Marsh Wren Cistothorus paludicus
is a common but local resident in Monterey County, and that generalized
description applies to much of North America. Marsh Wrens breed in
cattail or bulrush marshes across the northern U.S. and southern
Canada, and coastal marshes on both the Pacific and Atlantic coasts.
There is a complex pattern of local resident populations and migrant
populations, as shown on this map (right) from Kroodsma & Verner
(1997). Marsh Wrens winter in the southern U.S. and n. Mexico in many
locations where they do not breed, but these still tend to be marshes
or brushy wetlands.
Recent research into
vocalizations (Kroodsma & Verner 1987, 1997, Kroodsma 1989) has
shown that Marsh Wrens can be divided into two large evolutionary
types, a "western" type and an "eastern" type. These meet in a broad
intergrade zone (100 miles wide or so) from at least south-central
Saskatchewan to northeast Nebraska, shown roughly by the yellow line on
the map (right.) The two types appear to nest assortatively as one
crosses this zone, but genetically about 40% of adults in the zone show
mixed ancestry. The two types might represent two biological species,
or these populations may be in the process of becoming species (more
research is needed).
These facts draw our interest
as field birders, as we begin to wonder how different populations might
be identified in the field. Here in coastal central California, we know
from specimens that migrants from other populations have been collected
in migration and winter. This project is an effort to introduce the
topic from a local perspective. It has already captured the attention
of birders in Colorado, for example, who might look for both forms
within that State (see Leukering & Pieplow 2010).
photo 12 Mar 2001 Zmudowski SB, MTY © D. Roberson
Wrens are often much easier to hear than to see as they skulk through
the reeds. Kroodsma & Verner (1997) write: "Although the Marsh
Wren’s harsh, broad-band songs contain few pure musical tones that
resonate with our ears, careful analysis of this wren’s vocal behavior
has now shown a rich array of behaviors that rank it among the most
impressive of all North American songsters. During their early
sensitive phase, for example, males learn 50–200 song types. As adults,
neighboring males engage in complex countersinging duels and seemingly
sing almost continuously, day and night, in their bid for success."
best means of distinguishing the two forms is through vocalizations.
Leukering & Pieplow (2010) explain:
"The easiest and most reliable way to separate the two forms is to listen to the introductory notes. The introductory notes of Western Marsh Wrens almost always consist of two quick, low, noisy 'tuk' sounds. The second 'tuk' usually runs right up against the start of the trill: 'tuk tukRRRRRRRRRR.' Eastern Marsh Wrens, by contrast, very often start with a single nasal and/or buzzy note, which might be transliterated as 'beer' or 'bzt'.”
As to songs, "Western birds sing mostly noisy, unmusical notes, almost all of which sound like they could have been made by a typewriter or a stock ticker. The trills of an Eastern bird, meanwhile, tend to be more musical,
although it’s a stretch to apply the word 'musical' even to an Eastern
Marsh Wren, since, at their loveliest, their trills tend to sound like
someone rapidly shaking a fistful of coins. Nevertheless, listen
closely to an Eastern trill for semi-musical 'clinking' sounds or 'piping' notes mixed in with other kinds of sounds."
You can listen to examples on Nathan Pieplow's Earbirding web site.
is one additional separating point between Eastern and Western forms:
Eastern birds have a complete molt after nesting (in late summer/early
spring) but also most seem to have a pre-alternate body molt in spring.
In contrast, Western birds have a complete molt only once a year (after
nesting; late summer/early fall) and apparently do not have a spring
molt of any kind; Kroodsma & Verner (1997). This means Western
birds would continue to become increasingly worn and faded through
breeding season — and that may account for just how faded some Western
birds in July can appear. Pyle (1997) mentioned additional eccentric
molt involving replacement of upperwing coverts between western and
southern populations, and more eastern populations. This does not yet
appear to be fully sorted out.
vocally there are two (2) "evolutionary groups" of Marsh Wrens,
visually there are three (3) Groups of subspecies. Pyle (1997)
summarized these Groups as (a) Coastal Pacific Group — medium in size,
dark rufous-brown, (b) Interior Western Group — large, pale brownish
with a rufous tinge, and (c) Eastern Group — small, dull brown tinged
rufous. Pyle outlined 13 subspecies to be assigned within the three
Groups (plus another race in Mexico), but Kroodsma & Verner (1997)
rearranged the subspecies and their assignments. They also have 13
subspecies (plus one more in Mexico) but they merged one of Pyle's
races, added a new subspecies from coastal southern California (clarkae, based on Unitt et al. 1996), and readjusted boundaries. Today we might think of them as the Pacific, Interior West, and Eastern
Groups. The generalized breeding range of each Group is shown below.
Pacific birds and coastal Eastern populations are mostly resident, but
large numbers of Interior West and the interior Eastern birds migrate
south in winter.
PACIFIC — 2 Mar 2008 Coyote Hills, ALA © Tom Grey
INTERIOR WEST — 12 July 2011 Sierra Valley, PLU
© Tom Grey; ssp pulverius
EASTERN — 29 May 2011 Massachusetts
© Ryan Schain; ssp dissaeptus
trio of songsters above illustrate examples of the three Groups on the
breeding grounds. They are not posted at the same proportional size —
the Interior West birds are largest. Also, they were not photographed
at exactly the same time of year (although all are spring/summer
birds), but perhaps one can gain a generalized feeling about (to use
Pyle's headings) the "dark rufous-brown" Pacific bird, compared to the
"pale brownish" Interior West bird, next to the "dull brown tinged
rufous" Eastern bird. Or not. What I see is that the "large pale brown"
Interior West bird is the most different — being plain and lacking much
contrast — while the Pacific and the Eastern individuals are more
similar to each other — being rufous (especially on rump) and
'contrasty' and with a more prominent supercilium that contrasts with a
darker crown. Indeed, the Eastern bird looks essentially "black-capped"
in this shot — sometimes said to be a point to notice — but, as we
shall see, that is variable in both the Pacific and Eastern
problem in comparing Marsh Wrens is that the state of molt and wear
affects color and contrast perceptions. The panel below is of four
birds of the widespread California subspecies aestuarinus,
but in different months. The first bird (top left below) is in fresh
plumage — note the primaries are black with tiny white tips — and the
plumage is fresh, with well-defined black back patch with white
streaks; crisply patterned tertials; and brown overall with rusty
shoulders, scapulars and rump [I don't have a month for this photo, but
it most be soon after the complete molt in late summer/early fall]. The
next individual (top right below) is from mid-December. It is still
rather crisp and colorful, with rufous shoulders, scaps and rump;
tertials somewhat less crisply patterned; and a well-defined white
throat, but note that the primaries are wearing to brown and have lost
their white tips. The bottom two photos are of birds in March/April.
The primaries are more worn and dull brown. The rufous shoulder, scaps
and rump are duller, and the overall underpart pattern is less
contrasty and dull.
problem arises in juvenal plumage, which is held only briefly in summer
(May-Aug, depending on location), but birds in juvenal plumage can
disperse away from the breeding grounds. The top two photos in the
panel below illustrate juvenal plumage, but this time in one of the
Interior West races, pulverius. The bird in juvenal plumage
(below, at top left in that set) is still on the breeding site but,
compared to adults, is plain and dingy, without a full supercilium and
with a dull darkish crown. Pyle (1997) says that juvenal-plumaged Marsh
Wren "has a relatively dark crown, less distinct or no white streaking
on the back, a less distinct supercilium, and loosely textured
undertail coverts." Most of these points are apparent on the netted
individual shown top right (below). This bird has a much darker crown
than adults, almost entirely lacks the white-streaked black triangle on
the back, and has a very short post-ocular supercilium
|July 2011 Sierra Valley, PLU © Mark Eaton (juv)
|July 2011 Devils Postpile NM, MAD © Sacha Heath (juv)
20 Apr 2012 Camas NWR, Idaho © Netta Smith
12 July 2011 Sierra Valley, PLU © Tom Grey
|Contrast the juvenal-plumaged pulverius Marsh Wrens (top row above) with full adult pulverius
Marsh Wrens (bottom row above) from April and July. Adults of Interior
West populations have a mostly brown crown, a less well-defined
supercilium that those in the Pacific or Eastern Groups, and are
overall pale tan in color. There is, though, a rufous wash to the rump
that can be seen on the April bird (left) with is lost by July (right).
these overall concepts in mind — (a) there is a distinctive juvenal
plumage that needs to be considered in summer birds, and (b) that wear
can significantly affect colors and patterns — we can turn to a look at
photos from different populations in hopes of extracting something
useful. This is on the next page in this set.
All photos © Don Roberson, except those attributed to © Mark
Eaton, Tom Grey, W. Ed Harper, Sacha Heath, Greg W. Lasley, Tony
Leukering, Dennis Paulson, Netta Smith, Ryan Schain, Dan Singer, and
Brian Sullivan, used with permission; all rights reserved. Dan Singer
also provided useful editing comments.
D.E. 1989. Two North American song populations of the Marsh Wren reach
distributional limits in the central Great Plains. Condor 91: 332-340.
Kroodsma, D.E., and J. Verner. 1987. Use of song repertories among Marsh Wren populations. Auk 104: 63-72.
Kroodsma, D.E. and J. Verner. 1997. Marsh Wren (Cistothorus palustris) in The Birds of North America Online (A. Poole, Ed.). Cornell Lab of Ornithology, Ithaca.
Leukering, T., and N. Pieplow. 2010. Eastern and Western Marsh Wrens. Colorado Birds 44: 61-66.
Pyle, P. 1997. Identification Guide to North American Birds, Part I. Slate Creek Press, Bolinas, CA.
Roberson, D. 2002. Monterey Birds, 2d ed. Monterey Pen. Audubon Soc., Carmel, CA.
P., K. Messer, and M. Théry. 1996. Taxonomy of the Marsh Wren in
southern California. Proc. San Diego Soc. Nat. Hist., No. 31.