The double observer approach with marked nests was used in southern France for estimating the detection probability of nests of the Squacco Heron (Ardeola ralloides), a cryptic species in its breeding behavior. Results indicate that the detection probability varied from 0.518 to 0.700 between observers. The overall detection probability, i.e., the probability that a nest was detected by at least one observer, was 0.856. Given a total of 66 nests found by both observers during the surveys, the breeding population was estimated at 77 nests. Since previous estimates of the Squacco Heron breeding population size did not take into account detection probability, we suggest that the breeding population is larger than previously thought in the Camargue and in other European breeding localities.