In many areas surveyed during the North American WBPHS, ground surveys are conducted in conjunction with the aerial surveys to derive annual visibility correction factors (VCFs) for the aerial survey crews, which are used during analyses to adjust for detection bias. Due to the remoteness of strata 1–12, ground surveys are not conducted there. Instead, a set of fixed VCFs by species and basic habitat type (strata 1–7 = boreal forest; strata 8–11 = tundra; stratum 12) are used. The fixed VCFs that were used through 1991 were derived from work done in WBPHS prairie strata. During the years 1985–1991, efforts were made by the FWS Division of Migratory Bird Management-Alaska Region (MBM-Alaska) to develop more appropriate fixed VCFs for strata 1–12. Helicopter/fixed-wing comparison surveys were flown, using the helicopter counts in place of ground counts. As a result of this work, new VCFs for strata 1–7, 8–11, and 12 were established and have been used since 1992. The new VCFs are applied to all years of the data when conducting analyses.
WBPHS_VCF.csv is a table that lists the VCFs that were adopted in 1992 for strata 1-12.
