Mergansers: Identification, ageing, and sexing · 2020. 4. 24. · Colorado Birds │ Spring 2018 │ Vol. 52 No. 2 97 I N T H E S C O P E sexing birds. Unlike humans, in which female

Colorado Birds │ Spring 2018 │ Vol. 52 No. 2 92

I N T H E S C O P E

1 Molt and plumage terminology and definitions follow Howell (2010) and Pyle (1997, 2008). For additional detail about molts and plumage, please refer to Howell (2010) or Pyle (2008)— both of which I have cribbed from extensively—or to the “In The Scope” column that presented a molt-and-plumage primer in a previous issue of this journal (Leukering 2010). I also follow Pyle (2005, 2013) in considering the “bright” plumages of male waterfowl to be basic plumages.

In the Scope: Mergansers: Identification, ageing, and sexing TONY LEUKERING

Ah, April in Colorado: The beginning of the manic birding season and a period

of transition for most of the state’s birders from paying attention to water birds,

including lots of ducks, to a fairly tight focus on land birds. April is also a time of

transition for many birds, what with prealternate molts to look attractive to the

opposite sex and immatures of various bird species molting into adult or, at least,

more-adult-like plumages. While many of us might note those White-crowned

Sparrows molting from the brown-and-tan immature head plumage to the white-

and-black adult head plumage, how many of us have noted the many first-year

male mergansers that still look mostly like females in spring?

Mergansers also seem to create identification headaches for many birders. Thus,

in the couple of years that I have been pondering writing this essay, the focus has

broadened greatly from just first-spring male mergansers to the entire

assemblage of post-downy plumages presented by the three Colorado

representatives of Lophodytes and Mergus.

While Hooded Merganser (genus Lophodytes) is distinctive enough from

Colorado’s two large mergansers (genus Mergus), female-plumaged birds create

identification challenges for many birders. Even more importantly, the two large

mergansers in female-like plumages engender much confusion. Although the

main thrust of this essay is to provide details on ageing and sexing mergansers,

that very task provides more certain identification to species by expanding the

number of identification features available in birders’ ID toolboxes.

After an introductory treatment about molt and molt timing – yes, it is critical to

understanding the various plumages, I describe each species’ progression from

first basic plumage (=juvenile plumage) through definitive basic plumage1. Most

points are illustrated via the figures and their captions, including the photos

presented on the back cover of this issue.

Molt timing in mergansers

Quite unlike most dabbling ducks and somewhat different from that of many

other diving ducks, the plumage transition of first-year mergansers can be

protracted, with males still wearing mostly female-like plumage into summer

(when a year old). This protraction of plumage transition was the initial point


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2https://ebird.org/media/catalog 3www.flickr.org 4https://tinyurl.com/COME-COplains - scroll down to mergansers

that I wished to make in this essay, and it remains one of the chief foci of this

expanded essay. The take-home message is that, just because a merganser in

April looks like a female, it does not necessarily mean that it is a female.

Pyle (2008) notes that juvenile mergansers can initiate molt out of first basic

plumage (=juvenile plumage) as early as August of their first year, with that molt

– the preformative molt (resulting in formative plumage; see Leukering 2010,

Pyle 2008) – continuing as late as March of the following year. That source also

notes that some first-summer (one-year-old) mergansers conduct a limited first

prealternate molt (in Hooded, possibly only in non-breeding first-cycle birds,

and in all species, possibly only in males) June through August.

I have seen many individual mergansers, particularly of Red-breasted Merganser,

wearing female-like plumage late in spring (that is, after March) that are

obviously first-year males (see below). Thus, I suspect that the molt period

presented by Pyle (2008) is too brief. In fact, a one-year-old Hooded Merganser

that summered at Cape May Point, Cape May Co., NJ, in 2008, and identified by

nearly all as a female, eventually started growing in male-like plumage in

August (pers. obs.). I do note that Pyle (2008) does suggest this slower transition

by noting that the crests of first-cycle male Hoodeds are “increasingly mixed

with black and white feathering in Dec-Sep….” Additionally, as part of the

research into this topic and in search of suitable illustrative material, I looked

at >500 photos of Common Mergansers in the eBird/Macaulay Library photo

archive2 (Sullivan 2009) and at Flickr3 and found <8 photos of Common

Mergansers that I could determine (via plumage, eye color) were immature

males, even when restricting my search to November through May. Conversely,

there are numerous photos of immature male Red-breasted Mergansers

discoverable with similar searches. I suspect that many or most immature male

Common Mergansers make the transition to adult-like plumage quite late, if at

all. One of the causes of this relative lack of photographic material of immature

male Commons in plumage transition may be due to its relatively earlier spring

migration (personal observation, eBird 20124), which could mean that the molt

takes place mostly on or near the breeding grounds, where they are less

accessible to most photographers.

Adult mergansers conduct two molts per year, a complete prebasic molt August-

October and a partial prealternate molt May-July. However, it has been shown

with many other duck species that males and females of individual species have

differing molt schedules, particularly for prealternate molts and Pyle (2008) does

indicate that Hooded Merganser prealternate molts are poorly known and

require more study.


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5 see https://tinyurl.com/HOME-redeye for a female with red eyes

Eye Color

As with many other duck species, eye color is a useful indicator of age in

mergansers (with caveats), at least from late summer through fall and varyingly

deep into winter. Independent juvenile Hooded Mergansers have brown eyes,

while those of Common and Red-breasted mergansers have yellowish to yellow-

olive eyes. Additionally, as with some other duck species, eye color varies by sex

in adults. In Hoodeds, females have brown to olive eyes5, males have bright

yellow eyes (Dugger et al. 2009). Common Mergansers have dark eyes in both

sexes (brown in female, dark brown in male; Pearce et al. 2015), while adult Red-

breasteds have paler eyes, bright red in males and yellow-orange or reddish-

brown to dull red in females (Craik et al. 2015).

While you are looking at the eyes, note the color of the nearby plumage. The

plumage around the eyes is among the first adult head plumage grown by first-

year males transitioning to adult plumage, and any distinctly dark plumage

there identifies the bird as a male.

Bill Color

The bills of Common and Red-breasted mergansers are quite similar in color

with extensive orange, reddish, or salmon on at least the mandible, but creeping

up onto the maxilla. Adult males, however, have bills almost entirely bright red.

Hooded differs quite strongly in this regard: adult males have black bills and any

brown merganser with yellow on the bill is a Hooded (female or immature male).

Leg Color

The legs of the two large merganser species are bright orange or reddish, while

those of Hooded are often dull and/or dark, but can be fairly bright yellow.

Alternate Plumage

In all three species, adult males attain a variably female-like plumage May-July

and molt out of that plumage August-October (Pyle 2008, Dugger et al. 2009,

Craik et al. 2015, Pearce et al. 2015). Though their crests are reduced in alternate

plumage, they retain their adult wing and tail plumage, which can allow ageing

and sexing. Adult males in the two large species also develop a juvenile-like

white loral stripe in alternate plumage.

SPECIES ACCOUNTS

The back-cover photos depict first-year males of all three species of Colorado-

occurring mergansers (Figs. 1-3), presented in taxonomic order (as per AOS 2018).

All individuals are in plumage that is readily confused with that of females of

their respective species. I do not present illustration of adult males, either in basic

(bright) or alternate (dull) plumages.


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Hooded Merganser (Figures 4-7)

Figure 4. This Hooded Merganser is wearing the typical juvenile (=first basic) plumage

of the species. This plumage presents an appearance that causes many birders fits.

Perhaps most critical of the angst-causing features is the lack of the typical crested head

shape of Hooded Merganser. However, the long, narrow bill should get us to the

mergansers and, once there, the yellow on that bill identifies the bird as a Hooded.

Hennepin County, MN; 6 September 2017. Photograph by Brad Argue.

Figure 5. Unlike the bird in Fig. 4, this bird has more of the crested head-shape that we

expect out of Hooded Merganser. The presence and coloration of the brown crest

contrasting with the gray of the rest of the head, the thin white stripes on the tertials, the

dark eye, and the late June date all point toward the bird being an adult female. Tompkins

County, NY; 29 June 2017. Photograph by Jay McGowan.


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Figure 6. This bird’s bright yellow eye might be one’s first clue that this Hooded

Merganser is not a female. Throw in the streak of white behind the eye, the black plumage

around the eye, black back, and black bill and its identification as a one-year-old male is

certain. Note that the formative tertials with distinct white stripe is also typical of a first-

year male; formative tertials of first-year females have no or indistinct pale stripes.

Sacramento County, CA; 1 June 2015. Photograph by R M Yoshihara.

Figure 7. This bird provides a peek into another confounding process in ageing and


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sexing birds. Unlike humans, in which female is the “default” sex (females are

homozygous, that is, they have two identical chromosomes – XX, unlike the male

heterozygous state – XY), the default sex of birds, when it comes to plumage, is male.

That is, female hormones typically suppress the expression of male-like plumages in some

birds, including ducks. As estrogen concentrations decline with a female’s age, the

suppression of male-like features is reduced, resulting in male-like appearances in older

females (Pyle 2008, page 72, and references therein). This Hooded Merganser is probably

one such bird, i.e. an older female. While this bird’s plumage might be similar to that

shown by some immature male Hoodeds, the November date is too early in the plumage

cycle for a first-year male to have molted enough to sport this appearance. A one-year-old

Hooded Merganser of this appearance in June or September might be determinable as a

one-year-old male, but this bird’s relatively dark eye should rule out that possibility at

that time of year, as should other plumage details and the extensively yellow bill. Toronto,

Ontario; 23 November 2014. Photograph by Ricky Wong.

Common Mergansers (Figures 8 – 10)

Figure 8. While the sharp meeting of the reddish nape and paler gray neck helps identify

this as a Common Merganser, the extensively white foreneck might be considered

anomalous. However, this plumage is typical of a first-winter bird and closely resembles

one in juvenile plumage. Note the yellowish eye, the distinct white subloral stripe above

the brown gape stripe, and near lack of the species’ typical bushy crest. While it is

probable that the preformative molt has begun, it is not safe to sex this bird on this date.

Cape May County, NJ; 1 December 2013. Photograph by Sam Galick.


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Figure 9. Avoid getting stuck in the adorableness of the photo and note that this adult

female Common Merganser lacks the white subloral stripe of first-year plumage (see Fig.

8) and sports a bushy crest and fairly dark eyes. Potomac River, Frederick County, MD;

23 March 2013. Photograph by Dave Czaplak.

Figure 10. Note the scattered white and clump of black feathers in this Common

Merganser’s right wing. Since females do not exhibit such plumage, and because it is

March, this must be a first-spring male. Alameda County, CA; 19 March 2018.

Photograph by David Yeamans.


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Red-breasted Merganser (Figures 11 - 13)

Figure 11. This Red-breasted Merganser is in formative plumage, as discerned by

yellowish eye and the white subloral stripe (running from bill to under eye).

Unfortunately, I could not find a suitable photo of a juvenile Red-breasted, so just

imagine that this bird has less crest and is less worn-looking and you will have a picture

of a juvenile in your mind’s eye. Pinellas County, FL; 22 December 2013. Photograph

by Tony Leukering.

Figure 12. This Red-breasted Merganser can be aged as an older bird by the lack of a

white subloral stripe. Given the date, the yellowish eye, and the lack of any obvious male

plumage, determination as an adult female is straightforward. Port Clinton, Ottawa

County, OH; 14 February 2018. Photograph by Mike and Mary Caldwell.


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Figure 13. This first-spring male Red-breasted Merganser has only a few very dark

feathers around the eye; they would not be noticeable at much more than point-blank

range. However, the bird’s red eye and, particularly, the incoming black-and-white

plumage on the breast and among the scapulars provides proof of age and sex, along with

the date, of course. This photo also nicely illustrates the point that there are often many

clues as to age and sex of mergansers (and many other bird species), not just the one

or two noted in field guides. Learning the appearance of common birds in great detail

allows more-accurate ageing and sexing… and more certainty in determining whether

the ogled bird is a common species or not. Barnegat Light S. P., Ocean County, NJ; 1

March 2015. Photograph by Sam Galick.

ACKNOWLEDGEMENTS

I greatly appreciate a thorough review of a previous draft of this manuscript by

Peter Pyle. Peter also pegged the bird depicted in Figure 7 as an old female. Any

remaining errors are mine.

LITERATURE CITED

American Ornithological Society [AOS]. 2018. Checklist of North and Middle

American birds. http://checklist.aou.org/taxa/ (Accessed 16 March 2018.)

Craik, S., J. Pearce, and R. D. Titman. 2015. Red-breasted Merganser (Mergus

serrator), v. 2.0. In The Birds of North American (P. G. Rodewald, ed.). Cornell

Lab of Ornithology, Ithaca, NY.

Dugger, B. D., K. M. Dugger, and L. H. Fredrickson. 2009. Hooded Merganser

(Lophodytes cucullatus), v. 2.0. In The Birds of North American (P. G. Rodewald,

ed.). Cornell Lab of Ornithology, Ithaca, NY.


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eBird. 2012. eBird: An online database of bird distribution and abundance [web

application]. eBird, Ithaca, New York. Available: http://www.ebird.org.

https://tinyurl.com/COME-COplains

Howell, S.N.G. 2010. Molt in North American Birds. Houghton Mifflin, New

York.

Leukering, T. 2010. In the scope: Molt and Plumage. Colorado Birds 44:135-142.

https://cobirds.org/CFO/ColoradoBirds/InTheScope/24.pdf (Accessed 11 March

2018.)

Pearce, J., M. L. Mallory, and K. Metz. 2015. Common Merganser (Mergus

merganser), v. 2.0. In The Birds of North American (P. G. Rodewald, ed.).

Cornell Lab of Ornithology, Ithaca, NY.

Pyle, P. 2005. Molts and plumages of ducks (Anatinae). Waterbirds 28:208-219.

http://www.birdpop.org/docs/pubs/Pyle_2005_Molts_and_Plumages_of_Duck

s.pdf (Accessed 11 March 2018.)

Pyle, P. 2008. Identification Guide to North American Birds, part II. Slate Creek

Press, Bolinas, CA.

Pyle, P. 2013. Molt homologies in ducks and other birds: A response to Hawkins

(2011) and further thoughts on molt terminology in ducks. Waterbirds 36:77-81.

http://www.bioone.org/doi/full/10.1675/063.036.0111 (Accessed 11 March 2018.)

Sullivan, B.L., C.L. Wood, M.J. Iliff, R.E. Bonney, D. Fink, and S. Kelling. 2009.

eBird: a citizen-based bird observation network in the biological sciences.

Biological Conservation 142: 2282-2292.

Tony Leukering, 1 Pindo Palm St. W, Largo, FL 33770 ([email protected])

Figure 1 (Back Cover). The right bird appears to be an adult female Hooded Merganser

by the dark plumage without mixed feathers, although caution is necessary as some first-

spring males may lack male-like feathers on this date. The left bird, however, is not. While

the bit of extra white in the face provides a reasonable clue, this bird’s splotches of black

on the chest are proof that the bird is a first-spring male. The March date of the photo is

then proof of the bird’s age as something less than a year old. San Diego, San Diego Co.,

CA; 13 March 2018. Photograph by Chad King.

Figure 2 (Back Cover). This merganser can be identified as a Common by the browner

aspect to the head plumage, the distinct and well-defined white chin, and the sharp

transition from brownish-red head plumage to white neck. However, ageing and sexing

this individual is a bit trickier. The pale eye color identifies it as a first-year bird, while

that bit of dark plumage intruding into the white chin patch is all that we need to

determine that it is a male. Obviously, at any distance, the bit of adult-like head plumage

would not be visible, suggesting that caution is warranted in determining the sex of any

Common Merganser in fall and winter that is not wearing adult male plumage,

especially those not seen very well. Socorro County, NM; 18 January 2017. Photograph

by Jerry Oldenettel.


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Figure 3 (Back Cover). While looking female-like, this Red-breasted Merganser is a

first-winter male, a determination that can be made by the bright red eye and the dark

patch of plumage around that red eye. Adult male Red-breasteds would have been

wearing obvious adult male plumage for many months by this photo’s date. In addition to

the red eye, this bird can also be safely differentiated from Common Merganser by the

more-orange aspect to the head plumage (see Fig. 2) and the ill-defined border between

reddish head plumage and the paler neck. Cape Ann, Essex County, MA; 2 February

2014. Photograph by George L. Armistead.

Figure 1

Figure 2

Figure 3

In the Scope:

Mergansers: Identification, ageing, and sexing.................................92

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Mergansers: Identification, ageing, and sexing · 2020. 4. 24. · Colorado Birds │ Spring 2018 │ Vol. 52 No. 2 97 I N T H E S C O P E sexing birds. Unlike humans, in which female