Redwings and Irruptive Migration
January 15th (2024) saw the beginning of a short, severe cold spell which brought flocks of Redwings from the countryside to towns, where conditions are less exposed and there is a plentiful supply of berries to feast on.
Views of Redwings in Hastings
We had a flock of more than twenty birds regularly feeding on the berries of an old holly tree at the bottom of our garden in St Leonards (near Hastings). They would spend a lot of time rummaging in the thick of the tree, while individuals would occasionally emerge to sun themselves for a while at the top. On the odd occasion when they were startled we saw the full extent of the flock.
Identification
Redwings are our smallest member of the Thrush family, similar to the Song Thrush in appearance, except the Redwing has a strong pale stripe over the eye, and has a flash of deep russet brown under the wing.
Breeding and Migration
Redwings breed in Iceland, Scandinavia, and Russia and migrate south in large numbers during the winter. Some will travel as far south as the Mediterranean.
The majority of Redwings that overwinter in the UK come from Russia. Their influx can be quite spectacular, as those that are destined to remain here mix with those that will be flying across the channel to winter in France, Spain and Portugal.
In record years, tens of thousands of Redwings, and the closely related Fieldfare, are counted in a single day funnelling along their main migration routes (flyways).
The massive flocks show up on weather radar assembling in the southern tip of Norway waiting for the right weather conditions to cross the North Sea. When they judge conditions are good, they cross en masse, some flying directly south, to reach the Dutch coast, and others south west to reach the north east of Britain. From here they head inland and then turn directly south, down through central and west England, to cross the channel.
The majority of these birds winter in Europe, but around 70,000 are estimated to remain in the UK for the duration of the winter. Their preference is for open rural areas, feeding on worms and berries, but when conditions get tough they move into urban areas to feed.
Richard Smith gives a wonderful description of the spectacle that was observed in October 2022, in the north east of England, during a record year. His analysis of the migration flyways is fascinating: http://www.deeestuary.co.uk/news1222.htm#first
How decisions are made on which flyways to use, and where to overwinter is still not well understood. Clearly weather conditions are an important factor, but research on the migration of thrushes from Scandinavia indicates that a certain amount of individual preference on the day is part of the equation1.
In many species, migratory behaviour is hardwired in the individual bird’s DNA. When they leave, the direction they take, and the distance they fly are very much predetermined. Redwings have many of the characteristics of “irruptive migrants” that are far less predictable in their behaviour.
Irruptive Migration
Irruptive migration occurs in many parts of the world. In the Palearctic region the phenomenon is usually associated with birds that nest in Scandanavia and Russia – birds such as the Brambling, Crossbill, and Waxwing.
The winter diet of these birds is seeds and berries from particular trees e.g. the Brambling favours beech nuts, Crossbills depend on pine nuts, and Waxwings on Rowan berries.
The trees they depend on are cyclic in their production of fruits and seeds, with a year of super abundance (mast years) often followed by poor or even no production. This cycle is synchronised among the same tree species across large regions.
The birds will over-winter in or near their nesting area as long as there is sufficient of their favoured seed or fruit. When the crop depletes the birds leave in large numbers in search of a new supply. (Similarly, vole populations are cyclic, with years of abundance and scarcity. This affects the behaviour of some raptors that favour voles as a food source.)
In times of very poor or no production the majority of the birds will migrate early; in poor to average years migration will occur late; and in times of abundance the birds will often remain in and around the breeding area. When they do depart, they disperse in a variety of directions and do not necessarily follow the same flyway as in previous years. The distances they travel also vary considerably between years. Much depends on where they find a good food source.
Redwings as Irruptive Migrants
The southern migration of the Redwing has been described above, and in itself doesn’t suggest they are irruptive migrants. It is, however, their highly mobile and unpredictable movements that have all the characterisitcs of irruptive migration. In particular Ian Newton sites studies that show that:
“Of Redwings ringed in winter in Britain, dozens have been recovered in subsequent winters as far east as Italy, Greece, Turkey, and some as far east as Israel and Iran, at localities upto 5,000 kms apart … even birds from the same brood have been recovered in widely separate localities in the same winter.” 2
As with other irruptive migrants, one year you will see them in a locality in large numbers, and the next there will be no sign of them. You may have to wait several years before the appear again.
The holly tree in our garden is loaded with berries this year (2026) and we are still waiting their arrival.
……………………………………………………………………..
Notes
1. “To cross or not to cross – thrushes at the German North Sea coast adapt flight and routing to wind conditions in autumn” Vera Brust, Bianca Michalik, Ommo Hüppop; Movement Ecology volume 7, Article number: 32 (2019)
2. “The Migration Ecology of Birds”, Ian Newton, 2024 (second edition), Academic Press, pp 420.