The relation between variability and abundance shown by the measurements of the eggs of British nesting birds

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Abstract
From measurements of 100 eggs of each of 180 spp. made by F. C. R. Jourdain, 5 statistics were calculated, mean length and breadth, variance in length and breadth, and covariance in length and breadth. Tables of these for each species are given. Since the variances in length and breadth are greater in larger eggs, and within any group of birds both large- and small-egg species occur, allowance was made for these size differences by replacing the means and variances by their common logarithms, and calculating from the sums of squares and products within groups the regressions of the log. of variance in length and log. variance in breadth as dependent variates, on the log. of mean length and the log. of mean breadth as independent variates. The analysis is given in an appendix. Comparisons within natural tax-onomic groups showed the most abundant species exhibiting the greatest variability, the rarest the least, supporting Darwin''s generalization that "widely distributed, much diffused and common species vary most." Amongst the few exceptions to the general finding was the house sparrow, in the abundant class, which was less variable than the average of the finches. The magnitude of the differences was about 20%. Evolutionary progress thus appears to be more rapid among the more numerous species in each group. The passerines were more variable than the non-passerines.

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