Grasshopper Warbler photographed at Claire Mare [CLA] on 28/7/2011 Photo: © Christopher Mourant (visit site)

The Grasshopper Warbler is found across the Western Palaearctic in the middle, mostly temperate latitudes. Although poorly know, the birds which pass through the Channel Islands probably winter in sub-Saharan West Africa. Grasshopper Warblers have undergone a marked decline in north-west Europe, probably due to the intensification of marginal agricultural habitats and drainage of wetland habitats.

Surprisingly, despite now annual records, Grasshopper Warblers were unknown in the Channel Islands until the mid-1900s. Smith makes no mention of this species and Dobson only mentions a possible record from 1936 and two records from Sark in 1939 and 1949. This is puzzling as Grasshopper Warblers were much more common in north-west Europe than they are currently and it may be that fewer birds stopped off in the Channel Islands during that period. Smith was certainly familiar with Grasshopper Warblers and their song at his home in Somerset and it is unlikely he would have overlooked it, if they were present.

Grasshopper Warblers are now regular spring and autumn migrants, averaging 3-4 records per annum in spring and 2-3 records in autumn. Over the past 30 years, only once in 1974 were no birds recorded in Guernsey. Numbers in spring have varied widely from zero to 18 records between years. During fall conditions on a early misty morning in mid-April, the sound of Grasshopper Warblers 'reeling' on the south coast cliffs or West Coast headlands is one of the highlights of a spring in Guernsey. During these falls, the arrival is sudden and 1-2 birds can be heard from almost every scrubby headland.

In spring, Grasshopper Warblers occur widely across the islands. The first birds arrive in the second week in April. Numbers peak during the last week of April and the first week of May and the majority of migrants have passed through by the end of the second week in May. Most records are of singing birds on common land dominated by scrub, especially along the low-lying coastal areas and escarpment along the west coast and also along the south coast cliffs and in Herm. Birds are also regularly recorded in wetland areas including La Claire Mare, the Reservoir, Silbe Nature Reserve and the Marais Nord. Birds in spring tend to only stay for one day but occasionally for two or three days.

There have been few mid-summer records. During the early 1970s in Guernsey, birds stayed throughout the summer at the Grand Mare. In 1970 a bird was heard singing on 29 June and then on 1 and 5 July at La Grande Mare and in 1971 a bird was heard singing in the same location on 28-29 July although this could have been a very early returning passage migrant. A similar argument could also be applied to a bird heard singing at La Pomare on 29 July 1972. There are two mid-summer records from Sark which indicate that a male bird was holding a territory. A male sang from the hedgerow at La Vaurocque meadow for at least 26 days to 26 May 1966, another was heard singing in the hedgerow of a hayfield at Beau Sejour for 25 days to 4 June 1968 and finally, a male was heard singing intermittently from a hedge at La Vaurocque for 31 days to 11 June 1968. In Alderney a bird was heard reeling at Val de L'Emauve from 31 May to 6 June 1980 and in 1982 a bird was heard singing from 27 July to 3 August although at this late date it may well have been a passage migrant.

During autumn, when birds tend to be silent, most records come from ringers mist-netting in the island's reedbeds and there are proportionately fewer records from coastal headlands. Where effort is put in at sites, birds are generally caught annually in small numbers (1-3 birds) from the beginning of August onwards. The passage is more extended than in the spring, beginning at the end of July, peaking at the end of August and continuing through to the end of September. There have been three records of birds in October, including two from the Casquets Lighthouse.

The situation in other islands in the Bailiwick is probably similar. In Sark Rountree describes Grasshopper Warblers as a not common passage migrant and a possible breeding species. Between 1966 and 1972, over 40 records were accepted by the Sark Rare Birds Panel. At an average of five records a year, this is similar to the current number of records. There are fewer records from Alderney and Grasshopper Warblers are by no means annual. However, the pattern of records remains the same and it is a predominately spring migrant averaging one record every two years. This is undoubtedly due to lower coverage by birders compared with other islands.

There have been no recoveries or controls from the 125 birds caught in the Bailiwick. Most birds passing through the islands in autumn are juveniles and, as with many warbler species, adults presumably migrate in long hops rather than a series of shorter movements.