You can find the very same post on Charlie's blog here, by the way, as that's how the cooperation works.
It's spring, migrants are everywhere, and you catch a glimpse of what looks like a small warbler walking in a damp, shaded area. As you move closer you get better views- it's a brownish bird, streaked below, possibly a waterthrush, but - crucially - it's not calling. Your prize may fly off at any moment, so just what features do you need to concentrate on to make a positive ID...?
The two species of Waterthrush, Louisiana Waterthrush Seiurus motacilla and Northern Waterthrush Seiurus noveboracensis, are very similar to each other but are themselves easily distinguished from other species within the North American wood warblers. They basically form their own genus Seiurus, with the only other member being the strikingly different Ovenbird Seiurus aurocapilla. The Ovenbird though is so different from the two waterthrush species (in multiple aspects) that many authorities consider a review necessary that would then place the waterthrushes in a genus of their own. All three species, Ovenbird, Louisiana and Northern Waterthrush, share the following field characters that can be used to separate them from other North American wood warblers:
- They are brown above and pale (buffy or white) below with bold brown streaks.
- They are largely ground-dwelling and only seldom seen on branches.
So basically, unless a vagrant Redwing Turdus iliacus from Eurasia is taken into account, a ground-dwelling small song bird that is brown above, pale below with brown stripes and shows a pale supercilium is bound to be a waterthrush.
So far so good, but unfortunately (or fortunately, depending on how much you enjoy a good challenge) this is where easy ends and not-so-easy begins: the identification and separation of the two species of waterthrush... When confronted with a waterthrush, these are the field characters which we have found can be used to distinguish one from the other (field-guides do mention other characters ie Louisiana's buffy supralorals and buffy rear-flanks but in our experience these are difficult to see under normal viewing conditions):
The movement of the tail and rear body
Both species constantly bob their tail and rear body, but there is a slight yet distinct difference between the two.
Northern Waterthrush: a plain up-and-down bobbing of the tail and rear body.
Louisiana Waterthrush: a more "sinewy" bobbing that includes also moving the tail/rear body to the sides.
The shape and colour of the eyebrow
Northern Waterthrush: narrowing at the rear and very often yellowish or buffy white.
Louisiana Waterthrush: broad, bold and white, and not narrowing.
The shape and length of the bill
Northern Waterthrush: rather short and thin, the lower mandible being thin and straight as well.
Louisiana Waterthrush: long and strong, the lower mandible often appears strong and bent upwards, (very remotely similar to a Yellow-billed Loon Gavia adamsii!).
Patterning of the chin/throat
Northern Waterthrush: most birds have a finely striped chin, but some can show a white chin comparable to Louisiana.
Louisiana Waterthrush: always without fine stripes, pure white.
A striped chin therefore is diagnostic for Northern Waterthrush, but an unstriped chin cannot be used to definitely identify a bird as Louisiana Waterthrush!
Colour of the breast and belly
Northern Waterthrush: varies between whitish and yellowish, but never shows a contrast between the tone of the breast and the flanks.
Louisiana Waterthrush: always whitish with buffy rear flanks.
Pattern of stripes below
Northern Waterthrush: often relatively dense, usually more heavily striped than Louisiana.
Louisiana Waterthrush: often relatively few.
An additional difference that we came to notice but still needs to be tested in the field is the patterning of the stripes on the rear flanks: Louisiana Waterthrushes tend to show two parallel and straight lines along the rear flanks, very nicely patterned as the small individual stripes are completely in line. In Northern Waterthrushes, these lines on the rear flanks still show the individual stripes but they are not parallel but in a pattern reminiscent of a spruce twig, pointing in different directions. There is a considerable amount of individual variation however and this field character will not work on all birds.
Northern Waterthrush: dull, darker pink.
Louisiana Waterthrush: bright pink, almost reddish.
In general, the Northern Waterthrush appears more as a "dull and dirty" bird, with its darker legs, tainted supercilium and often yellowish tinge below. The Louisiana Waterthrush in contrast always gives the impression of a neat, clean and stylish bird, based on its clean white supercilium, bright legs and white colouration of the breast and belly. This difference might not work in all birds, but describes the general appearance quite well as can be seen on the two images below.
All these field characters are well established and should allow the identification of (almost) all the waterthrushes one encounters. However, most of these field marks are of a relative character and therefore require a certain amount of experience with at least one of the species, most likely to be the more common Northern Waterthrush. How else is one to determine if the bill of a bird is long or longer, if the stripes on the breast and belly of a bird are numerous or less numerous and if a whitish supercilium is bright or obviously bright? Furthermore, some of the field marks mentioned above are subject to individual or geographical variation and not entirely foolproof as rarely single individuals of one species will show a character more typical of the other species.
We would therefore like to introduce here one more field character that might be useful in identifying all individuals, regardless of geographic origin or individual variation as the field mark we present here was shown in other species to be rather stable and reliable. However, this field character cannot be regarded as established because we have only been able to test it on a limited amount of individuals in the field and on photographs of both species we were able to study.
From these limited examinations it appears that the Louisiana Waterthrush shows a primary projection that is significantly longer than that of the Northern Waterthrush. Primary projection is the extent to which the primaries project beyond the tertials on the folded wing of a bird.
The primary projection can be measured by comparing the primary projection with the length of the visible tertials and estimating the relative difference in %, as shown in the following image. (This may sound complicated but it’s actually a rather easy process once you become familiar with which feathers are tertials and which are primaries.)
The identification of both species of waterthrush appears quite difficult at first but is a rather straight-forward affair in most birds once the following key features are checked:
- the movement of the tail and rear body (up-and-down only, or up-and-down and also sideways)
- the patterning of the chin (white or with stripes)
- the supercilium (small and often yellowish or bold and white)
- the colouration of breast and belly (yellowish/buffy or pure white)
- the leg colouration (dull or bright pink)
- the structure of the bill (slender or strong)
Due to individual and geographic variation, not all birds show the field characters normally typical for the species and the identification should thus always rest on as many of the field marks mentioned above as possible.
Based on our limited experience with both species, we present one further field character that might be less prone to vary and might facilitate the identification of even the more tricky individuals:
- the primary projection (length of the primaries projecting beyond the tertials on the folded wing) of the Louisiana Waterthrush is close to 100% of the visible length of the tertials whereas this value is closer to 50-75% in Northern Waterthrushes.
We hope this apparently new but still largely unconfirmed field character inspires birdwatchers in North America to specifically take a deeper interest in the identification of the two species and test our suggestion in the field. We welcome any feed back and opinion on the subject.
Jochen Roeder and Charlie Moores, May 2007Text by Jochen Roeder (www.belltowerbirding.blogspot.com), photos by Charlie Moores (www.charliesbirdblog.com).
Photographs taken in Central Park, New York, April 2007, except Ovenbird taken at Garrett Mountain, NY May 2006