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Egg data slips from museum collections were used to examine their reliability, especially bias in their seasonal distribution of egg-laying dates and clutch size and in overall mean clutch size of nidicolous altricial birds in North America. In general, egg data slips are useful. Suspect data, whether apparently falsified or not, are rare and usually detectable. Mean clutch sizes from egg data slips are usually not seriously biased upward, and are comparable with other nonoological sources. Thus, with some exceptions, Lack' s (1946) criticism that mean clutch sizes may be inflated is greatly exaggerated. Similarly, Lack' s criticism that the largest complete clutches in the middle of the nesting season are overrepresented lacks support. Lack' s remaining criticism is generally true; early dates from egg data slips are overrepresented. For a further review of the value of egg data slips, see McNair (1985).

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