MEASUREMENT ERROR IN AERIAL SURVEYS OF OSPREY PRODUCTIVITY

Reproductive performance of osprey (Pandion haliaetus) has been used as an indicator of health of populations and aquatic ecosystems. Reproductive output is usually assessed by a mid-May aerial survey of nest occupancy, followed by a mid-July aerial count of chicks, but measurement error margins have not been investigated. In 1992-93, we compared independent counts of chicks made from fixed- and rotor-wing aircraft by 2 observers in 2 study areas on Lake Huron, each supporting 30-40 occupied nests. Fixed-wing surveys underestimated total chick numbers (P = 0.03), but rotor-wing survey counts by both observers did not differ from actual nest contents (P = 0.7). Variation between observers in chick totals was greater on fixed-wing surveys than on rotor-wing surveys. From rotor-wing aircraft, we could differentiate some dead chicks from live chicks. Nestling mortality after mid-July was unexpectedly high in 1 study area and 18% overall. Osprey productivity data should not be interpreted without considering measurement error and post-survey nestling mortality.