Variation and heritability in egg size of the northern pintail

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 × breadth 2 ), was strongly correlated with weight of the fresh egg (r 2 = 0.89) and weight of the 1-day-old duckling (r 2 = 0.89). The body weight of pintail hens was weakly correlated with their egg size in the wild (r 2 = 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.