Reproductive success of bald eagles in interior Alaska

We compared productivity and nesting success of 2 adjacent populations of bald eagles (Haliaeetus leucocephalus) near the northern limits of their range in interior Alaska during 1989-94. Productivity (x̄ ± SE young fledged/occupied territory) and nesting success differed between populations; pairs in the Gulkana River basin had higher productivity (0.86 ± 0.05, n = 274) and nesting success (59%) than those in the Copper River basin (0.71 ± 0.04, 48%, n = 471; P < 0.02). Productivity varied both annually and spatially within each basin (P < 0.001). However, brood sizes of successful nests were identical for both basins (1.48 ± 0.03), suggesting that variability in productivity resulted largely from differences in nesting success. Patterns of variability in reproductive success within a territory also were similar for both populations. Pairs that were successful one year fledged more offspring, were more likely to be successful, were more likely to reoccupy the same territory, and were less likely to change nest locations the following year compared to pairs that were unsuccessful the previous year (P < 0.025 for all comparisons). Most nesting failure (92%) occurred during incubation when weather conditions tend to be most severe. However, reproductive success was not negatively correlated with severity of spring weather (temp or rainfall) or strongly correlated with prey abundance during brood rearing. We hypothesize that annual and spatial variability in reproductive success of these northern bald eagle populations may be associated with variation in prey availability, especially before and during incubation.