by Guillermo Rodríguez
Eiders have been a recurring topic of Birding Frontiers, and Martin really pushed the boundaries of Eider subspecies identification. However, most of the effort so far has focused on males, and females have gone largely forgotten. It’s time to change that! Along the East Coast of North America, where at least 2 subspecies regularly occur (the local breeder Dresser’s Eider S. m. dresseri and the arctic Northern Eider S. m. borealis), identification of females is relatively easy, and studying them here is useful for understanding the phenotypic variation. Massachusetts hosts tens of thousands of wintering Dresser’s, and in a few spots they are very close to shore, allowing great opportunities for study.
Given that both subspecies have been recorded in Europe, but only/mainly in adult male plumage, it’s interesting to take a look at some females to raise awareness about how distinctive these birds are. As far as I know, the plumage variation of borealis is poorly known due to lack of information from the breeding grounds, so this post contains a lot of speculation!
Dresser’s Eider dresseri
dresseri is the default subspecies in the US and southern Canada, with an extensive intergradation zone with borealis along the Labrador Peninsula. There are several features that differ from both borealis and mollisima, and identification might be possible in most individuals.
Bill lobe: reminiscent of the massive bill of males, female dresseri show very long lobes, almost reaching the eye, which often look thick and always present a blunt and rounded tip. If seen close enough, the pattern of the tip is absolutely diagnostic in almost all birds, and even at long distances they give a characteristic look. There is a huge variability in lobe length and thickness, apparently without correlation to age or geography.
Nostril position: as in mollisima, nearly all individuals show overlap of the nostril with the feathering at the bill base.
Bill color: dark blue, with a reduced and contrasting greenish tip, duller in first-winter birds. On average they have a small nail.
Head shape: typically flat-topped, with the forehead relatively short leading to a profile that is dominated by the straight contour of the large bill. My impression is that the crown is longer and the bill is shorter than in mollisima, leading to a more square-looking head, as opposed to the elongated bill (and consequently profile) typical of mollisima.
Plumage coloration: adult females are typically rusty or orange-tinged, with most first-winter females being browner and darker. Nevertheless, variation is extensive and adults vary from dark brown to creamy pale, whereas first-winter birds sometimes look very reddish and adult-like even in the first prebasic plumage. The cheek is often neat, lacking any dark streaking.
Tertials: richly colored in adults, typically with a rufous fringe and darker feather center, but the pattern is quite variable and sometimes they show vermiculated or almost entirely rufous tertials. Plain brown in juvenile and second-winter plumages.
Sails: some adult females show small black scapular sails, not as large as in adult males but definitely distinctive if present, compared to mollisima.
Northern Eider borealis
Nearctic borealis overwinters around Greenland, reaching Newfoundland but quite rare further south (see below). I have experience only with vagrants in Massachusetts, but Bruce Mactavish kindly allowed me to study and show here some of his pictures from Newfoundland. Males from Newfoundland show orange bill but rounded lobe tips, likely suggesting these birds are relatively southern breeders in Greenland, as the high Arctic breeders are expected to show more pointed lobes.
Bill lobe: short and sharply pointed, and thus very different from dresseri, but probably not very distinctive if compared to mollisima.
Bill color: extensive pale greenish tip, sometimes with a more gradual transition towards the dark blue of the lobe. They often present a marked and pointed nail.
Nostril position: often no overlap of the nostril with the bill feathering, but there is definitely some variation and birds showing overlap aren’t rare.
Head shape: the bill height is very short in borealis, sometimes strikingly so, giving rise to a peculiar head shape. The triangular bill meets the rounded head forming a marked angle and leading to a usually concave head profile. In addition, the bill is remarkably short, especially the distance from the bill feathering to the bill tip – this feature is sometimes quite eye-catching.
Plumage coloration: it seems that birds overwintering in Newfoundland present two main “morphs” – rufous and pale grey – with all sort of intermediates. Whether this difference in plumage coloration is due to some geographical variation or just represents the plumage variation within a single population is yet unresolved. The fact that the few available pictures from N Greenland show completely grey females – whereas dresseri is usually reddish – suggests to me a clinal variation, but with the current evidence it’s difficult to say.
Wing bars: as in all other Common Eider subspecies, adult females present white tips on greater coverts and secondaries, forming two parallel white bards in the wing. This feature is in general quite variable, but in borealis the bars seem to be on average thicker and more obvious than in dresseri and mollisima.
Tertials: usually dull brown, or with at most rufous on the fringes.
Sails: pictures from Newfoundland don’t show obvious sails in females, but see below.
Note that most/all individuals shown here are adults: juvenile borealis seem to be quite rare, or at least in much lower proportion in the population than in dresseri, perhaps as a consequence of strong hunting pressure in Greenland & Canada?
Out of range borealis – some examples from Massachusetts
Borealis is scarce or rare on the East Coast of the US, with most accepted records involving adult males. However, females seem to be more regular, and given the striking differences in lobe pattern and overall coloration, they are much easier to separate from the local form than in Europe, where the similarity with mollisima hinders finding vagrant borealis. But certainly they are out there; several females have been claimed alongside confirmed males in the UK.
Among flocks of dresseri, female borealis look slightly smaller and less bulky. The measurements given in the Reever guide also suggest a smaller size compared to mollisima (but may depend on the population). Most of the females seen in Massachusetts are of the striking pale grey morph, but it’s likely the proportion is also biased because they are the easiest to pick out from a distance. All five of the birds I’ve seen had neat pale cheeks and a distinctive pale eyebrow.
The bird below is a rufous adult female, quite similar to many of the birds from Newfoundland. Both the lobe length and the bill are fairly short, giving her a distinctive appearance.
This second bird is a more typical pale grey/creamy adult female. Note all the key characters – pointed lobe, extensive pale bill tip, clean cheek, pale eyebrow, dull tertials, and hint of sails.
Some of the females seen in Massachusetts and neighbouring states are very striking. Too striking, perhaps. For instance, take a look at this amazing bird found by Marshall Illiff and Ryan Schain off Cape Cod, MA, on December 2011. This adult female differs from the typical borealis from Newfoundland and other vagrants seen in Massachusetts – note the heavy head, flat crown, large size, and even the shape and length of the lobe. Is it possible that this bird shows some influence from sedentaria, the subspecies that breeds in the Hudson Bay and spends the winter in polynyas in the frozen sea? Very little is known about this subspecies, not to mention the potential intergradation with borealis at Baffin Island and its surroundings. Do some birds from this region actually take the eastern route and spread out into the western Atlantic? Hopefully we will discover more about this fascinating topic in the future!
I would like to thank Bruce Mactavish and Marshall Illiff for insightful discussion about nearctic eiders.