I'm interested to know how much observer's rate the brighter yellow undertail/vent of Green-winged Teal compared to Eurasian Teal? According to some sources, Green-winged Teals show a brighter lemon-yellow vent than do Eurasian Teal. Through my experience of the species, analysing photos and reading various literature, this is not case in at least some individuals.
Presumably some Green-winged Teal do show a brighter lemon-yellow vent than do Eurasian Teal, but it seems that they can also be equal in colour to Eurasian Teal. I have not however found any Eurasian Teal which show brighter vents than Green-winged Teal.
Could the vent colour be age related, or could the fact that paler feathers abrade faster than darker feathers be a factor (in which case a late spring adult drake teal might show a slightly darker yellow vent than an early-winter adult drake teal)?