Thursday, 7 January 2010

The vikings are coming

Hedgerows that a few short weeks ago were plump with berries are now empty, stripped by large numbers of wintering thrushes, including many blackbirds, together with song and mistle thrushes. Snow, thick on the fields in the run up to Christmas, added to the demand for energy-rich berries, pushing redwings and fieldfares off the fields to feed upon the now dwindling berry stocks. Many of the redwings will have been here since late autumn, arriving at night, their nocturnal migrations softly broadcast through their characteristic flight calls. In the mild weather of November and early December these birds will have fed on earthworms and other invertebrates, initially in farmland but later within the more sheltered woodlands where the effects of overnight frosts are greatly reduced.

Redwings are extremely vulnerable to prolonged periods of cold weather and, unable to find food, they quickly use up their limited energy reserves. In the coldest of weather they can suffer very high levels of mortality, something that may explain why the birds may push southwest from Britain into Spain and Portugal if the British Isles are hit by a pronounced cold snap. The birds will also turn to gardens, moving in to exploit any berries remaining on a cotoneaster or holly. Here they will face competition from the larger (and hence dominant) blackbirds and mistle thrushes. The redwings may also make use of windfall apples.

The arrival of these birds often triggers a run of phone calls to the offices of the British Trust for Ornithology (BTO), with people unsure of the identity of these small thrushes seeking advice. Slightly smaller than a song thrush, the redwing is an altogether darker looking bird, streaked below and dirty brown above. The key features to look for are the bold white stripe above the eye (the supercilium) and the red ‘staining’ under the wing which just extends down onto the flanks. Be aware that the song thrush has a rusty-buff underwing that might cause confusion.

Redwings are nomadic in their movements, to the extent that a bird wintering in a Norfolk garden in one year, could be wintering in Italy or Greece the next. Our birds come from two distinct breeding populations, one in Iceland and one extending from Finland through into Russia. Those from Iceland tend to winter in Ireland and northwest parts of Scotland, while those from Finland and Russia tend to winter in the southern half of Britain. The two ‘races’ are subtly different from one another, the Icelandic birds tending to be larger and darker. Interestingly, the species has bred in Britain on occasion, typically when populations elsewhere have been at their peak, something that is no longer the case.

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