British Leaf Warblers and the Wildlife Sound Recordist

By Philip Radford

This article was originally published in the Journal of the Wildlife Sound Recording Society Vol6 No1 Spring 1989

There can be few wildlife sound recordists who can always identify the calls of the three British leaf-warblers with absolute confidence - certainly at the beginning of the season. With the songs, of course, there is no difficulty, but it was not until the 18th century that Gilbert White, curate of Selborne and meticulous observer of nature, was able to state that there are three species of these little, green-brown birds; he wrote of "three species of willow-wrens which constantly and invariably use distinct notes.

In most seasons we hear the onomatopoeic song of the Chiffchaff Phylloscopus collybita towards the end of March although, especially with the recent cool springs experienced in the British Isles, it is now well into April before this migrant is fully distributed. Some Chiffchaffs undoubtedly over-winter but I do not know if these individuals are of British-bred stock or autumn migrants from Scandinavia. Anyway, the Chiffchaff is the smallest of our breeding leaf-warblers, showing short wings and near-black legs - features which are most easily seen early in spring before the leaves have fully opened. It can then be compared in appearance with the Willow Warbler Phylloscopus trochilus which arrives later in the season; however as both warblers are so restless in the foliage continued observation is always a difficulty. 

Gilbert White mentioned the Chiffchaff as the "chirper" and stated that it "utters two sharp piercing notes, so loud in hollow woods as to occasion an echo... ' .The Chiffchaff's notes are indeed loud and repeated but lesser, attenuated sounds are given quite frequently between the main phrases; in any case, 'chiffchaff' is not an exact rendering of the song, and different inflexions and spellings are noted in the various European languages. The same differences can be found in the renderings of the song of the Cuckoo Cuculus canorus. As Gilbert White remarked, the Chiffchaff's song is loud and may cause an echo in suitable hollows or valleys. There are few problems in recording the vocalisations and it is astonishing how the song can be heard clearly against a background of farm machinery or aeroplane noise.

Chiffchaffs, unlike Willow Warblers, require the presence of at least some tall trees in their territories as song-posts, associated with a certain amount of dense bramble or bush vegetation. Thick cover is needed for the
nesting site, with the domed nest being comnonly positioned about a foot above the ground. It is surprising that the structure is sometimes so loosely built as well as being thin-walled, but there is a compensation in a profuse feather lining which permits a sustained high interior temperature. Because the nest is often so flimsy in construction, it may get distorted by the rapid growth of a bramble or other strong shoot. I have known a nest with the usual side-facing entrance to be so altered in position that the entrance faced upwards; happily the brood flew successfully.

Gilbert White wrote of the songs of the leaf-warblers and related them to the shot specimen. It seems curious that, in those days of egg-collecting, he did not differentiate between the nests with their characteristic sites of building and methods of construction. Furthermore, he did not refer to the typical egg colours and patterns -again broadly distinctive according to the species.
As already mentioned, it is of ten the leaf-warbler calls, rather than the songs, which give the most problems to naturalists. The alarming Chiffchaff , for instance, utters a 'wheet' call which becomes louder and more frequent as the degree of urgency of the situation increases.

Essentially, the call is of one syllable, as compared with the rather similar cry of the Willow Warbler which has two syllables, 'hu-eet'.The Chiffchaff s 'wheet' is used as a contact call or an alarm note, becoming louder and more strident if a predator approaches the nest, particularly after the young have hatched. A perching magpie Pica pica will provoke these sounds and so will an intruding Cuckoo; ground predators such as a Stoat Mustela erminea or a Brown Rat Rattus norwegicus will do the same. The acoustic response. however, does vary with the individual warbler -some readilv alarm while, with others, vocalisation does not appear to develop easily. As with all the leaf-warbler sounds, it is easy for the recordist to overmodulate and distort them; this applies both to the call-notes and the song. 

Willow warblers have decidedly longer wings than Chiffchaffs and their legs are paler in colour. Their distribution in Britain is widespread and any bushy area will suit, provided that the cover is not too sparse; woodland verges or glades are favoured localities. In his writings, Gilbert White described the song on more than one occasion. In particular I like this little-known passage: "a pretty plaintive note; some call it a joyous note; it begins with a high note and runs down." Better known, of course, is the poetic line "joyous, easy, laughing note". Every naturalist must make his own assessment of these musical phrases of springtime. Even so, Willow Warblers are so common that few wildlife sound recordists can be without a worthwhile sequence of the song in their collection.

The call notes are also widely heard in the sound-scene of summer, and it must be unusual to record any alarm situation in the habitats already mentioned without a Willow Warbler joining in. The two syllable 'hu-eet' construction. however. is also shared bv certain finches. The Goldfinch Carduelis carduelis is a common example, with its call having a distinct nasal intonation. Other examples come from the Chaffinch Fringilla coelebs and the Green finch Carduelis chloris.The quality and tone of the call is really characteristic for each species. From the human point of view. continued practical experience and odservation is the only wayof achieving certainty in identification. 

Then, some chats produce a 'hu-eet' type of call, often admixed with hard, tacking notes; the Redstart Phoenicurus phoenicurus, Whinchat Saxicola rubetra or Stonechat Saxicola torquata may all be cited here. A prowling Fox Vulpes vulpes will elicit these calls, as will an investigating bird of prey like a Kestrel Falco tinnunculus.

But the most remarkable call of the Willow Warbler is a high frequency, squealing pipe, emitted particularly if a Cuckoo, of either sex, enters the breeding territory. It must be remembered that Cuckoos do perch for lengthy
periods of time, especially females in the late afternoon when they are intent on egg-laying. When a Willow Warbler at the appropriate stage of the breeding cycle spots an intruding Cuckoo, or hears one, it will fly into the tree and start giving these squealing calls; indeed, if such sounds are heard and the area is carefully scanned with binoculars, a Cuckoo will nearly always be found to be present.

Yet I do not think the squeal calls are specific by Willow Warblers for Cuckoos as I have heard the same notes as a Stoat or a Weasel Mustela nivalis passed close to the nest-site. Nevertheless, there is no doubt that Willow
Warblers loathe Cuckoos and yet it must be very unusual for a Cuckoo to lay in the warbler's nest. Possibly it is the strength of the Willow Warbler's reaction which deters the cuckoo from selecting and laying in its ground nest.
As the Willow Warbler is Britain's commonest summer migrant, I feel that the squeal call must have considerable biological value for the species.

Like the Chiffchaff, the Willow Warbler's domed nest is always lined with feathers; the chosen site is on the ground, often cleverly concealed by a growing tuft of grass or bracken, so that the nest becomes better hidden as
the season advances.

I am commonly surprised when experienced naturalists tell me that they have never heard the Willow Warbler's anti-Cuckoo squeal in spring. In the past I understand that few ornithologists seem to have noticed the spectacular
mating display of the Robin Erithacus rubecula and yet today it is considered a commonplace occurrence. Indeed the breast swaying behaviour, with head and tail raised, can be seen in almost any garden.

Somehow I think it is the lack of familiarity which prevents some observers from hearing this particular Willow Warbler call, but there is no doubt that the squealing piping may be detected in any bushy habitat where
both Willow Warblers and Cuckoos are breeding. But to react in this way, the Willow Warbler must probably have built a nest, with the response increasing as eggs are laid and, particularly after the young have hatched. A squeal call of the male will bring the female Willow Warbler off the nest to investigate and both will call together until the impulse fades. At times, a calling Cuckoo and a Willow Warbler singing its undulant song may be heard together in
the same tree; probably the male warbler is either unmated or the hen has not started to nest-build. The squealing Willow Warbler usually approaches the Cuckoo along a branch while flicking its wings or it may hover in the air,
calling even more intensely. If the reaction has been a strong one, the Willow Warbler may return to the same place again and again to demonstrate even after the Cuckoo has flown off; thus the eventmust have produced a marked impression on the warbler's mind. It is true that resting Cuckoos may stay perched for hours without making a sound, but the more one listens carefully in May and June, the more often the squeal notes of the Willow Warbler may be detected.

The largest of the British leaf-warblers, and the least common, is the Wood Warbler Phylloscopus sibilatrix. This is Gilbert White' s "little yellow bird" and his "largest willow-wren". White describes the bird further, stating correctly that the "yellowest bird is considerably the largest, and has its quill feathers and secondary feathers tipped with white. . . ". The Wood Warbler has a largely western distribution in England and Scotland, often being found in mature oakwoods in company with Redstarts or Pied Flycatchers Ficedula hypoleuca. These localities are usually rich in high quality spring song from their distinctive bird inhabitants and are fruitful places for the wildlife sound recordist. 

Gilbert White knew the Wood Warbler's territorial song as a "sibilant shivering noise in the tops of tall woods". With greater detail, he writes another passage showing his knowledge of the species in that it "...haunts the tops of trees in high beechen wood, and makes a sibilous grasshopper-like noise, now and then, at short intervals, shivering a little with its wings when it sings. . . " Moving high in large trees, the Wood Warbler does indeed
make an accelerating sound on one note which some may consider to resemble that of a grasshopper.

But there is another componeht to the Wood warbler's song which gets interposed at variable intervals. This is a series of plaintive, piping notes, suggestive of those of a Bullfinch Pyrrhula pyrrhula; I have noticed that some individual male Wood Warblers seem to utilise this form of sound in their songs much more than others. Then, regarding alarm and call vocalisations, the notes are again melancholy and Bullf inch-like, given with increased vehemence and frequency if danger is near; the quality and tone of these calls is decidedly different from analogous sounds of either the chiffchaff or the Willow Warbler.

These notes can be amazingly loud, and I recall being drawn some three or four hundred metres amidst large beeches to find out why a Wood Warbler was so upset. I found that the reason, at dawn that May morning, was a
vigilant Tawny Owl Strix aluco which seemed reluctant to settle at roost.

As with the other two leaf-warblers, Wood Warblers construct domed nests but they are situated in hollows on the woodland floor; surrounding cover is normally sparse because the habitat requirement is of well-grown timber but,
nevertheless, there is usually excellent concealment. Being a ground nester, the Wood Warbler will initiate distraction displays. I admit that I was very surprised recently when a Wood Warbler flew down from a branch to the forest floor in front of me. It then fluttered away, trailing a wing; there was, in fact, a nest with well-grown young not far away. One expects such behaviour from a Ringed Plover Charadrius hiaticula but I did not anticipate it in a Wood Warbler; certainly, I have never seen a distraction display from a Willow Warbler. Incidentally, feathers are not used to line a Wood warbler's nest; instead, dried grasses or rootlets are selected.

There is fascination in considering the varied songs and calls of the British leaf -warblers. Amongst naturalists, perhaps the wildlife sound recordist should be the best placed to interpret these sounds'under different natural circumstances and, clearly, there is still much to be learned as to their significance. To a large extent, Gilbert White succeeded in mastering the subject by his own observations ; there was no relevant literature, and tape-recorded sounds existed only in the imagination. The modem wildlife sound recordist is in a privileged position. Unfortunately, compared with Gilbert White's days, wildlife habitats have been seriously eroded but, pleasingly, one does not now have to ask a local marksman to go into the woods to procure a bird specimen. The limp, killed bird is voiceless and devoid of personality. Without the subject s ability to fly, react and vocalise, it is impossible, at least for me, to think of so lively a species as a leaf-warbler. 

REFERENCES:

WHITE, G. (1789)     Tne Natural History & Antiquities of Selborne, in the county of Southampton. LONDON.
WHITE, G. [I9791     The Garden Kalendar & The ~aturalist's Journal 1751-1793. selected by John Comander. THE SOLAR PRESS, ILKLEY.

Philip Radford

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