Cetti's Warbler photographed at Rue des Bergers [BER] on 18/10/2017 Photo: © Mike Cunningham (visit site)

The Cetti's Warbler is the only Western Palaearctic member of the bush warblers, essentially an east Asian genus. It has a widespread distribution and breeds in lower and mid-latitudes having spread north from the Mediterranean region to the edge of the Bay of Biscay, the English Channel and the North Sea. In the Channel Islands, it first bred in Jersey in 1973, one year after the first UK breeding attempt was suspected at Stodmarsh in Kent. The Cetti's Warbler has since established a strong foothold in Jersey, in particular around St Ouen’s Pond, but has failed to become fully established on the other Channel Islands tending to follow a series of colonisations and sunsequent extinctions a few years later.

The first Bailiwick record was a female trapped on 21 and 28 September 1971 on the marshy tract at the head of the Banquette Valley in Sark. A further bird was ringed in the same place on 2 May 1972 [Bull Soc Jers 1972]. The first Guernsey record was present at the Dell Nursery from 28 February-2 March 1975. During the same year, an adult and juvenile Cettis's Warbler were trapped at Barrackmasters Land in Alderney on 1 July 1975 and during the following year birds were seen on the Casquets Lighthouse on 6 August and 20 September 1976.

Since these first records in the mid-1970s, Cetti's Warblers have been an irregular migrant to the Bailiwick, tending to appear, stay and breed for a few years and then disappear after having failed to establish a themselves as a regular breeding species. During the first influx, several Cetti's Warblers appeared on all three main islands in the Bailiwick. This species was recorded breeding in Alderney in 1975, at La Fougeraie in Sark in 1977 and 1978 and probably bred in Guernsey in 1982, 1983 and 1984 (Bisson). Breeding was never conclusively proved in Guernsey but in1983 juvenile birds were trapped on 21 August (a female) at the Vale Marais and at the Grandes Pré on the 19th and, in 1984, a moulting female was trapped with a large brood patch at the Grandes Pré. These records strongly suggest that breeding took place in Guernsey but immigration, especially from the large population in Jersey, can not be ruled out.

After the initial records, Cetti's Warblers were regularly seen at either the Vale Marais, the Grande Mare or the Dell Nursery from the mid-1970s onwards. By spring 1980, there were two singing birds at the Vale Pond and Houge Pere. The following year, this rose to four singing males which were at the Dell Nursery, Marais Nord, Grande Mare and Jerbourg. During the autumn and winter of 1981/82, birds were recorded from the St Sampson's Marais and the Ivy Castle/Grandes Pre area as well as at the Vale Marais. In the spring and summer of 1982, birds were singing at the Marais Nord, Grandes Pre, La Claire Mare, Le Gele, Les Landes, St Sampson's Marais, Grande Mare and Le Bourg and to date this is the maximum number of singing birds that have been recorded in Guernsey.

In 1983, fewer singing birds were noted at four key sites, the Vale Marais, Dell Nursery, the Grande Mare and the Grandes Pre. Throughout 1983 and 1984, birds remained in the Vale Marais / Grandes Pre area with few birds seen elsewhere and the influx which had started approximately 10 years before was definitely waning. There was occasional interchange of ringed birds between these two sites. By April 1985, a long-staying female at the Vale Marais had disappeared and there were no further records in Guernsey for almost a year until another bird, presumably another migrant, was recorded at the Grande Mare in March 1986. A male was seen at the Vale Marais that spring, but was not recorded after the end of April.

No Cetti's Warblers were recorded in 1987, ending an unbroken thirteen year run of records. Over two years passed before the next record in October 1988 which was of a pair again at the Vale Marais. These birds did not appear to winter and for the next few years were followed by sporadic spring and autumn records. In May 1990, a singing bird at the Ivy Castle was referred to in the GBN as a 'typical May bird'. After a record of a singing bird at the Track Marais in April and May 1991, no birds were recorded in 1992 or 1993 until one was ringed at the Grandes Pré in September 1994. This was the only record for another two years before another influx which took place during the autumn and winter of 1996/1997. In spring 1997, birds were regularly recorded at the Vale Marais (summered) and in autumn at the Mare de Carteret, the Vale Pond, and the Grandes Pré. In 1998, singing birds were located at the Vale Pond, the Vale Marais and the Grandes Pré. It seems likely that another attempt at establishment was being made.

In the Bailiwick, Cetti's Warblers are associated with reedbeds but favour habitat which is marshy but with appreciable amounts of scrub, hence the attraction of the Vale Marais and the Dell Nursery. During the latter part of the twentieth century much effort was made in Guernsey to restore the island's reedbeds, especially at the Claire Mare and Vale Pond and this inevitably led to the reduction in the amount of scrub available to this species.