| Summary: | In many avian species with synchronously hatching young, incubation and embryonic development begin prior to the completion of the clutch. However, mechanisms regulating synchronous hatching and more rapid embryonic development in eggs laid later in the clutch are poorly understood. We measured physical characteristics of eggs from complete clutches of Canada Geese (Branta canadensis) to investigate whether variation in eggshell conductance, shell thickness, shell porosity, or rates of yolk accumulation during egg formation might support the observed increase in metabolic rate of eggs laid later. Canada Geese lay large clutches (7–9 eggs) and initiate incubation before completing them (approximately with egg 3), but the young hatch and leave the nest synchronously. Both conductance (from ∼15 mg H2O torr-1 day-1 for the first to ∼25 mg H2O torr-1 day-1 for the eighth egg in the clutch) and porosity (from ∼2.5 pores cm 2 for the first to ∼3.0 pores cm -2 for the eighth egg in the clutch) increased linearly with the sequence in which the eggs were laid. Neither shell thickness nor rates of yolk accumulation were related to the egg's position in the sequence. We hypothesize that, in waterfowl and other species that lay large clutches and initiate incubation prior to completion of the clutch, changes in the shell gland during egg laying lead to the production of progressively more porous eggs that promote vascularization of the chorioallantoic membrane of the embryo to sustain the higher metabolic and developmental rates necessary for synchronous hatching.
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