Mexican Duck, Anas (platyrhynchos) diazi, at First Dam, Logan, Utah.  Copyright Ryan P. O'Donnell.  

If you follow Utah Birdtalk/Birdnet, you already know that on February 5th, about two weeks ago, my friend Craig Fosdick found a very interesting duck at First Dam in Logan, Utah.  It was a Mexican Duck; probably the northernmost record of the species, and only the third from Utah.  Mexican Ducks are easy to overlook, and many readers of this blog might have never heard of one before.  Even the experienced birders Craig was with at the time hardly gave this unique vagrant a second look.

First, what is a Mexican Duck?  That question is not easy to answer.  According to the American Birding Association and the American Ornithologists Union, it is a subspecies of Mallard.  But that view is considered by some to be antiquated and inaccurate.  Recent genetic work has shown that the Mexican Duck may actually be its own species, and it is at least as unique as several other species such as the Laysan Duck and the Mottled Duck.  The picture is complicated, however, because Mallards contain several genetic lineages, and thus to have species boundaries that reflect mitochondrial gene phylogenies, one would either have to split Mallards based on strictly genetics, or else consider "Mallards" to include Laysan Ducks, American Black Ducks, Mottled Ducks, and many others, which obviously does not reflect biology well.  This is an area that is in need of additional research, particularly in generating phylogenies and assessing hybridization with nuclear molecular markers.

Phylogeny of Mallard-group ducks, modified from Johnson and Sorenson 1999, based on mitochondrial DNA sequences.  Note that Mallards appear twice here, having (at least) two different mitochondrial sequences.  Also note that Mexican Ducks are as distinct from Mallards as many other groups that we know of as species.

The definition of species within the Mallard group is confusing, but perhaps even more so in the case of Mexican Ducks, because our understanding of this species is clouded by hybridization with Mallards in Arizona and New Mexico.  A classic study of morphology in Mexican Ducks found that there are virtually no pure Mexican Ducks anywhere in the species range, according to a numerical scale of morphology that ranges from pure Mexican Duck to pure Mallard.  However, an alternative interpretation of the same dataset is that our definition of what identifies a pure Mexican Duck is too narrow, and that pure Mexican Ducks can show traits that have once been taken to be indicative of Mallards.

With respect to the duck seen recently in Logan, this appears about as close to a pure Mexican Duck as one can expect at the northern part of the range of the (sub)species.  There is very little green on the head.  The bill is bright yellow.  The tail shows no patches of white.  The speculum has green iridescence, and is bordered only thinly by white.  The rump and undertail coverts match the flanks well in color, showing no obvious indication of the black that a male Mallard has in these areas.  The belly is dark, matching the color of the rest of the bird well.  The only part that seems to show some obvious Mallard ancestry is that the central retrices (tail feathers) curl up slightly off the plane of the tail, hinting at the curled central tail feathers of an adult male Mallard.  But, with how little we know of "pure" Mexican Ducks, perhaps this is not outside the range of variation shown by them?  Only an extensive study of morphological variation and nuclear DNA across the range of the Mexican Duck and Mallard can really address this question well.

The Mexican Duck looks a lot like a female Mallard, but darker and (in a male, such as this one) with a bright yellow bill.  Copyright Ryan P. O'Donnell.

The Mexican Duck's speculum has more of a green iridiscence, compared to the purple of a Mallard's, although the color varies somewhat with the angle of view.  Also note here how, unlike a male Mallard, the rump is about the same color as the flanks and the tail is dark, without white.  Copyright Ryan P. O'Donnell.

Unlike a Mallard, the lower belly is not noticeably paler than the flanks or breast.  The bright white underwing coverts are a trait shared by male Mexican Ducks and male Mallards.  Copyright Ryan P. O'Donnell.

This angle can be tough to see in the field, but it shows how the back of the Mexican Duck is generally dark, and the speculum is only thinly bordered by white, not boldly bordered by white like a Mallard's speculum.  Note that at this angle the speculum looks more purple than green, like a Mallard's.  Copyright Ryan P. O'Donnell.

As rare birds go, this individual might be pretty easy to find.  It has been seen over the last two weeks at First Dam in Logan.  Watch for a duck that looks a bit like a female Mallard, but is darker and has a bright yellow bill.  While I don't advocate feeding park ducks, it is a common practice at this park, and if you happen to time your visit when a local is feeding them, this duck might come right out in the open and fight with the other local domestic breeds for bread.  Otherwise, you might get lucky and see it swimming around on the water, or it might be sleeping on the far shore of the lake.  If it's not out in the open when you get there, try patiently scanning the sleeping ducks along the shoreline.

The Mexican Duck being seen at Logan's First Dam seems to have paired up with a female Mallard.  Note in this view how this Mexican Duck's central retrices lift slightly off the others, perhaps indicating a small amount of Mallard heritage.  Copyright Ryan P. O'Donnell.