During 1997 and 1998, we studied American Kestrels (Falco sparverius) using nest boxes at Kesterson Reservoir (Merced County, central California), a site with elevated soil selenium concentrations. We radiotagged male kestrels to determine their foraging ranges. Nest-box use was lower than expected; eleven nest boxes at Kesterson were used during the two years (16% and 20% occupancy) and five offsite nest boxes were used (60 and 40% occupancy). Kestrel pairs in 60% and 67% of onsite nest boxes that were used successfully fledged chicks in 1997 and 1998, respectively; pairs in offsite nest boxes had 100% (1997) and 50% (1998) success. Concentrations of Se in blood and eggs of kestrels at Kesterson were similar to concentrations in offsite kestrels. We analyzed 31 pellets and we used previously measured selenium concentrations in prey items from Kesterson to estimate dietary Se exposure of 8.1 µg Se/g for kestrels at Kesterson. We estimated that radiotagged male kestrels foraged onsite 58% of the time in 1997 and 53% of the time in 1998 and that they foraged from 0 to 3.4 km (maximum foraging distances observed: 0.5–3.4 km) from the nest box during foraging bouts.
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Vol. 43 • No. 4