SIRS,--I have delayed answering the letter on the above subject from Mr. P. H. T. Hartley (British Birds, xli, pp. 254-255) because the data then available only served to justify his remark that " more investigation is needed." I examined the two sets of winter frequencies statistically, and assuming the validity of both methods, could only conclude that (statistically) he and I were working on " dif...

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