This adult male Mexican Duck (front, with a molting male Mallard, rear) surprised the author in the heart of the Rocky Mountains. Are Mexican Ducks expanding north into Colorado and the rest of the Interior West of the U. S.? Or have they always been present but undetected? Frisco, Summit County, Colorado; Sept. 2019. Photo by © Jack Bushong.

It is an unusually warm August evening in the mountains of Colorado. A squall has just cleared. I am out searching for any fresh migrants that may have trickled in on the heels of the storm. Arriving at a beaver pond, I scan the local Mallard flock, not expecting much. But there, concealed among the ordinary, is something unusual. It ticks all the pertinent field marks: dark brown body, turquoise speculum, and brown tail. For a moment I pause and admire my surroundings. Craggy peaks thrust skyward from the forests around me, and the air is fresh and pure. A snowmelt-fed pond at over 9,000 feet is certainly not the conventional habitat of the Mexican Duck, yet here it is. This bird is the second Mexican Duck I’ve seen in the past several weeks in Colorado’s high country. The first came from farther south in the Arkansas River Valley, another male. Earlier this year, I had also found one in a residential pond along the Blue River 10 miles north of the beaver pond. And along the Front Range, I happened upon a female with young this past spring. Has this denizen of the Southwest undergone a dramatic range expansion? Or has it been long overlooked? Or both?

My excitement in finding these birds stems from the species’ limited distribution in the ABA Area. The Mexican Duck’s core range is in central and northern Mexico, where the species inhabits lakes, marshes, rivers, and irrigated farmlands. In the U. S., the species can be found from western Texas through Arizona to the far southeastern corner of California. Some wander north into the southern Rockies, although the species’ occurrence there is likely minimal. When present in the U. S., these ducks are often sparse and considered to be genetically impure.

In my home state of Colorado, at the northern limit of the bird’s range, the Mexican Duck is generally rated as a vagrant. Most detections are March–April in the Denver region, due in part to the abundance of birders there. Elsewhere in Colorado, records are few, although with an unsurprising concentration in the San Luis Valley, directly north of New Mexico and in the Rio Grande drainage. Yet this past summer, I encountered the species on four separate occasions in the mountains. All but one were outside the expected date range of early spring. Perhaps, I was beginning to speculate, the Mexican Duck is more prevalent than most birders realize. Is the bird increasing in Colorado, or has it been here but undetected (see Floyd and Floyd 2017)?

When the Mexican Duck is mentioned in ABA birding circles, it typically conjures an image of an obscure, dark, Mallard-like inhabitant of southerly latitudes. It can seem that nearly all aspects of the species’ biology are shrouded in ambiguity. Variously regarded as either a subspecies of the Mallard or its own species (see Taxonomic History, below), the Mexican Duck presents a daunting task to identify, especially given the regularity of apparent hybrids. Along with other “brown Mallards” in the ABA Area (Mottled, Hawaiian, and American Black ducks), this is an ID challenge on par with empids and gulls.

When searching for Mexican Ducks in the Southwest, I have found that it is best to scan duck flocks consisting of at least a dozen Mallards. All the birds I have recently observed in Colorado, save for females with young, were in the immediate company of a sizable waterfowl flock. This need not be at a reservoir or large lake; Mexican Ducks are often reported at local duck ponds or water treatment plants, and might even prefer smaller water bodies. Due to the scarcity of Mexican Ducks in the U. S., they are rarely observed in groups, except in far southern Arizona and New Mexico, where the species occasionally congregates in homogenous flocks. At Sweetwater Wetlands Park in Tucson, for example, the species may outnumber the Mallard in winter.

The Mallard super-species complex comprises five species in the ABA Area: Mallard, American Black Duck, Mottled Duck, Hawaiian Duck, and Mexican Duck. Mallards are the most widespread of the complex, and can be found throughout the region. Ubiquitous in virtually all aquatic habitats, they frequently hybridize with other waterfowl. For this reason, it was long thought that Mallards were the closest relatives of Mexican Ducks, although recent findings suggest that Mottled Ducks are more closely related to Mexican Ducks. Introgression of Mallard genes is troublesome for the genetic viability of certain species, especially Mexican and Hawaiian ducks, which have small, vulnerable core populations. Concerns have arisen that, due to high gene flow with Mallards, the Mexican Duck may lose genetic “purity.” However, in regions of central Mexico there are likely still substantial populations of relatively pure Mexican Ducks, with one study indicating that at least 55,000 individuals may have existed in the highlands alone (Scott and Reynolds 1984). Even American Black Ducks, which have a considerably larger range, have long been considered threatened by introgressive hybridization; a recent study, however, reevaluates that threat (Lavretsky et al. 2019; and see Ottenburghs 2019).

Despite plumage differences among Mallards, Mexican Ducks, and American Black Ducks, these species are surprisingly difficult to distinguish with genetics alone. This is likely a testament to extensive hybridization among members of the super-species complex and incomplete lineage sorting, which has contributed to phylogenetic uncertainty. In addition, the genetic inseparability of these birds appears to indicate an adaptive radiation affecting plumage but not overall genetic diversity (Lavretsky et al. 2014). This complicated phylogeny is part of the underlying cause of confusion among scientists and birders alike, and is compounded by relatively minimal research on the topic. This is also a contributing factor to the continuing controversy of Mexican Duck classification.

Since 1983, the Mexican Duck has been considered a subspecies of the Mallard. Due to this classification, it has largely been disregarded by field birders. At present, the American Ornithological Society (AOS) has chosen not to regard the Mexican Duck as a distinct species due to a broad hybridization zone and lack of concrete genetic evidence. Meanwhile, the Mexican Duck was elevated to species status in 2018 on the eBird/Clements Checklist due to findings regarding genetics of the Mallard complex. According to a study by Philip Lavretsky and coauthors (2014), Mexican Ducks have been diverging from Mallards for about 325,000 years. Meanwhile, the American Black Duck has been diverging for roughly 180,000 years. In addition, Mexican Ducks appear to not be hybridizing more extensively with Mallards than is the case for other taxa in the super-species complex. As is evident from this disparate treatment, the status of the Mexican Duck continues to be in flux. Future alterations to Mexican Duck classification will likely be based on genetic testing, as well as aspects of its natural history.

Along with the Mexican Duck’s confusing classification, difficulty in field identification is a central reason that birders have shied away from this bird. In the continental U. S., there are three primary species with which the Mexican Duck is often confused: Mottled Duck, American Black Duck, and Mallard. All are prone to some degree of vagrancy, and extralimital birds may sometimes occur within the range of Mexican Duck—and vice versa. All members of the super-species complex are of the same general size and structure, and their vocalizations are very similar.

Field Identification

The Mallard is the beginning point for this identification conundrum simply because of its sheer abundance throughout the Southwest. Especially in summer, Mexican Ducks can seem exasperatingly difficult to separate from male Mallards. However, when one knows what to look for, they can be reliably distinguished from their ubiquitous congeners.

Compared to Mallards, the male Mexican Duck is considerably darker overall, with a sharp demarcation between the breast and lighter, more tan-colored throat. The head sports a dark crown and eye-line which stands out noticeably on the pale face. Summer Mallards often have splotchy facial patterns with a poorly defined eye-line. Bill coloration on male Mexican Ducks is highly variable, ranging from yellow to olive–green. But it is often more muted than on a male Mallard. Another aspect to look for is the speculum, which tends to be more turquoise blue than purple, although this is highly influenced by light. The white borders to the speculum are thin on pure Mexicans, whereas on Mallards they are noticeably thicker.

Given that male Mexican Ducks are presumably overlooked outside their core range in the ABA Area, it is reasonable to assume that females are even more ignored. On this adult female Mexican Duck in the Denver metro region, note the nearly uniformly colored bill with little dark splotching; note also the head-to- breast contrast and that the tail is mostly concolor with the rest of the plumage. Louisville, Boulder County, Colorado; Nov. 2019. Photo by © Jack Bushong.

Perhaps the most useful and reliable feature is the color of the tail and under-tail coverts. On all Mexican Ducks (male and female), the tail is largely brown, as is the under-tail. On Mallards, it is mostly white with a variable but small amount of brown intrusion. This is especially helpful in summer, when male Mallards can resemble Mexican Ducks in plumage. Although male Mexican Ducks and Mallards can be relatively straightforward to separate, those of the opposite sex are much more daunting.

Many females are indistinguishable from their Mallard counterparts, and female hybrids are virtually undescribed due to the difficulty of identification. However, the same basic features apply to females as to males: darkness of plumage, turquoise speculum with limited white edging, and brown tail and under-tail coverts. All female Mexican Ducks show body plumage considerably darker than that of typical Mallards. Similar to males, there is a noticeable demarcation between the dark brown breast and paler buy/dusky throat. The facial pattern also tends to be more pronounced. Although such characteristics often contribute in combination to an identification, there is considerable overlap with Mallards.

The key mark is the mottled brown tail, which on a pure Mexican Duck lacks white— except for pale edging on the sides of the tail on some worn birds. As well, there should be no hint of upturned tail feathers, a male Mallard field mark. The under-tail coverts, too, should be rather uniformly dark or dusky, rather than speckled brown and white. The bill of the female Mexican Duck is highly variable, ranging from brilliant orange to a duskier orange–yellow; most, however, retain at least a hint of a black saddle. Although there is much overlap, the bill of the female Mexican Duck is more consistently orange and has less extensive black splotching than that of its Mallard counterpart. With females in particular, the entire suite of characters should be present for a confident identification. This entails close views and careful examination, as well as photographs.

Although the core ranges of Mottled and Mexican ducks overlap only in a narrow sliver of south Texas, distinguishing the two (and their hybrids!) can be necessary. Adult male Mottled Ducks sport a small but diagnostic black spot at the gape, which is lacking in Mexican Ducks. Mottled Ducks also tend to have a bright yellow bill, whereas those of Mexican are often duller olive–yellow. The face of the Mottled Duck is clean, unmarked buffy, while that of the Mexican Duck appears duskier due to fine black feathering.

Further, the eye-line of a male Mottled Duck is noticeably shorter than others of the Mallard complex. Another important feature on pure Mottled Ducks is that the speculum lacks any white edging, whereas all Mexican Ducks possess some amount of white. Female Mottled Ducks are similarly difficult to distinguish from their Mexican counterparts. The bill color on the female Mottled is more reliably solid orange, the face is a brighter, unmarked buffy, the eye-line is shorter, and the speculum lacks white edging. Female Mottled Ducks also have the distinctive dark gape spot of males.

American Black Ducks might also be confused with Mexican Ducks and their hybrids. Their ranges do not overlap, as American Black Ducks occur primarily in the Northeast, but vagrants of both could pose an identification problem. This is especially true given that American Black Ducks are sometimes released in regions outside their indigenous range.

American Black Ducks of both sexes can usually be distinguished from Mexican Ducks based on coloration alone. American Black Ducks are very dark (approaching blackish), whereas Mexican Ducks are richer and browner. Also, the scapulars and back of Mexican Ducks have internal markings, whereas those of American Black Ducks do not. Furthermore, pure American Black Ducks lack any white edging on the speculum. Females often have dull olive or dusky bills rather than the brighter orange–yellow of Mexican Ducks. American Black Duck x Mallard hybrids typically retain blackish body plumage, although they are variable and can appear similar to Mallard x Mexican Duck hybrids.

Hybrids, Intersex Individuals, and Simply Unknowable Birds

Although much remains unknown, we are beginning to unravel the complexities of Mexican Duck identification. Yet, right when you begin to feel reasonably sure in your abilities, a hybrid or some odd second-generation backcross prompts you to second-guess yourself. Just as birders from the Pacific Northwest can justly assume that most Glaucous-winged Gulls are not pure, one wonders whether most Mexican Ducks in the U. S. have some Mallard genes. That said, there are always birds that appear phenotypically pure, especially in southern Arizona and southern New Mexico (Sibley 2011). Mexican Duck x Mallard hybrids frequently have a combination of white in the tail or curled tail feathers and green in the head. In addition, males often show a strong reddish breast reminiscent of Mallards, rather than a dark brown one. These hybrids are highly variable; some, such as F2 and F3 backcrosses, are nearly identical to male Mallards save for some brown on the head or flanks. Others strongly resemble a pure Mexican Duck, but have some white in the tail feathers. Still others simply cannot be identified.

First-generation adult male hybrids in basic (breeding) plumage, like this bird (foreground), are generally identifiable by their blending of Mexican Duck and Mallard characters. Backcross males and hybrid females of any generation are considerably more difficult, and the existence of intersex individuals (often older females) further complicates matters. Louisville, Boulder County, Colorado; Oct. 2019. Photo by © Jack Bushong.

When in doubt, it is best to report confusing birds as “Mallard/ Mexican Duck” or even “duck sp.” This has happened to me on multiple occasions. In one instance, a bird that I had initially identified as a Mallard x Mexican Duck hybrid was determined by identification experts to be an American Black Duck x Mallard, then Mallard x Mexican, and possibly even Mexican x Mottled. After much deliberation, the birding community remained undecided. In a related vein, Ted Floyd and Andrew Floyd told me about a specimen they examined in the collection of the Denver Museum of Nature and Science; over the years, it was labeled Mallard, Mottled Duck, Mexican Duck, and Mallard! Unfortunately, without genetic testing some birds must remain unidentified.

An interesting factor complicating Mexican Duck identification is the existence of the intersex Mallard (Stiver 2019). As waterfowl (and likely other birds) mature, certain features of their plumage may change to resemble the opposite sex. It has been suggested that this development may be related to estrogen shortages or ovarian abnormalities. Although much remains unknown, it seems that in intersex Mallards, females gradually develop masculine traits such as green head plumage. As a result, they are often confused with hybrids, especially Mallard x Mexican Duck, Mallard x Gadwall, and Mallard x American Black Duck. However, most intersex birds retain a female-like bill (orange with a black saddle), whereas hybrids typically have the yellow or dusky bills of males. Detailed analysis of an intersex Wood Duck is discussed by Tanaka and Pyle (2018).

In Colorado, I have encountered numerous birds that struck me as candidate Mexican Ducks. Most of these have been females with one or two of the aforementioned traits—especially dark body plumage and the turquoise speculum. Many such intermediate birds are impossible to identify with certainty, and particular caution is warranted in regions north of their typical range where the extent of hybridization is unknown. In regions of the Southwest, it seems there is a Mexican Duck or hybrid in almost every sizable Mallard flock. Although Mexican Duck identification can be complicated, it is a worthwhile endeavor for both the bird and the birder to search for them. The Mexican Duck is a vulnerable species due to extensive Mallard introgression and a relatively limited range, and a changing climate will likely exacerbate its precarious condition. By contributing their observations of Mexican Ducks, birders enable scientists to better understand and conserve this enigmatic species.