Twites Wintering in Caithness
The Twite Carduelis flavirostris is one of the commonest small seed-eating birds in Caithness throughout the winter months. Twites can be found in flocks of several hundreds in various agricultural habitats. They can often be seen, for example, in very noisy flocks on telegraph wires above particularly weedy Turnip fields. There are several other traditional local names in Scotland for the Twite, the commonest being Heather Lintie and Hill Lintie, for the Twite is very similar in appearance to the Linnet Carduelis cannabina, from which it is easily distinguished at close quarters by its bright yellow bill compared with the dull grey bill of the Linnet.
Within Britain the Twite breeds in 2 main areas, the Peak District and southern Pennines in England, and in northern and western Scotland from the Mull of Kintyre to the Western Isles and through the Highlands to the Northern Isles (Orford 1873, Sharrock 1976), with only small numbers breeding elsewhere in Scotland. In Caithness, however, Twites are far more common in the winter than in summer.
Over the past 19 years, we (2 or 3 other birdwatchers and myself) have been studying Twites in Caithness in winter; we have been studying their habitats, their abundance and their origins.
Abundance and Habitats
We have carried out 6 surveys of Caithness to attempt to estimate the Twite’s winter abundance (Clark and Sellers 1997, Clark and Sellers 1999). Five of these surveys were carried out in the period 1992-1994 and the last survey was carried out in Dec. 1998. Each of the 6 surveys was carried out over 2 consecutive days and involved up to 5 volunteers checking fields and other ground visible from all public roads in Caithness. Earlier exploratory fieldwork had shown the Twite’s preferred habitats to be almost exclusively fields of Turnips Brassica napa and Oil Seed Rape Brassica napus, with a few birds in Marram Grass Ammophila arenaria near sand dunes and on other weedy areas such as roadside verges. So in our surveys we attempted to check all such locations in Caithness; this frequently involved getting out of the car and walking through the area to put up the birds which were feeding intently on the ground. For all Twite flocks located, the number of birds was estimated and the habitat recorded.
The most important habitats by far were confirmed as weedy Turnip fields, where Charlock Sinapsis arvensis was the most common weed, and fields of uncut Rape or Rape stubble. 31% of all birds were found in Turnip fields and 63% were found in Rape fields. Naturally these preferred habitats were found in the agricultural areas of the county, with the peat and bog land of south and west Caithness having no birds. The results of the 6 surveys are summarised in Table 1,where they are also compared to the availability of Rape.
Our survey showed that in years when farmers had been unable to harvest their spring sown Rape crops due to bad weather, or to plough in the Rape stubbles, again due to bad weather, the number of Twites wintering in Caithness increased. The winters when most Rape seeds were available to the birds, 1992 and 1998, were the winters with the most birds.Origins
In the hope of being able to determine the origins of the winter Twites, we have ringed over 900 birds with rings supplied by The British Trust For Ornithology who administer the scheme for bird ringing in Britain. We caught the birds in mist nets set up in Turnip or Rape fields, with birds being encouraged to the vicinity of the nets by the playing of a tape recording of Twite’s song. The ringing of these birds involved us spending many hours in the field, because in the windy weather of Caithness many birds just bounced off the nets, resulting in daily catches sometimes being only 2 or 3 birds, while on calm days up to 40 birds could be caught.
Only 4 of these ringed birds have been seen again away from their site of ringing. A bird ringed at Killimster, near Wick, in December one year was found dead near Poolewe, Wester Ross (165 km WSW) the following May and a bird ringed at Lynegar, near Loch Watten, in November was found dead on Lewis in the Western Isles (180 km W) in July, 3.5 years later. In addition, a bird ringed at Lynegar in January was found dead the following April at Dunbeath (29 km SSW) and another ringed at Northfield, near Wick, in December was found tangled in sheep’s wool at Hempriggs, near Wick (2 km SSE) in June 2.5 years later. The last 2 birds mentioned seem to have been Twites from our own Caithness population which would have summered and wintered in the county, whereas the first 2 were birds which bred or summered in north west Scotland but wintered in Caithness.
We have also examined the records held by the British Trust for Ornithology of all documented Twite movements and noted that, of 4 birds found in Orkney, at the onset of winter, 3 were ringed in Fair Isle and one in Shetland, all in summer. In addition, a Twite ringed in summer in Fair Isle was found in winter near Ellon in NE Scotland. So it would seem highly likely that some Twites from the Northern Isles move south for the winter into agricultural areas of northern Scotland, including Caithness. Our continued ringing of Twites in Caithness may one day confirm this.
So at this stage in our study, the Twites which winter in Caithness appear to consist of birds from our own breeding population, plus birds from NW Scotland and the Northern Isles.
The total British and Irish midwinter population of Twites has been estimated at 100,000-150,000 birds (Lack 1986). The Twites wintering in Caithness thus represent roughly, in typical years 2%, and, in exceptional years, up to 5% of the British and Irish wintering population, emphasising the importance of the area for Twites.
Throughout Britain there is a concern over vanishing habitats and the consequent decline in the numbers of small seed-eating birds including the Twite. Indeed, national bird organisations such as the RSPB and the BTO now class the Twite as a species of high conservation concern. Although Twite numbers seem reasonably healthy in Caithness at present, there are some worrying changes in farming practices which could soon alter that situation. For example, over the past 50 years there has been a huge decline in the acreage of Turnips grown in Caithness and, by implication, the associated weed flora. According to data supplied by the Scottish Office Agricultural and Fisheries Department, the acreage of Turnips in Caithness has decreased by a factor of 8 over the past half century or so; from 3662 hectares in 1937 to 439 hectares by 1991. Furthermore some farms whose Turnip fields contained many weeds, especially Charlock, 10 to 15 years ago, now have virtually no weeds due to improved methods of weed control. So the food available to Twites from Tunrip fields will have been reduced by much more than a factor of 8 in the last 50 years.
The continued availability of Rape crops could have been a boon to the Twite, but there are signs that this crop may be about to decline sharply in Caithness due to a change in farming subsidies and the falling price being paid to farmers for their Rape seed crops. However, on a more positive note, the number of fields of weed strewn ‘set-aside- appears to be on the increase in this area.
Whether Caithness continues to be an important wintering area for the Twite, and other small seed-eating birds such as the Linnet Carduelis cannabina, Chaffinch Fringilla coelebs, and Greenfinch Carduelis chloris with which the Twite often associates in winter, will clearly depend on farming practices and the crops grown in the area.