Comparative feeding ecology and reproductive performance of ospreys in different habitats of southeastern British Columbia

We compared the general breeding and feeding ecology of ospreys (Pandion haliaetus) in the Creston and Nelson areas of southeastern British Columbia. In the Creston Valley, ospreys nested atop tall trees surrounding a shallow and productive warm-water marsh. Prey species taken by male ospreys included black bullhead (Ictalurus melas), pumpkinseed (Lepomis gibbosus), and yellow perch (Perca flavescens). In contrast, near Nelson, ospreys nested on man-made structures along the narrow West Arm of Kootenay Lake. Osprey prey species in the Nelson area included longnose sucker (Catostomus catostomus), largescale sucker (Catostomus macrocheilus), and mountain whitefish (Prosopium williamsoni). Prey captured at Nelson were larger and contained significantly more energy than at Creston Valley, and hunting from a perch was used for 26% of all captures. All Creston Valley prey were caught by flight hunting. The strike success of foraging ospreys at Nelson was significantly higher than at Creston, and the net yield of flight hunting was 3 times higher. In spite of these differences, the breeding performance of ospreys in the two areas was very similar. Average clutch size was 2.8, brood size at hatching was 2.0, and the average pair fledged 1.4 young. The nest failure rate did not differ between the two areas. Most clutches were initiated in early May, with Nelson-area ospreys laying, on average, 4–7 days later. Egg volumes were smaller at Nelson (66.6 vs. 69.1 cm3). However, the rate at which nestlings gained mass was significantly greater at Nelson. We discuss reasons why the breeding performance varies so little in spite of the great differences in feeding regime.