Temporal and sex-specific variation in growth rates of Marabou Stork Leptoptilos crumeniferus chicks

Abstract

The nesting success of Marabou Storks Leptoptilos crumeniferus breeding in north-eastern Swaziland is closely associated with rainfall, with nests started late in the season exposed to higher rainfall and showing lower success. This may be related to lower food intake and slower growth of the chicks. This study set out to determine whether hatching date and sequence of laying affected the growth rate of chicks. Chicks were also sexed, as Marabou Storks show sexual size dimorphism—males are on average 20% larger—and this trait is often associated with differing patterns of growth between sexes. Nestlings were measured weekly from hatching until they either died or fledged. Nestling development is described in detail and photographs of different-aged chicks are presented. The nestling period was significantly shorter for female chicks, at 94 d, than for male chicks at 104 d. Male and female chicks differed in growth rate and asymptote for both mass and wing length. Unusually, females showed higher instantaneous growth rates for much of the nestling period. Chicks surpassed adult mass before fledging. Date of hatching had an effect on growth rates, with chicks at late nests having slower growth, consistent with a decline in food availability. Marabou Storks appear to be slower growing than expected for the Ciconiidae, the taxonomic family to which they belong.