There is nothing quite like encountering an atypical bird to kickstart the learning process. I was at Hillman Marsh (Essex, Ontario) on June 1, 2022 when a "dark" female Anas duck flew into the Shorebird Cell with a male Mallard (MALL). Once it touched down, a quick study did not seem to directly line up with any of my expected Anas species or hybrid combinations. This bird did not compute.
I worked closer, took photos, and eventually lucked out in having a close flyby when the bird was flushed by a 2nd male MALL - revealing a Mottled Duck (MODU) / American Black Duck (ABDU) type wing pattern - lacking any notable white. In flight, it gave a much stronger impression of MODU - so that was where I started researching.
Mottled Duck Commentary:
Subspecies: there are two; paler / more buff birds in peninsular Florida (Anas fulvigula fulvigula) and darker / more streaked birds along the the Gulf Coast (Anas fulvigula maculosa) from Alabama west into Mexico. There is also an introduced population in South Carolina consisting of both (?) subspecies. Each is reported to have gene flow with wild and/or domestic MALL, particularly in Florida.
Further Regional Variation: while reviewing media of Gulf Coast birds, I perceived greater regional variation than I was expecting. For example, birds in extreme southern Texas, moving west inland from the coast, start to give more of a subtle "Mexican Duck" vibe (beyond true MODU X MEDU hybrids, which have been recorded). Another example was a stronger sense of Mallard gene flow with nearly MODU-like birds in more developed counties.
Hybrids with Mallards: much attention has been given to Mottled Duck X Mallard hybrids in Florida, leading to an explosion of reports and media on eBird. This includes some birds that only have a few feathers that seem different than what might expect a pure MODU to show. There are less reports and associated media from the northern Gulf Coast, despite a paper which suggests hybrids are occurring with some frequency. The line between "pure" and "hybrid", particularly with female MODU, becomes blurry (and perhaps arbitrary) with additional research suggesting that the similarities between these species may be more due to ancestral variation and ancient gene flow than recent hybridization (yikes).
Individual Variation: if the above points weren't enough, there seems to be notable individual variation within a given region and it becomes very difficult to determine if there is potential gene flow with MALL (wild or domestic). The problem may be easier to determine in male MODU due to the more distinctive male MALL genes (green on the head, gray on the tertials, and especially the curled tail feathers); however, there are still some difficult individuals. Seasonal variation due to feather molt or wear may also lead to slight changes in what the "expected" appearance of MODU is. Finally, some individuals are just weird.
Commentary on the Jun 1, 2022 dark Anas:
Pro-MODU features include the slender structure, overall darkness, gape spot, pale buffy unstreaked throat and the wing pattern. Atypical marks include the white edges to the tail feathers, pale areas in the undertail coverts, the extent of streaking on the face and the concentration streaking/marks extending as a line out from the gape - which is present, but possibly accentuated due wet plumage from constant feeding. The overall darkness, less mottled appearance and extensive face streaking removes the Florida MODU from consideration.
While researching Gulf Coast MODU variation and hybrids, I noted that white edges to the tail and notable levels of overall face streaking were (in my opinion) well within the expected range of variation for Gulf Coast MODU. Less common were the concentrated gape-line streaking; however, there are a number of examples where this was shown on possibly pure MODU, including from more wild habitats, paired with equally pure looking males.
Finally, MODU-type birds with white or pale areas on the undertail coverts were genuinely harder to find, with birds showing this feature often looking atypical in other ways such as lacking a gape spot, being heavily streaked or just looking a bit "off" from the classic female MODU. With that said, potentially pure MODU with these pale areas are being documented and the sample size (media where you can see the undertail coverts of a female MODU - especially during the nesting season) is small.
So where does this leave the identification? Are the atypical traits due to introgression with domestic or wild Mallard genes? And is that introgression recent or historical? Or is it just variation within the female plumage variation (regional or seasonal) for MODU? A New York Times article on MODU hybrids in the gulf coast has a great quote: "The whole idea of speciation is like a mountain range. It’s easy to tell the differences between peaks, but saying where one starts and another begins is very difficult."
My biased opinion: she is within the range of the Gulf Coast MODU but is not a classic or typical example which would make for an easier identification. Interestingly, I could find images of female MODU in southern Louisiana, at a higher rate / in a lower sample size, that looked more like the Hillman bird than elsewhere in the Gulf Coast MODU range. See the composite below of the Hillman bird (top left) with birds labeled as MODU from Louisiana:
It's not hard evidence or a field mark, but it at least made me feel like I wasn't (entirely) sitting in a hopeful MODU echo chamber during my research. Alas, it still leaves the question of "Are the above birds pure MODU? Or do they have MALL genes?"
I sometimes find myself disappointed when a bird (both ones I observe and other peoples observations) are "rejected" for not being a typical or perfect example of a species. Especially when you consider that birding by nature is not always perfect (e.g., viewing conditions, duration, distance, seasonal appearance, etc.). Setting standards are required for scientific and competitive purposes, but I would encourage everyone who also birds for fun to try to not let it get you down and enjoy the highs and lows of seeing something unusual and the potential learning opportunities it can provide.
If anyone has any thoughts on the ID of this bird, please let me know!