Friday, 20 October 2006

Where have all my birds gone?

Over recent weeks, increasing numbers of garden birdwatchers have contacted me to ask where all their birds have gone.  Familiar species like robin and blackbird have deserted gardens, leaving birdwatchers concerned that something untoward may be happening. Fortunately, this exodus is an annual event and one that forms part of the regular seasonal pattern of garden use. Autumn is a time of plenty, when Nature’s larder is packed full with wild fruits, seeds and nuts. As such, many birds will shun the food provided at garden feeding stations, in favour of beech mast, rose hips, berries and haws. The extent of this sudden switch in where to feed can be seen very clearly from the weekly records collected by the 16,000 BTO/CJ Garden BirdWatchers. Examination of the results for a species like blackbird ( shows an abrupt decline in the use of gardens, something which extends over several weeks. Once the supply of wild fruits is exhausted, the blackbirds return to gardens and the Garden BirdWatch reporting rate increases again.

You only have to wander out into the Norfolk countryside to see that many fruit or seed-producing plants have produced a bumper crop this year. Crops of acorns and beech mast are particularly large; that for beech being the largest since 2002. With such an abundance of food, seed-eating birds (such as coal tit, chaffinch, brambling, jay, nuthatch and woodpigeon) are likely to make very little use of garden feeding stations over the course of the winter months.  Work carried out by David Glue of the British Trust for Ornithology has demonstrated this link between seed availability and the use of gardens by coal tits.  During winter, when insect food is in short supply, coal tits switch to feeding on beech mast. When there is little beech mast available, more coal tits move into gardens to feed on peanuts and sunflower seeds. When there is a bumper crop of beech mast, they can remain within their favoured woodland habitats.

Many trees vary their production of seeds from one year to the next, using environmental cues to synchronise the size of the crop across large areas. This results in a run of years when seed crops are quite small, followed by the occasional year when they are very large. This large crop is known as a “masting” event and the strategy itself is designed to reduce the amount of seed taken by seed predators. If trees produced a similar crop each year then seed-predator populations would increase and the amount of seed eaten would be high. By having a series of small crops, followed by one big one, the trees effectively swamp the predators during the masting year, insuring that more seed survives to germinate.

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