03 May 2016

Birder Beware - Bleached Pigments

It's Spring. Large white-headed gulls have been wearing their juvenile flight feathers for nearly an entire 12 months. By now, many are showing primaries that are worn, faded, and yes, bleached.

1st Cycle Ring-billed Gulls. Cook County, Illinois. 01 May 2016.
The individual to the left is showing typical, dark primaries that appear to be in very good condition. The bird to the right has primaries (and tertials) that have a light brown coloration. These feathers appear diluted in color because of bleaching. Both birds are the same age, except the individual to the left hasn't suffered the effects of bleaching.

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.