Week-end bias, length of stay of rarities and proportion of rarities undetected Sirs,--Although the effect of 'week-end bias' (the increased watching at week-ends) is often commented on, there appear to be no figures to show exactly how marked it is. In an attempt to determine this, I have analysed the annual 'Reports on rare birds . . .' for the eight years 1958-65 (Brit. Birds, 53: 153-173, 4Q9-431;...

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