Central parts of avian colonies provide better safety against predators and, thus, are likely to be occupied by pairs of high quality. Therefore, spatial variation in the quality of breeders and predation rate within colonies should be directly reflected by the gradients of breeding success and chick survival. We have investigated relationships between nest location characteristics and survival of Whiskered Tern Chlidonias hybrida chicks in the colony at Jeziorsko reservoir, central Poland. We found that pairs nesting in the central, densely occupied areas of subcolonies had higher chick survival rates in comparison to edge pairs. There was also strong support for the positive effect of egg size and the negative effect of hatching date on the chick survival. Since pairs breeding in the central parts of subcolonies laid eggs of greater size and initiated laying earlier, we suggest that within-colony patterns of chick survival could be primarily attributed to the spatial variation in parental quality rather than to the higher predation pressure in the peripheral zones of the colony. There was also a moderate support for age variation in the survival rates of chicks, which ranged from 0.72 ± 0.06 to 0.91 ± 0.07 in different 5-day intervals of pre-fledging period. The survival of chicks over the entire pre-fledging period was estimated at 0.29–0.33, depending on the model.
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Vol. 48 • No. 2