Variation and heritability in egg size of the northern pintail

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Abstract
Sources of variation in the egg size of wild and captive northern pintails (Anas acuta) were examined in southern Alberta. Egg size, estimated by volume index (length × breadth2), was strongly correlated with weight of the fresh egg (r2 = 0.89) and weight of the 1-day-old duckling (r2 = 0.89). The body weight of pintail hens was weakly correlated with their egg size in the wild (r2 = 0.11) but was not correlated with egg size in captivity. Captive birds fed a 29% protein diet laid larger eggs than did those fed a 14% protein diet and egg size increased from first to second clutches in birds maintained on a constant diet. Egg size was not related to laying date or clutch size, and did not differ between adults and yearlings. Individual hens tended to lay eggs of a consistent size but there was no significant heritability for egg size.