Back from the sun
Sixteen species of warblers have succeeded in returning to Britain since the last Ice Age 10,000 years ago. Long providers of delight to birdwatchers, the warblers deserve special study. In this article, IAN WALLACE seeks to spark your enthusiasm for them with a word and paint essay on their lives and forms.
The mainly insectivorous warblers Sylviidae form one of the most diverse bird families in the Old World, but their distribution has been changed by the Ice Ages. During each of these, Britain lost all its warblers. Their return has been hampered by the loss of a land bridge, the spread of maritime climate and man’s destruction of natural habitats. This time round, only 16 of Europe’s 37 breeding warblers reproduce regularly in Britain; no more than 11 rear substantial new generations.
Nevertheless, the British birdwatcher eagerly awaits his annual reunion with the warblers. In these years of cold springs (and few Sand Martins) it is usually a brave Chiffchaff or Blackcap that sounds out the approach of summer.
Of the 16 British warblers, 12 are long distance migrants, making up to 4.5 thousand mile treks to as far as Africa and back. Each spring, 4.5 to 5.25 million adult pairs invade Britain, providing around five per cent of the total breeding bird community. The year’s broods may briefly add 20 million or more young but heavy juvenile loss — up to 70 per cent of the crop – means that the August exodus of adults and juveniles is less large, perhaps the equivalent of 8 or 9 million pairs. About half of them will fail to return.
The British breeding warblers fall roughly into three groups — first, six skulking species in the genera Locustella (Grasshopper warblers), Acrocephalus (Reed warblers) and Cettia (Cetti’s warbler); second, five less shy members of the genus Sylvia (Whitethroats and relatives); and third, five often quite approachable birds in the genera Phylloscopus (leaf warblers) and Regulus (crests or kinglets, the tiniest of all British birds).
The 16 breeding warblers are neither evenly distributed nor completely settled in Britain – see maps 1 and 2 – but they make fascinating subjects. The best book on them is Eric Simm’s British Warblers (Collins 1985).
The regional diversity of British breeding warblers. Note the restriction of the maximum of 16 species to coasts nearest Europe and their increasing absence in the far north. Ireland enjoys only half the British community.
Three recent spreads of British breeding warblers.
1 General advance of Chiffchaffs into Scotland.
2 Colonisation by Firecrests from NW Europe.
3 Colonisation by Cetti's Warblers from France.
IN MARSHES AND DAMP THICKETS…
the most widespread warbler is the Sedge. It wears the most patterned plumage and utters a vibrant, sweet then swearing song. Its notoriously similar, plain congeners, the Reed and the Marsh Warbler, are restricted to England and Wales, the latter mainly to the Southwest Midlands. The other two brown wetland warblers are the softly-marked Grasshopper and the uniform Savi’s, the first widespread but often shifting its ground, and the second confined to a few south-eastern coastal reedbeds.
The Sedge and the Reed Warbler present themselves readily; finding the other three requires knowledge of habitat preferences and song character. The Marsh Warbler, fond of osiers, warbles astonishingly richly and always mimics other songbirds (even as different as Golden Oriole). The Grasshopper Warbler, attracted to bushes or young trees among grass tussocks, buzzes like an angler’s reel. The Savi’s Warbler, lurking in tall reeds, utters a less fast, lower-pitched, ticking trill. All five species build open nests, the first three weaving them between plant stems – the Marsh adding characteristic “basket handles” – and the Grasshopper and the Savi’s placing them on or near the ground. All lay four to six eggs, dull white with variable speckles. In southern areas, two broods are reared; elsewhere (or everywhere by the Marsh) only one is attempted.
AMONG SCRUB AND WOODS AND ALONG HEDGEROWS…
the dominant warbler was the Whitethroat, a cheery, restless sprite with a scratchy warble often delivered in flight. Sadly the years of sand drought have withered its nearest winter quarters and its numbers have fallen dramatically. Nowadays it is often the richly chortling Blackcap or the rattling Lesser Whitethroat that birdwatchers find first. Both use high song-posts. Always less obvious is the Garden Warbler, subdued in looks with a rather quiet, bubbling warble. All five are essentially English and Welsh birds, petering out in the Scottish highlands and rarely reaching islands.
These Sylvia warblers nest off the ground, in dense foliage and at heights of 1 to 5 metres. The nests are robust, mostly of grass. The generic quirk of early “cock’s nest” construction is commonest in the Whitethroat and the Garden Warbler.
All lay four to six eggs, cream or stone-white with variable blotches and spots in the Blackcap and the Garden Warbler, and more liberal speckling in the whitethroats, especially the Lesser. A single brood is the rule, but the Whitethroat raises two.
AROUND TALL BUSHES AND TREES...
There are two truly common warblers. The Willow and the Goldcrest swarm in millions and almost everywhere in tall cover. Post-war coniferous re-afforestation has particularly favoured the latter. The sad-sweet, falling cadence of the Willow is diagnostic, quite different from the monotonous tick-tock-ed notes of the Chiffchaff and the shivering, yet pulsed trill of the Wood. The very high-pitched warble of the Goldcrest ends in a final flourish, lacking in the Firecrest’s more evenly-paced phrase.
Of the three less common species, the Chiffchaff needs tall trees – as song posts above nesting cover – and is mainly found south of the Forth-Clyde strath; the Wood thrives in mature beech and oak forests; and the Firecrest likes most middle-sized Norway spruces.
The leaf warblers construct domed nests on or close to the ground, but the almost wholly arboreal ‘crests attach moss balls to densely-leaved branches. The former lay six or seven eggs, off-white with red-brown speckles (purple in the Chiffchaff). The Goldcrest produces eight or nine eggs – exceptionally up to 13 – white with fine brown spots. The leaf warblers raise single broods, the ‘crests two or three.
THE BRAVE COLONIST…
is the Cetti’s Warbler. Twenty years ago, few ornithologists forecast that it would jump much of France and the English Channel to reside in England. Yet in 1971, birds reached three marshes and the Kentish immigrés soon bred. Now the chortling song-bursts unique to the Cetti’s sound out from marsh thickets from Devon to East Anglia! The grass, hair-lined nest is often set in brambles. The four brick-red eggs are unlike those of all other British warblers. Typically a bird of Mediterranean swamps and barrancos, the Cetti’s in Britain rub shoulders happily with Dunnocks and Wrens. It looks as if only a series of icy winters will threaten its new status.
THE FOOLHARDY RESIDENT...
is the Dartford Warbler, essentially an Iberian species able to shrug off British rain but unfortunately subject to decimation in an icy winter, when its heathland food sources are too long deep-frozen. Thus if ever such a season stretched into a similar spring, the few birds left alive in England’s still reducing gorse and long heather might not meet to raise their next generation! The compact heath and grass nest is built low and contains three or four pale green or stone-grey – or brown – spotted eggs. In warm summers, two, even three broods are reared. It is only this occasional, high productivity that has allowed the British Dartfords to cling on!
is the Firecrest, the most beautiful small bird in Britain. Long an autumn delight to migrant watchers in south-east England, it became a proven breeding species amidst the New Forest in 1962. In the next 20 years, birds bred in or occupied up to 30 sites in southern Britain. Recently, the Firecrest has added to its act both close association and proven pairing with the Goldcrest. To recover winter losses, male ‘crests change partners and thus overlap their brood cycles. Has “more haste” led to “mixed marriage” in the two congeners?! Typically, the Firecrest suspends a moss nest under a conifer spray, lays around 10 faintly pink eggs and rears a family in just over a month.
DIM Wallace wrote this article in the mid-1980s, and although his advice remains excellent, much has changed in UK bird populations in recent decades. Here are some of the more significant changes since this feature was originally published in BW
Dartford Warblers have spread and expanded their range thanks to recent milder winters, with a breeding population now in excess of 3,200 pairs.
Cetti’s Warblers underwent a massive crash in population after some very cold winters in the mid-1980s. In the last couple of decades they have expanded massively across the UK with more than 2,000 pairs breeding in England and Wales.
Marsh Warblers are even rarer than they were in the 1980s and are only sporadic breeders, as are Savi’s Warblers.
Willow Warblers have declined greatly in the UK in recent years, except for in Scotland, but there remain about 2.4 million breeding pairs. The Chiffchaff now has a population of about 1.2 million (double that of the Goldcrest).
These days, many taxonomic authorities have separated the ‘crests into their own family, the kinglets or Regulidae. It is thought there may now be more than 1,000 pairs of Firecrest breeding in the UK.
Article taken from Bird Watching magazine March 1986