Prolonged Incubation Behaviour of Red-Winged Blackbird Incubating Several Egg Sizes

[New records for prolonged incubation of eggs are summarized. The advantages for adaptation to a prolonged period of incubation only a few days longer than the normal incubation period is discussed. Non-passerines tend to remain on unhatched eggs for a relatively longer period of time compared to normal incubation period length than do passerines, with such notable exceptions as Columbids. Experiments were performed on wild incubating Red-winged Blackbird (Agelaius phoeniceus) females to determine some of the roles of the egg as a stimulus in prolongation of incubation. Five sizes of eggs of normal shape and color were introduced into nests after the normal clutch was complete. Very small or very large eggs were nearly always abandoned immediately. Normal size eggs and eggs ½X and 1½X the dimensions of normal eggs were usually incubated. More of the ½X eggs were abandoned on the first or second day than in the 1X and 1½X sizes and the mean length of incubation was 15.06, 19.38 and 19.13 days, respectively. There was no difference in the mean length of incubation on the 1X and 1½X eggs. There was a significantly longer incubation on iX and 1½X eggs than on ½X eggs. It is concluded that 1½X eggs were not super-normal stimuli but that ½X eggs were sub-normal stimuli for prolonging incubation. Hypotheses are developed for explaining the differences in the stimulating capacities of different egg sizes. The incubation patch as a receiver of stimuli may play a primary role. The physiological condition of the incubating female especially with regard to levels of different hormone titers may be most important in maintaining the incubation patch as an effective receiver of stimuli. The shape of the original nest, size of eggs and size of the female also may determine how much surface area of the brood patch is in direct contact with eggs., New records for prolonged incubation of eggs are summarized. The advantages for adaptation to a prolonged period of incubation only a few days longer than the normal incubation period is discussed. Non-passerines tend to remain on unhatched eggs for a relatively longer period of time compared to normal incubation period length than do passerines, with such notable exceptions as Columbids. Experiments were performed on wild incubating Red-winged Blackbird (Agelaius phoeniceus) females to determine some of the roles of the egg as a stimulus in prolongation of incubation. Five sizes of eggs of normal shape and color were introduced into nests after the normal clutch was complete. Very small or very large eggs were nearly always abandoned immediately. Normal size eggs and eggs ½X and 1½X the dimensions of normal eggs were usually incubated. More of the ½X eggs were abandoned on the first or second day than in the 1X and 1½X sizes and the mean length of incubation was 15.06, 19.38 and 19.13 days, respectively. There was no difference in the mean length of incubation on the 1X and 1½X eggs. There was a significantly longer incubation on iX and 1½X eggs than on ½X eggs. It is concluded that 1½X eggs were not super-normal stimuli but that ½X eggs were sub-normal stimuli for prolonging incubation. Hypotheses are developed for explaining the differences in the stimulating capacities of different egg sizes. The incubation patch as a receiver of stimuli may play a primary role. The physiological condition of the incubating female especially with regard to levels of different hormone titers may be most important in maintaining the incubation patch as an effective receiver of stimuli. The shape of the original nest, size of eggs and size of the female also may determine how much surface area of the brood patch is in direct contact with eggs.]