Reproduction within a recently re-introduced resident flock of Whooping Cranes ( Grus americana) in Florida during 1992–2007 was poor compared to an established wild flock. Pairing and nesting increased with average age of the flock, but fertility, hatching and fledging were variable among years, suggestive of an environmental influence. For climatic variables measured during the incubation period, only maximum soil temperature was associated with the failure of late nests in dry years. However, pre-nesting winter precipitation and water elevation were positively correlated with an index of nesting effort. Winter precipitation was associated with fertility and hatchability, whereas winter marsh water levels were associated with earlier nesting dates and increased egg volume. Both winter precipitation (>8 cm mean monthly) and water elevation (>20.3 m above mean sea level or 68% of full marsh surface area or perimeter) greater than a threshold level appeared to be good predictors for successful reproduction in central Florida, but occurred together in only four of ten study period years. Pairs delayed nesting, had smaller eggs, and hatched and fledged fewer chicks in years with low winter water elevation. Low winter precipitation associated with decreased fertility of eggs may explain the failure of fully incubated nests to hatch in some years. Insufficient stimulation of the neuroendocrine system due to limited rainfall and poor physiological condition due to poor food resources in low water marshes are proposed mechanisms for low fertility and delayed nesting and egg size, respectively. Drought appears to encourage birds to use lake edges for nesting which have increased hazards. Thus, conservation of wetlands for cranes should include deep marshes and lake edges with controlled boat usage during the nesting season. Further investigation is recommended for pre-nesting and incubation behaviors, diseases of eggs, and egg volumes.
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Vol. 32 • No. 4