Observing birds on one of the most remote islands in the land for well over half a century has revealed dramatic and unexpected changes in the behaviour of some long-distance migrants from Africa.

It also shows those coming from Siberia have increased significantly.

Continuous records made by the Fair Isle Bird Observatory for more than 60 years show that spring migration has got much earlier in recent years for many species, such as the swallow, which is arriving up to three weeks earlier. But, much more surprisingly, for some species, such as the willow warbler, spring migration has got much later.

In addition, considerable changes are apparent not just in the timing of spring bird migration, but also in the timing of migration in the autumn. The autumn migration of house martins has got progressively later while that of the swallow has got earlier, again by up to three weeks, compared to 60 years ago.

Dr Will Miles, of the Fair Isle Migration Project, will present some of his initial findings at RSPB Scotland's Big Nature Festival, Musselburgh on Saturday. The Migration Project, which he is working on, is a scientific collaboration between the Fair Isle Bird Observatory Trust and Aberdeen University.

Dr Miles said: "The natural world is changing, including the timing and numbers of birds on migration and a crucial challenge facing scientists is to identify and understand these changes and their causes.

"It's too early to say for sure why some birds are arriving so much earlier on Fair Isle, while others seem to be delaying their arrival, but possible causes include changing climate and weather patterns, also changes in the summer breeding range and population size of many species across Europe."

Fair Isle is Britain's most remote inhabited island, lying between Shetland and Orkney. It is an internationally renowned hot-spot for bird migration and the Fair Isle Bird Observatory has made daily census counts of migrant birds. This written record was recently digitised,.

It also reveals the number of scarce migrant birds, arriving in Scotland from Eastern Europe and Siberia in the autumn, has risen sharply over the last 60 years, and is continuing to do so. Birds such as the yellow-browed warbler and the barred warbler in particular have been arriving in greater numbers.