Researchers (Huppop and Huppop, 2003) studying bird migratory patterns have tagged birds and monitored migration timing over Helgoland, an island in the southeastern North Sea. Birds passing over this island breed in Scandinavia and spend the winter in either continental Europe or Africa. Observations have been made since 1909, and methods of trapping birds have not changed since 1960. Twenty-three species have been trapped sufficiently often to provide reliable evidence of trends over this time period. For these species, migratory times are 2 to 12 days earlier now than 40 years ago (van Noordwijk, 2003).
References
Huppop, O., and K. Huppop, 2003: Proceedings of the Royal Society of London, B270, 233-240.
van Noordwijk, A. J., 2003: The early bird. Nature, 422, 29.