|The mirror on p9 merges with a white inner web. © Martin T. Elliott|
This pattern is found on multiple taxa - not only Thayer's - but is only visible when the primaries are sufficiently spread. Below is a short series of the same bird demonstrating this:
|The inner web to p9 is sufficiently exposed with the primaries spread.|
|As the wing begins to fold the inner webs are less exposed.|
|No thayeri pattern is detected here as the inner web to p9 is covered by the outer web of p8.|
With many contemporary papers on large gull ID relying less on "birds in the hand" and museum specimens, it's imperative that fully spread wings are obtained. Digital photography has made capturing these details relatively simple, and it allows a closer look at what would otherwise be fleeting glimpses in the field.