Thursday, 9 December 2010

Birds of a feather roost together

The bird table and hanging feeders are busy first thing in the morning, underlining the urgency with which many of the smaller birds need to replenish energy reserves lost overnight. Winter is a difficult time for small birds, the long nights and low temperatures placing strain on the small fat reserves these birds carry. If temperatures hover around freezing, or dip below, then these reserves are quickly depleted, something that can be a particular problem for our smallest birds, like Wren, Long-tailed Tit and Goldcrest. Research has shown that both Blue Tits and Great Tits lose some 5-10% of their body weight over the course of a typical winter night, possibly a lot more if the conditions are particularly poor.

Heat loss is, in part, dependent on where you choose to roost. Pick somewhere warm and you’ll be able to maintain your body temperature at a safe level more easily than if you pick somewhere cold. It is for this reason that some of our birds will roost communally, seeking the warmth from street lighting (Pied Wagtails) or from huddling together in a nest box or roosting pouch (Wren, Blue Tit or Coal Tit). Others roost within vegetation, a behaviour which sees Long-tailed Tits form up in a line along a branch or stem, huddled together within thick cover and out of the wind.

The use of nest boxes for roosting is something that is easily overlooked, especially if the birds pile into the box just as it is getting dark. There are some records of observers witnessing these arrivals; in one case at least 60 Wrens squeezed into a single box, but the behaviour is likely to be more common than these occasional reports would suggest. It is for this reason that the British Trust for Ornithology (BTO) has asked us to look at our nest boxes over the course of the next few weeks. If you take a look at your nest box one afternoon and then another the following morning, you should be able to tell if it has been used as a roost. This is because you are likely to find fresh droppings come morning if a bird has used the box overnight.

You might be fortunate enough to have a nest box camera attached to your box. If so, turn it on each evening to see if a bird is using the box for roosting. More details will shortly appear on the BTO website: Other birds form roosts that are more obvious. Starlings, for instance, gather together in very large flocks, which pulse and whirl about the sky prior to the birds dropping down and into the chosen site – often a large conifer.

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