Tuesday, December 18, 2012

ID Feature: Knots

Following on from one of our previous ID features on Pectoral Sandpiper identification (cf. Sharp-tailed Sandpiper), we now discuss the separation of another tricky wader pair; the two knot species, Great Knot (Calidris tenuirostris) and Red Knot (C. canutus). Both species are regular visitors to much of the Western Australian coast, with very large numbers occurring in the north at locations like Roebuck Bay, near Broome. Identification of the two species in breeding plumage is trivial (providing you’ve correctly identified the bird as a knot), but with the exception of northern areas early and late in the wader season, knots are not commonly encountered in breeding plumage in Western Australia.

Red Knot in breeding plumage (right), easily identifiable from Great Knot by extensive red on face and underparts.

Great Knot moulting out of breeding plumage, easily identified from Red Knot by extensive black breast markings and lack of red colouration on underparts.

Identification of birds not in breeding plumage can be more problematic, and these plumages are the most commonly encountered in WA, particularly in the south. The following features are useful for separating the two species in non-breeding plumage. Structural features mentioned below, like bill length, are still applicable in breeding plumage, but plumage features aren’t. Several plumage features are also applicable to juvenile plumage, generally mention will be made of differences. As always, there can be significant individual variation in many of the features mentioned here, so we advise observers to use a combination of features to confirm identity.

Great (left) and Red (right) Knot in non-breeding plumages, showing similarities and differences. Identification is generally easier in situations where the two species can be compared side by side. See end of Key Features section for an annotated version. 






Key Features

Bill
Bill size is an important feature in separating the two species, but the absolute length is often underestimated in illustrations, particularly for Great Knot (e.g. Message & Taylor’s wader guide and Morecombe’s field guide). This has led to longer-billed Red Knots being misidentified as Greats, and  longer-billed Greats being mistaken for dowitchers or Curlew Sandpipers. Bill length in Great Knots is typically noticeably greater than head length, compared to Red Knots where bill length is usually roughly equal to head length, or shorter.


Red Knot, showing relatively short bill length, typically equal to or slightly shorter than head length (compare with Great Knot in following image). Note also the pale, even grey upperparts, chevron patterning on the flanks, and dark scalloping on the the pale rump (just visible beneath folded wings). 

Great Knot, showing long bill (often longer than shown in many guides), significantly longer than head length. Note also the darker, less evenly grey back, and more extensive dark streaking on the mantle.

Head
The facial pattern of the two species varies and is a useful identification feature – the differences between the species are similar to the differences in facial pattern between Sharp-tailed and Pectoral Sandpipers. Red Knots show a slightly darker loral stripe, combined with a more prominent pale supercilium. By contrast, the dark loral stripe on Great Knots is generally broader but paler and more diffuse, and the pale supercilium is noticeable weaker.

Plumage
There are several other plumage features that can be useful for identification in non-breeding plumages. Firstly, Red Knots in general show more patterning on the flanks, typically dark chevron marks. Great Knots generally show cleaner white flanks, and any dark markings that are present are typically streaks or spots rather than chevrons.

Two Red Knots, showing extensive chevron markings on flanks. The flank markings are not always as extensive as this, but most Red Knots will show at least some chevrons along the flanks, and typically show more patterning than most Great Knots. Note also the evenly pale grey upperparts, scalloping on the rumps, and relatively short bills (the left hand bird has a bill at the long end of the spectrum for Red Knot).

Great Knot (front) with Sharp-tailed Sandpipers at Lake McLarty, showing relatively clean white flanks compared to most Red Knots. Note also the long bill and darker, less evenly grey back.

In addition, the mantle and scapulars of Red Knots are a much more even, and paler, grey colour. By contrast, Great Knots tend to show dark patterning on the mantle, and darker, less evenly grey scapulars (scapulars sometimes almost black in juveniles). Beware of juvenile Red Knots which show quite extensive pale fringing to the scapulars which give them a more ‘scaly’ (less even) appearance, however the scapulars should still be paler than in Great Knots. Great Knots often also show a few heavy dark spots on the breast, which Red knot do not show.

Great Knot showing diagnostic dark spotting on breast. Note also the long bill, darker and less evenly grey upperparts, weak supercilium, and pale, diffuse loral patch.

In flight, Red Knots show a pale rump with extensive dark scalloping or uneven barring, whereas Great Knots show a much cleaner (though narrower) white rump, typically showing only a few dark spots. In breeding plumage, Great Knots develop more extensive dark spotting on the rump, but such birds should show at least traces of breeding plumages to help with identification.

Red Knots in flight, showing dark scalloping on a pale rump. Note also the relatively pale, evenly grey backs and mantles, and relatively short bills. 
Great Knots in flight at Woodman Point, WA (a regular site in summer). Note the paler and relatively cleaner white rump (marginal markings only), and relatively long bills.


Great Knot with wings partially raised, showing a cleaner white rump lacking the extensive scalloping of Red Knots. Note though that in breeding plumage, Great Knots can acquire extensive dark spotting on the rump. 

Size
Great Knots are slightly larger than Red Knots, and this difference is usually quite easily noticeable when the two species are together. However, it is of limited use if the species are seen individually.

A rather poor photo showing the the size difference apparent when both species are seen together. Note also the darker, less evenly coloured back of the Great (right) and the scalloping on the rump of the Red (left), just visible beneath the wingtips.

Annotated comparison between Great (left) and Red (right) Knots, showing identification features.

Flock of knots at Nairns on the Peel Inlet. The flock comprises mostly Red Knots, but there is one clear Great Knot near the front - can you pick it? (Look for the longer bill, larger size, and weaker supercilium)

Great Knots with other waders, including Bar-tailed and Black-tailed Godwit, Sharp-tailed Sandpiper and Grey-tailed Tattler, indicating the size of the knots in comparison to other species.

Further Reading

Chandler R. (2009) Shorebirds of the Northern Hemisphere. Christopher Helm (also published as Shorebirds of North America, Europe and Asia by Princeton University Press). Pages 213-220.

Geering A., Agnew L. & Harding S. (2007) Shorebirds of Australia. CSIRO Publishing. Pages 118-121.

Hayman P., Marchant J. & Prater T. (1986) Shorebirds: An Identification Guide to the Waders of the World. Houghton Mifflin. Pages 183-184 & 363-365.

Message S. & Taylor D. (2006) Waders of Europe, Asia and North America. Christopher Helm (also published as Shorebirds of North America, Europe and Asia by Princeton University Press). Pages 100-101 & 184-185, but be aware that the illustrations in this guide underestimate bill length in both species.

O’Brien M., Crossley R. & Karlson K. (2006) The Shorebird Guide. Houghton Mifflin. Pages 134-138, 281-282 & 396-400.

Paulson D. (2005) Shorebirds of North America: The Photographic Guide. Christopher Helm. Pages 205-212.

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