Perhaps it's harder to detect because birds with their ZW sex chromosomes can produce both male and female offspring, whereas other species with XY chromosomes

Perhaps it's harder to detect because birds with their ZW sex chromosomes can produce both male and female offspring, whereas other species with XY chromosomes would only produce females, e.g. the "lesbian lizards". So, we very easily notice cases of parthenogenisis if we see an imbalance of the sexes in a population, but we that wouldn't happen in birds so we don't notice until we sequence the DNA.