Cetti’s Warbler (Cettia cetti)

Our First Atlas period ended with Cheshire’s first Cetti’s Warbler, ringed on 20 April 1984 near Frodsham (SJ57J), and all seemed set for their colonisation of the county. Their population was growing, the species was expanding to the north and there is plenty of suitable habitat available for them, especially along the Mersey valley and parts of Wirral. However, it was not to be, and Cetti’s Warblers have still not bred in Cheshire and Wirral.

The only records in the period between our Atlases were one at Rostherne Mere (SJ78M) on 8 and 9 May 1989, and a female ringed at Woolston (SJ68U) on 26 September 1998, present until at least 19 December 1998. Two birds ringed in October 2001 and October 2002 in the Wirral were not reported to CAWOS.

This Atlas survey has coincided with a rise in records, including the first in the breeding season since 1989. One or two birds were at Neston (SJ27Y) from 15 February 2005 to 31 March 2005, unfortunately not sexed but not reported to sing. Another was reported, seen and heard calling, although not singing, on four dates from 29 March to 10 April 2006, also at Neston (SJ27Y). Finally, a male was ringed at Woolston (SJ68U) on 20 July 2006 and sang there to the end of the year.

Although the highly-publicised and most intensively-watched Cetti’s Warblers in the county have been in Neston reedbed (Williams 2005), they are not really birds of phragmites. Detailed study has shown that their best territories are in damp, dense scrub, especially willow carr, with song-posts in small bushes, and reedbeds rarely visited (Bibby 1982). A particularly favoured habitat is where willow or bramble scrub is invading the edge of extensive reedbeds (Wotton et al 1998). Their diet comprises small flies and other insects, notably caterpillars and beetles, with small snails, spiders and occasional earthworms.

This species is strongly sexually dimorphic, with males significantly larger than females. In established breeding areas, many males are polygamous, males holding large territories with up to three females. Males spend most of their time singing and defending the territory but take no part in nest-building or incubation, and only some of them even bother to feed their young. Females paired with polygamous males lay larger clutches and successfully raise more young than those in monogamous pairings, however, suggesting that the polygamous males select the best-quality territories. Because of this habit, breeding surveys of the species count singing males, rather than ‘pairs’.

A national survey in 1996 found a total of up to 574 singing males (Wotton et al 1998) and their continued strong increase is shown in the records of the Rare Breeding Birds Panel. By 2004 the population of the UK had more than doubled to at least 1,137 singing males, but no farther north in England than Warwickshire and Leicestershire, with breeding birds also in north Wales, in Anglesey and Caernarfon (Holling et al 2007). These are the northernmost breeders in Europe, and continued close monitoring of the species will be important for measuring climate change.

In winter, adult Cetti’s Warblers stay on or close to their breeding territories, while some first-year birds disperse in all directions (Migration Atlas). Two of the Woolston birds detailed above stayed in the same area for several months. Another bird, a first-year female, was ringed at Woolston (SJ68P) just outside our defined Atlas winter dates, on 4 November 2006.

They are vulnerable to hard weather because of their year-round dependence on insect prey, often scarce in winter. Reedbeds are probably more important to them at this time of year when birds can forage amongst the reed litter (Brown & Grice 2005).

It is notable that most of the county’s birds have first been found in a ringer’s net, without their presence being detected beforehand. The species is skulking and could be overlooked, although their ‘chip’ calls are quite loud and distinctive, and the males’ explosive song is used at any time of year.

As well as those listed earlier, two Cetti’s Warblers have been caught at Shotton, just outside the Cheshire and Wirral boundary: one of these had been ringed as a juvenile female in June at Chew Valley Lake, Avon, and wintered at Shotton in two successive years, giving a fascinating insight into the origins and behaviour of one of the region’s birds.

In memory of Jean Martin