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Correction

Posted by KateRock on 25 Jul 2012 at 06:24 GMT

20-minute period
http://plosone.org/article/info:doi/10.1371/journal.pone.0040622#article1.body1.sec2.sec2.p1

Observers report the maximum number of birds seen at one time within a week, there is no time limit on garden bird surveys.

No competing interests declared.