Counts of migrating raptors are done worldwide and are seen as a useful tool for hawk-population management. Rigorous monitoring of animal abundance, however, requires an evaluation of the detectability associated with the survey methods. Incorporating detectability into abundance estimates has never been done in the case of counts of migrating hawks. The goal of this study was to evaluate the extent to which variation in weather and raptor behavior can affect the detectability of migrating hawks and, consequently, the assessment of long-term population trends. We used a double-observer approach to evaluate detection probability at the Observatoire d'oiseaux de Tadoussac in Québec, Canada. We then did a population-viability analysis, adjusted for detectability, on historical raw counts to evaluate how detectability affects the assessment of extinction probability of diurnal raptors' populations. The detectability of migrating raptors ranged between 75% and 90% and was influenced mainly by weather and flight altitude. High-flying hawks were easier to detect against a cloudy sky than against a blindingly sunny sky. Optimal detection probability was associated with hawks flying at eye level. Probability of detection was higher for larger hawks and for hawks migrating in a cluster. No observer effects were found. No differences were found in the quasi-extinction probability of a species when detectability was accounted for, but this could be the result of the highly variable weather conditions experienced at our maritime site coupled with short time series. We suggest repeating our study at both maritime and continental monitoring sites if our results are to be generalized.