T H E F I R S T British bird observatory was founded in 1933, when R. M. Lockley made a small Heligoland-type trap in his garden on Skokholm, Pembrokeshire (Lockley and Russell 1953); a big Heligoland trap was built some two hundred yards from it two years later (Lockley 1947). These two traps still catch most of the passerine migrants that are ringed on Skokholm. The object of this paper is to an...

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