ShuiEro ... from vague up to sometimes ;) crystal clear drop by drop ...

The bill length measurement is used to determine the sex of yellow-crowned parakeets; bill width is less useful.
In a study of yellow-crowned parakeets in Fiordland (Elliott et al. 1996), 99 parakeets were measured and weighed. Males were found to have bill lengths of ≥ 13 mm, and female bill lengths were < 13 mm. This rule was found to also hold true for the Te Kakaho and Mana Island populations (Fig. , but note that there may be some overlap between male and female measurements in a small number of birds. ... Note that some females had larger bill width measurements than some males, but there is no overlap in bill length between the sexes in this case.