|1st Cycle Ring-billed Gulls. Cook County, Illinois. 01 May 2016.|
Birders should keep this in mind especially when assessing white-wingers (i.e., Thayer's and Kumlien's in particular) and suspected hybrids. A bleached Thayer's in California at this time of year may tempt one to call it a Kumlien's. Similarly, a dark Kumlien's in Toronto may be suspected of being a bleached Thayer's, and so on and so forth.
The implication here is that some birds may not be safely identifiable at this time of year only because of this ambiguity of "true" pigment.