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Synchronization of hatching within clutches of precocial bird species can be achieved either by acceleration or retardation, i.e. by shortening or prolonging the incubation period. The ability of mallard (Anas platyrhynchos) and ring-necked pheasant (Phasianus colchicus) embryos to accelerate or retard hatching was tested by incubating separate clutches, of which three eggs had 2 d longer or shorter incubation time than the others, and observing their individual time of pipping (breaking of the shell). Mallard embryos were able to delay hatching by on average 0.6 d (43% of the eggs delayed at least 1 d), but were better at acceleration (on average 1.3 d; 91% of the eggs accelerated more than 1 d). Conversely, pheasant embryos were only able to accelerate by 0.4 d (50% accelerated more than 1 d), but were better at delaying the hatching (1.2 days; 77% delayed more than 1 d). This difference between the species may depend on different degrees of relatedness within clutches in pheasants and mallards. It may also be an effect of the more developed sensory and neuromuscular systems in galliforms; a reduction of the incubation period would mean that the development of, for example, locomotion would be insufficient at hatching.