Female scaup at Willow Tank (Photos by Narca)
So... is she a Greater or a Lesser Scaup? This scaup is showing some ambiguous markings. She has the tiniest hint of a white cheek patch, which female Greater Scaup can develop rather strongly in summer. But a whitish cheek patch isn't a diagnostic mark.
The shape of her head appeared good for Lesser Scaup in Alan's photos, yet when another birder studied her, he thought that the head shape seemed flatter and rounder. (He agreed that the shape in Alan's photos looked better for Lesser.) When I saw her, some head feathers seemed matted down, while others were dry and puffy, distorting the head shape––as you can see in these photos.
Her bill does seem more delicate and narrower than a Greater should show.
My favorite field mark can be tough to see. It's the width of the nail on the bill, and can require a very careful look, in good light. So on Wednesday Alan and I headed to Willow Tank, with scope in tow. The early morning light was exquisite, and eventually we routed the scaup from her cover in the waterweeds, and she swam to the center of the pond, where it was finally possible to see that the nail on her bill was narrow. Lesser Scaup. (And true––Lesser is by far the more common species in Arizona. Yet in early July neither should be here!)
Female Lesser Scaup at Willow Tank
What in the world is she doing in southeast Arizona in summer? Did an injury prevent her migration?
A postscript... she did finally fly for Jon Dunn and his WINGS group, confirming Lesser Scaup. This bird was tricky, though––Jon said that his first thought was "Why isn't this a Greater?"