified: 08 August 2005 |
Flower borders in front of RSPB The Lodge reserve, Bedfordshire - Andy Hay (rspb-images.com, Ref: 1999_0503_009 )
Family Birdwatching through living room window - Andy Hay (rspb-images.com)
Nestbox on tree, RSPB Wood of Cree reserve - Andy Hay (rspb-images.com, Ref: D_2006_11968_0009 )