Where and when Both species occur in England and Wales, and in s. Scotland, but are in serious decline. Great care needs to be exercised in their identification, particularly Willow Tit Poecile montanus, which is now extremely rare in much of s. England (occurring mainly in the Midlands and n. England). Habitat differences are useful: Marsh Tit P. palustris prefers extensive mature deciduous woodland, especially beech and oak, whereas Willow prefers damp woods, carr (fenland overgrown with trees), scrubby habitats and lowland coniferous forest. However, the two species can occur in the same area and, of course, wandering birds present a complication.
General approach Marsh and Willow Tits must be separated by a combination of features, the most important of which are their calls. In fact, unless they call, they can be very difficult to separate (not until 1900 was it realised that Willow Tit even occurred in Britain). Fortunately, the calls most frequently given are diagnostic, but it must be stressed that, like all tits, both species have a variety of calls. Because of the difficulty in transcribing them, it is recommended that observers familiarise themselves by listening to recordings. Note that there is overlap in most individual plumage and structural features but, despite this, the two species do look different, although their separation becomes much easier with practice.
Calls Willow Tit has an emphatic, loud, full, deep, scolding djur djur djur or a more nasal chay chay chay, sometimes given as si si chay chay or jip jip… jee jee jee jee. Marsh has a very distinctive high-pitched, sneezing call, usually described as pitch-u. It is perhaps more accurately transcribed as a double-noted si-soo, si-swee, swe-oo or squee-soo, often extended into, for example, swip swip zu zu or squit zee zee zee zee. The important point is that the first part of the call – a rising si, swip, squee or squit – has an abrupt, rather explosive quality. Confusion may arise when this first syllable is omitted, leaving only the second part: for example, a zu zu, a zee zee zee or a zwee zwee zwee zwee, which may suggest some transcriptions of Willow Tit’s call. However, the quality of the calls is totally different, the faster, higher-pitched, rather jaunty calls of Marsh Tit being quite unlike the emphatic slow, deep, sombre tones of Willow Tit. As a final point: beware of the occasional mimicking Great Tit Parus major. When feeding, Willow Tits may give soft see see see contact calls and very high-pitched sit calls. Marsh Tits give a swit swit or quite hard tip tip tip tip in flight, more in keeping with the explosive quality of their other calls.
Song Marsh has a variable song (one study identified 37 song types!). All are typically tit-like, such as a rapid swe swe swe swe swe swe swe…, not unlike a Great Tit but more of a whistle and less ringing; or a disjointed rising and falling si swoo si swoo si swoo etc. These songs have a fast delivery: c. 5–10 notes per second in bouts of 8–20 notes. Willow has a descending, slow, clear, mournful sui-swee-swee-swee-swee-swee, recalling the piping song of Wood Warbler Phylloscopus sibilatrix. This is given at a slower rate of about three notes per second in bouts of 2–7 notes. Willow gives various other songs, including a series of thin, high-pitched phrases, and a thin, wistful, descending si si si soo soo soo. In late May and June, young Willow Tits utter a descending three- or four-note begging call jzee jzee jzee. In contrast, begging juvenile Marsh Tits give a fast, soft, thin, squeaky si-li-li or sid-it.
Structure and plumage Differences in plumage and structure are subtle and should be used with care. Head shape and facial pattern Willow has a larger head and appears bull-necked (with ‘less mantle’) while the head plumage appears more loosely textured. This bull-necked impression may be emphasised by the facial pattern: a large, swept-back swathe of white curves up behind the eye to the nape, the black of the crown becoming rather narrow on the nape. Note that some Willow Tits are tinged buff across the whole face, sometimes quite obviously so. On Marsh Tit, the white face does not curve up in such a broad swathe behind the eye, giving it a more strictly capped appearance. Also, the front of the face is white but the rear is duskier, sometimes producing a ‘two-toned’ appearance. Crown colour Willow has a dull, matt crown and tends to show a larger throat patch, the black broadening out diffusely to the sides (like a ‘bow-tie’). Marsh has a glossy crown and tends to show a smaller, neater, ‘Hitler moustache’. Remember, however, that the cap gloss is light dependent (see also ‘Juveniles’ below). Wing panel Willow has pale creamy fringes to the secondaries and tertials which, in fresh plumage, produce a distinctive pale wing-panel; however, this may be lacking in worn plumage, particularly in late summer. Marsh Tit is plain-winged, although fresh-plumaged birds may also show a subtle panel (occasionally obvious). Underparts Adult Willow tends to have buffier underparts, especially on the flanks. Bill colour Both species show pale cutting edges to the mandibles, but Marsh Tit also has a pale mark on the upper mandible, on the sides of the bill below the nostril (but this can be difficult to see in the field). Tail shape As a consequence of its marginally shorter outer-tail feathers, Willow has a slightly rounder tail tip, visible in good views. Juveniles Juvenile plumage is retained from the time of hatching in late May or June until the post-juvenile moult in late September. The following differences from adults should be noted, most of which will impact on the identification process: (1) like Willow Tit, juvenile Marsh Tit has a dull crown; (2) juveniles of both species have a wholly whitish face, juvenile Marsh Tits lacking the adult’s grey-brown on the neck-sides and (3) both species are paler below than adults.
Behaviour Both tend to feed at low levels, but Willow Tits can be found feeding very high in pine trees, often on larch cones. Willow Tits excavate their own nest hole from rotten wood, whereas Marsh Tits never initiate a hole from scratch. Note, however, that Willow Tit excavations may be taken over by Marsh Tits; also, Marsh Tits may enlarge their holes, carrying away chippings like Willow Tits.
There are a few British records of the northern borealis race, which occasionally occurs in northern areas as an irruptive vagrant from Scandinavia. Such birds are larger, paler and greyer than the British race kleinschmidti. Willow Tit appears to be more irruptive than the sedentary Marsh Tit, so any Marsh/Willow Tit seen at a coastal migration site is more likely to be Willow. Migrating Willow Tits may be located by an unfamiliar high-pitched si si si si or si-sisit flight call.
References Broughton (2009), Sharrock & Nightingale (2010).