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Tardy geese

by Malcolm Ogilvie

The arrival of the wintering geese on Islay this autumn was one of the latest ever recorded. Normally, the first few Barnacles and Greenland Whitefronts come at the end of September, about half the eventual 35,000 Barnacles and 12,000 Whitefronts arrive around 5th-10th October, and the remainder well before the end of the month. This year, the first arrivals were 80 Barnacles on 6th October, then no more until c.7,000 on 11th, plus a few hundred Whitefronts. Our first island-wide goose count on 24th/25th October found only 13,000 Barnacles and 2,000 Whitefronts. The comparable figures for 2000 were 34,000 Barnacles and 10,000 Whitefronts. It was not until 27th October that another 15,000 Barnacles flew in, together with more Whitefronts. Both species were still trickling in throughout the first week of November, a good two-three weeks later than usual.

The late arrival of the geese was a good demonstration of the effect of weather on bird migration. Both species of geese arrive in Iceland during September, the Barnacle Geese from east and north-east Greenland, the Whitefronts (left) from west Greenland. They settle in the farmland areas to feed on the grass, putting on weight, especially the goslings, in preparation for the flight to Scotland and Ireland. The distance from Iceland to Islay is between 650 and 800 miles depending from which area the geese depart. This is a journey, without allowing for the wind, of perhaps 15-20 hours duration. And the wind is a crucial factor in bird migration, capable of providing considerable assistance as a tail wind, but undoubted hindrance as a head wind, while cross winds can blow birds hundreds of miles off course.

Autumn winds in Iceland are strongly linked to temperature. Southerly winds will be mild, keeping the weather open and the grass available for grazing. Northerly winds will be cold, bringing frost and snow. This pattern governs how the geese behave. Southerly winds allow them to stay and feed, while if they chose to depart would mean a head wind on their journey south. Northerly winds not only bring frost and snow to reduce the availability of the grass, but give the birds a tail wind boost to their migration, capable of knocking several hours off their flight time. It makes sense, therefore, to remain in Iceland during mild weather and to leave as soon as it turns cold. What we have seen this year is the result of an unusual mild October, not just in Britain as we all experienced, but also in Iceland, with temperatures in Reykjavik still a relatively balmy 12 degrees Celcius right up to the last few days of October. Only as the weather turned cold did the geese finally think it was time to leave.

Good conditions for departure from Iceland cannot always guarantee equally good conditions throughout the journey and there are many recorded instances of migrating birds, including geese, flying into bad weather and being set badly off course. Fortunately, northern Scotland and the chain of the Hebrides presents quite a large target to birds coming from Iceland. However, Barnacle Geese occasionally find themselves well astray, as a recovery of a Greenland-ringed bird in the Azores some years ago showed.

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Dr Ogilvie is a natural history writer and editor, formerly a research scientist with the Wildfowl and Wetlands Trust, and resident on the island of Islay since 1986. Until 1997, a member of the 'British Birds' editorial board and also one of the editorial team which produced 'Birds of the Western Palearctic'.