Chapter 4: If that’s a Bibby’s Warbler I’ll eat my hat

Killian sketched dartfordiensis on Hartland Moor, where Colin Bibby made his series of classic studies that set him on the path to become the head of conservation science for the RSPB, and later head of BirdLife International’s research team. Killian avidly read Bibby’s papers on Dartford Warbler, having developed a particular interest in the species following his discovery of the third record for Ireland when he was 14 years of age.

Colin Bibby was 20 when he started his study of Dartford Warbler ecology (1977). The work, for which he gained a PhD, looked carefully at the status and habitat needs of the species, translating the results into a plan for its conservation. He showed that Poole Harbour’s Dartford Warblers were dependent on dry, open heathland with mature gorse, and needed the right management to thrive. It was 1968 and there were 31 pairs. Not many, but 50% more than in 1967. It was Bibby who pointed to Stonechats associating with Dartford Warblers. He believed that the Dartfords took advantage of the Stonechats’ vigilance.

Colin Bibby died in 2004, and his obituary in the Daily Telegraph mentioned his reputation for being brusque. “Once, during a study of merlins (small, normally ground-nesting falcons), a team member telephoned to report finding a nest in a tree. Bibby replied: ‘If that’s a merlin’s nest, I’ll eat my hat.’ After seeing for himself that at least one pair had indeed built a nest in a tree, he joined the team for a meal, and a hat was brought to him on a silver salver. He promptly bit off a chunk, chewed and swallowed it.”

Bibby found that Dartfords don’t go far in their lives. He followed this up with a study starting in September 1974, again at Hartland, where he colour-ringed adults and metal-ringed young for two years. He discovered that once paired, the adults will be faithful to the breeding site through snow and fire, and it is the young males that venture further afield. Most often they turn up along the coast in Kent and Sussex, rather than risking a sea crossing. Adults only move a few metres from a favoured gorse bush, and this may explain why their numbers get hammered each bad winter.

So dartfordiensis doesn’t travel far, and has at times been reduced in numbers to a mere handful of birds. This is the classic mechanism for the evolution of an endemic species. Killian jokingly called it ‘Bibby’s Warbler’. Meanwhile I had Greensleeves playing in my head as I thought about the Holy Grail: a real English endemic…. ‘English Warbler’.

However, there is a problem with our new English ‘endemic’. There are quite a lot in France. Distribution maps vary, and it is difficult to know where dartfordiensis stops and undata begins, but Arnoud investigated and it seems that dartfordiensis is the western, maritime race, going right down the Atlantic coast. Curious, Magnus and Killian visited Eugene Archer, a birder friend living in Nantes on the west coast, to find out whether the Dartfords there sounded the same as in Dorset. They found the brown-backed dartfordiensis occupying small patches of habitat much the same as in Dorset (only now they were in the company of Melodious Warblers and Cirl Buntings). Next, Magnus and Killian flew down to Portugal, taking a road trip from Cabo da Roca on the west coast to Monfragüe in central Spain. They found no dartfordiensis, but watched and sound recorded obvious undata in the company of Sardinian Warblers, Spectacled Warblers and Griffon Vultures. 

Back home, Magnus was able to analyse sonagrams from a large number recordings of both dartfordiensis and undata. He found some interesting differences. The main call of Dartford Warbler is a low, descending churr, typically a single note, or a two-note call with the second shorter and slightly lower than the first. Much less commonly, three or four-note versions can be heard. The calls vary in pitch, length, number of notes and how frequently they are repeated, depending how excited the bird is.

Comparing calls of dartfordiensis and undata, Magnus limited himself to males in their second calendar-year or older, to make sure any differences he found were not a matter of sex or age. This left him with 13 recordings of dartfordiensis from Dorset and western France, and 34 of undata calls from Iberia, southern France and Corsica. The result was that male dartfordiensis calls average lower-pitched and slightly longer, with a more gradual descent in pitch and a slightly more ‘grainy’ timbre. Listen to these two examples from the UK (CD1-09) and France (CD1-10), and compare them with calls of undata from Portugal (CD1-11). Male undata calls average slightly higher pitched and shorter, with a steeper descent in pitch and a less grainy timbre. The latter two characteristics give them a somewhat more whining quality. The differences are subtle, and dartfordiensis can sound more like undata when excited. 

A more basic problem is to tell Dartford Warbler from Common Whitethroat calls. Listen to the whitethroat in CD1-12 and see if you can hear what sets it apart from Dartford. The most helpful difference is the level pitch, not descending as in Dartford, and another important difference is that whitethroat calls are never doubled in the way that Dartford calls often are. Calls of undata are less whitethroat-like, being higher pitched and purer, so less likely to be confused. 

We were curious to know whether we could tell calls of males and females apart. When Magnus compared calls of both sexes in three undata pairs, along with a pair of dartfordiensis, it soon became clear that an often-quoted difference in pitch between males and females (V C Lewis in Cramp et al 1992) is unreliable. The only consistent but very subtle difference he found was that males had a clearer voice while the females were more husky or hoarse-sounding. Listen to female dartfordiensis (CD1-13) and undata (CD1-14) and see if you agree. 

Adult Dartford Warblers also have some other important call types besides contact calls. We already mentioned that they vary the number of notes in a call according to how excited they are. Well, when they are really alarmed, eg, by the presence of humans or potential predators near their brood, you start to hear short but rather loud rattles interspersed with the usual calls. Typically, the final note of a normal two-note or three-note call is replaced by a rattle, which can be the length of the replaced note, or up to a second long (CD1-15). Occasionally the first note is omitted altogether, so you just hear a series of short, loud rattles. 

Alarm rattles are typically heard at the height of the breeding season, but another, much quieter and generally shorter, twittering trrrt… trrrt... can be heard at any time of year. This can be heard in a variety of contexts, typically from rather excited but not especially alarmed birds, and is often given during short flights. In CD1-16, both members of a pair are giving them. Magnus and Killian also heard these calls from a male repeatedly making short flights, trying to persuade his brood to follow him away from the human intruders.

When following families of Dartford Warblers during the summer, it can be difficult to tell who is who. Some calls of juveniles can be similar to adults. However, when they are begging for food, juveniles use some quiet calls that give away their age. In CD1‑17, listen to the long, low, churring notes, followed by higher-pitched, whining notes. It is the frequent alternation of these two types of sound that readily identifies a juvenile.

The song of Dartford Warbler, both dartfordiensis and undata, is very fast with many notes squeezed into a short song that is surprisingly low-pitched for a bird of Dartford’s small size. Here is a typical example of dartfordiensis, recorded in Poole Harbour (CD1-18). 

Stonechat song can sound superficially like Dartford Warbler but is higher-pitched, more whistled, less hurried, and typically sounds a bit slurred. Stonechat also has shorter songs than Dartford. Have a listen to this Stonechat recorded in the dunes along the Dutch coast (CD1-19), followed by another example of Dartford Warbler, this time a dartfordiensis from western France (CD1-20). 

In southern Europe, other small Sylvia warblers add to the challenge of identifying Dartford Warbler song. For example, the range of undata is almost exactly the part of Dartford’s European range that overlaps with Sardinian Warbler. The two can often be found together, and at times their songs can be difficult to distinguish. Sardinian usually has a drier, more staccato quality, sings slightly less hurriedly and with clearer articulation, and nearly always starts the song with a high-pitched, fairly pure-sounding whistle, which seems to be optional in Dartford. Here is a typical example of Sardinian Warbler song from Portugal (CD1-21).

Undata can sometimes sound rather like Sardinian Warbler, and may even adopt rattles of Sardinian Warbler in its songs. So one way to tell undata songs from dartfordiensis may be to listen for similarities to, or borrowings from, Sardinian Warbler. For example, undata may have a slightly more generous scattering of high-pitched notes and rattles in the song CD1-22 is an example of undata song, recorded at Monfragüe in central Spain, at a location where both species occur in good numbers.

Like several other Sylvia warblers, Dartford has a specialised songflight that it uses in moments of higher excitement. The strophes delivered during these flights tend to be longer than in perched song. Greater length can be achieved by including many repetitions (near or exact) of short fragments. The dartfordiensis in the next recording (CD1-23) gives a songflight 38 seconds into the recording, which lasts for 5.5 seconds. Typical song lengths when perched are in the order of 2-3 seconds, though much shorter songs in flight can also sometimes be heard.

Now listen to the undata in CD1-24, which performs a 4 second songflight, with more whistles than its English counterpart.

Dartford Warbler is a species that sings through most of the year, and it is common to hear song in late summer and autumn. At this time, many of the singers are young of the year. In CD1-25, a first-winter in mid-September in Portugal, you can hear a slow transition from a very quiet, hesitant and indistinct subsong to a more confident plastic song. Typically for plastic song, there are many imitations, here including very brief fragments of Barn Swallow Hirundo rustica, Eurasian Wren Troglodytes troglodytes, and two species that can also be heard in the background: Sardinian Warbler and European Greenfinch Chloris chloris. Now listen to another recording of a first-winter in Portugal, made in mid-October (CD1-26). It contains several imitations of Meadow Pipits Anthus pratensis, which arrive in Portugal around the beginning of that month.

Despite having lost any hope of an English endemic, I still find it an amazing story, these maritime dartfordiensis apparently having stayed close enough to the sea to cling to life despite hard winters and heath fires in the summer. The plumage and sound differences between them and undata certainly merit further investigation. And should dartfordiensis ever be split, it would be nice to think that a future researcher might honour Colin Bibby. It would be a tragedy to call it Nantes Warbler.

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