Adult and Juvenile Osprey (Pandion haliaetus) Foraging Behavior on San Juan Island, WA

Abstract

The osprey (Pandion haliaetus) is a widespread piscivorous raptor with unique adaptations for plunge-diving to capture live fish. In the Pacific Northwest, breeding occurs in late spring throughout summer, during which juveniles learn how to hunt before leaving the nest and becoming independent. This study examined the foraging behavior of adult and juvenile ospreys at False Bay, San Juan Island, WA, with attention to how tidal conditions influenced foraging activity. We conducted 10 hours of observations over four days in August 2025 on a nesting pair with their two fledged offspring. Behavioral data were collected continuously and analyzed with respect to foraging trip timing, dive attempts, strike success, and tidal conditions. In total, 14 foraging trips were recorded, with adults making 11 trips and juveniles making 3. Adults were consistently more efficient, capturing prey in nearly three-quarters of trips, while juveniles did not capture any fish despite diving more frequently per trip. Adults made fewer but more precise dives, whereas juveniles often aborted dives or failed to recover altitude between attempts. Foraging occurred most often at mid-tide heights, around 2 to 3 ft, consistent with osprey diving limits. Ospreys also foraged at high tide heights during ebb tides compared to flood tides, suggesting that fish movement with tidal currents shapes prey availability. These findings highlight the learning curve juveniles face before achieving independence and emphasize the role of tidal dynamics in shaping foraging opportunities. Continued study of juvenile development and prey-tide relationships will clarify how dynamic ecosystems influence osprey foraging behavior.