The discrepancy between migrating populations and populations present at the overwintering sites suggests a significant adult mortality during the migration.[62] The Commission for Environmental Cooperation has determined that population variations require a long-term and large scale monitoring effort[60] Population estimates of adults, or of eggs and larva, and milkweed abundance, should correlate with the censuses at the overwintering sites. Data are currently unavailable at this time to determine these censuses but a current study by The Monarch Larva Monitoring Project is designed to determine whether or not population censuses in Mexico match the population censuses in the Midwestern United States and Canada.[63]