Thursday, 15 December 2005

Long nights hard on small birds

The long nights that we experience at this time of year can make things difficult for many of our birds. A combination of low temperature, reduced amounts of daylight in which to find food, and a general decline in the amount of food available, can all have a dramatic impact on smaller species. Small birds may need to expend more energy to keep warm and, with only small reserves of body fat, they have to spend much of a typical winter’s day searching for food. This food is vital to replace losses from the night before and to prepare for the night ahead. Great tits may easily lose five percent of their body weight overnight, with the amount of weight lost being greatest during periods of cold weather.

Watching my bird feeders has revealed a pattern to the order in which the different species arrive in the morning. As one of some 2,000 licensed bird ringers nationwide, I am frequently up before dawn to set the nets in which I catch small birds for ringing. At this time of day, I can already hear the calls of blackbirds and robins up and about before it is truly light. With the nets set, I operate a regular pattern of checking and emptying the nets every 20 minutes or so, fitting rings and taking various measurements before releasing the birds. The first birds to be caught are usually blackbirds and other thrushes, followed by various tits and the occasional dunnock. It is not until an hour or so after dawn that the greenfinches, goldfinches and chaffinches begin to appear in any numbers. Birds continue to arrive to feed through until mid-morning then it all goes rather quiet, with numbers not increasing again until the hour or so before dusk, presumably when individuals come in to top up before going to roost.

Intuitively, you might expect those species that most urgently need to top up their fat reserves to arrive first but other factors also come into play. For example, roosting behaviour may be important. Some species roost communally, sharing body warmth and reducing the amount of energy they have to expend to keep warm. While wrens can use nestboxes or roosting pouches, long-tailed tits prefer to roost together on a branch in the centre of a thicket. Dominance also plays a role, with less dominant species forced to feed at times when feeders are not being dominated by more aggressive birds like greenfinch or starling. Such factors influence the basic pattern and will change over time as birds respond to changing weather conditions. Recent nights have been relatively mild but, as we move further into the winter, lower temperatures may bring birds to our feeders that much earlier relative to dawn.

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