British Birds VOLUME 70 NUMBER 3 MARCH 1977

Cetti's Warblers in east in 1975 W. G. Harvey

The colonisation of Britain by Cetti's Warblers has been one of the major events of the decade. There were no records in east Kent until 1971, yet by 1975 this was probably the third commonest warbler in parts of the Stour Valley

he results of a census of Cetti's Warblers Cettia cetti in east Kent in Tsummer 1975 are here described in detail. The census confirmed that Cetti's Warblers were still spreading in east Kent and that numbers were building up rapidly, following initial colonisation late in 1971. Hollyer (1975) documented the history of Cetti's Warblers in the whole of Kent up to 1974, on the basis of records submitted to the Kent Ornitho­ logical Society. Additional records were available to Bonham and Robert­ son (1975), who summarised the situation up to 1975. The following is a more detailed breakdown of the 1971-74 records, which form a background to the 1975 census results: 1971 Up to three singing males, November 1973 In Stour Valley, at least eight singing to December, in the valley of the River males east of and at least one Stour, east of Canterbury (first substanti­ pair bred; up to four singing males west of ated record for east Kent). Canterbury. Outside the Stour Valley, 197a In Stour Valley, breeding strongly probable passage birds north and south of suspected at two separate sites (3J km Sandwich, possibly near Ashford and, in apart) east of Canterbury, with a third central Kent, near Maidstone. singing male to the east and a fourth to the 1974 In Stour Valley, up to 13 singing west of the city for much of the season. males east of Canterbury and two in Outside the Stour Valley, up to two south Canterbury; west of Canterbury, only two of Sandwich in May, October and Novem­ records, both suggestive of passage. One ber were probably on passage. probable passage bird near Ashford.

[Brit. Birds, 70: 89-96, March 1977] 89 90 Cetti's Warblers in east Kent Most of these records and, indeed, those in 1975 referred to singing birds, and pairs were not often seen. There is some evidence that females may sing on very rare occasions but, to avoid ambiguity, all singing Cetti's Warblers included in this census are recorded as males. It is particularly hard to prove breeding because of the difficulty of observing Cetti's Warblers in their often impenetrable habitat (usually thick scrub, and often marshy) and the desire of most observers to keep disturbance of a rare breeding species to the absolute minimum. Circumstantial evidence was strong, however, both during 1972-74 and in 1975, that many singing males were paired and holding breeding territories. One effective way of locating pairs is to look for the characteristic aerial display flights, but these are usually to be seen only at the beginning of the breeding cycle. Chases have been observed by several observers on a number of occasions in April and May. The male (distinguished by his slighdy larger size) pursues the female, at a distance of up to 1 m, in a rapid, zigzagging flight between 5 and 50 m above the ground for up to 5! minutes (usually one to two minutes). Both birds follow exactly the same course, around and through thick scrub in the breeding territory. The course is usually roughly circular or oval, but may describe a figure of eight. On some occasions, the male has been heard to make a soft 'twic' note, as well as his more common hard 'chic'. A modified version of the chase, of shorter duration and over a simpler course, has been observed in September and October over Phragmites reed-beds where Cetti's Warblers held winter feeding territories. It seems likely that the first Cetti's Warblers entered east Kent, between Thanet and Deal, in late 1971 and then moved inland, up the Stour Valley, towards Canterbury. Four months later (in April 1972), the first pairs were probably breeding in the Stour Valley and, in the ensuing three years, numbers were augmented by locally bred birds and by new arrivals. By 1974, all singing males which were apparently holding territories were in the Stour Valley, within 8 km of the city centre of Canterbury. Records farther west, and also north and south of Sandwich, almost certainly referred to migrants.

Methods Between 19th May and 30th August, I travelled widely in east Kent (the area of Ordnance Survey sheet 179). I visited all known and possible Cetti's Warbler sites within 15 km of Canterbury and, by the end, had made 41 visits to places where at least one singing Cetti's Warbler was found. My records were supplemented by those of C. Bignal, M. Daven­ port, P. J. Grant, J. N. Hollyer and D. B. Rosair, which usually dated from earlier in the season and covered a wider area of east Kent. The results of an independent survey by JNH in 1975, which included a number of records from other observers, were made available later. In view of the dangers of overcounting and the impossibility of checking records after the breeding season, I have incorporated only his personal records, two of which were additional to those gathered by the other five observers. Inclusion of the other records would have raised the final total by a maxi­ mum of only five (8%), probably comprising mainly passage birds. The Cetti's Warblers in east Kent 9i aims of the census were to discover the minimum number of singing Cetti's Warblers in east Kent, how many were holding territories and paired, and how many were on passage. Records were kept of the position of each site in relation to the Stour Valley, its aspect and, whenever pos­ sible, the dominant vegetation and the presence or absence of surface water during all or part of the year. The number of sites in each i-km square was recorded. All recorders had a considerable local knowledge, both of the area and of the history of Cetti's Warblers within it, so that most sites were known at an early stage of the census, and an estimate of 60 singing males, made in mid May, was very close to the final minimum count determined in August.

Numbers The final total was a minimum of 61 singing males. In areas of high con­ centration (in particular, four contiguous squares to the east of Canter­ bury) , much time was spent ensuring that overcounting did not take place, and it is likely that the final count was conservative. Five males were thought to be on passage, as they were heard for only short periods. At least 27 of those singing were paired for at least part of the season and five pairs were proved to breed two by JNH, one by DBR and two by me). Cetti's Warblers were recorded in a total of 30 i-km squares in east Kent. Three single squares held eight, six and four singing males, five squares contained three, six had two and the remaining 16 squares each had a single singing male. Obviously, many territories crossed square boundaries, but, in analysing the results, records were'placed in the one containing most of the bird's territory. There was very little evidence of wandering by Cetti's Warblers during the summer, but some territories were large, often linear in shape and extending along lake shores, dykes and strips of scrub. Singing Cetti's Warblers make full use of their terri­ tories; this, together with their secretive habits between widely spaced songsites, can lead to confusion. Careful watching is needed to elucidate the full extent of a territory, but the song sequences are often highly individual and can help to ensure that overcounting does not occur.

Locations The main concentration was found in the area where initial colonisation occurred in 1971 and 1972: east of Canterbury in the Stour Valley. A contiguous block of 15 i-km squares contained a total of 43 singing males and at least 21 pairs, of which four were proved to breed. This represented 70% of the east Kent total, and an average density of almost three singing males per i-km square. A total of 38 sites (62%) was actually on the floor of the Stour Valley, with a further 13 (21%) along the valley sides. Of these 51 sites, 49 were between Chilham and Grove Ferry, a distance of 19 knj. Of the ten out­ side the Stour Valley, one was south of Sandwich, in the presumed zone of entry from the Continent and close to sites where probable passage birds were recorded in 1972 and 1973. Four were less than 1 km outside 92 Cetti's Warblers in east Kent and two were 2 km and one 3^ km outside. These last three were certainly only on passage, as were probably the Stour Valley birds farthest from the centre of concentration (near Minster and Ashford). Of those on the edge of, or outside, the Stour Valley, 15 sites faced south or west and eight faced north. Thus, although 1975 saw a very large increase in the total number of Cetti's Warblers in east Kent, they followed the pattern of distribution set since 1971, and the Stour Valley remained very much the main area of concentration, with surprisingly few elsewhere. Within 15 km of Canter­ bury, in the valleys of the and the Nailbourne, and the low­ lands of the , Sarre Penn and between Thanet and Deal, there are many apparently suitable sites which were, however, unoccupied in 1975. Conversely, the attraction of the Stour Valley was such that Cetti's Warblers were found in apparently marginal sites within it: in dry scrub on coal slag, in scrub hawthorn Crataegus monogynaj laevigata in a busy village and well within Canterbury city boundaries. It would be interest­ ing to know if the Cetti's Warblers in these marginal sites included new colonists.

Habitats The favoured habitat of Cetti's Warblers in east Kent was described by Hollyer (1975) and confirmed, with some modifications, by this census. It can be summarised as marshy scrub or willow Salix carr, often associated with well-established flooded gravel workings and, to a lesser extent, areas of mining subsidence. I do not agree with Bonham and Robertson (1975) that the last of these are the focus of colonisation: well-established flooded gravel pits are more attractive to Cetti's Warblers in east Kent, perhaps because they support more scrub on land that is not under water all year round. The abundance of this habitat between Chilham and Grove Ferry in the Stour Valley is probably one reason why this area was so quickly and successfully colonised. Cetti's Warblers are, however, catholic in their tastes, as the acceptance of marginal sites in 1975 indicates. Those in 1975 were not always found in optimum habitats; rather, they appeared to prefer to spread into a greater variety of habitats close to the original centres of colonisation. Most sites are wet for at least part of the year and over parts of their areas, but the presence of surface water all year is not, apparently, an essential component in the habitat. Only 28 sites (46%) were considered to have some permanent water for all of the year. A further 22 sites (36%) had surface water for at least part of the year, but were no more than damp for most of the breeding season. At least 11 sites (18%) were normally completely dry for all of the year. Several gravel pit sites included spits or islands of overburden covered with scrubby vegetation and including only a small area of true marsh or swamp. A territorial male would have to fly over deep and permanent open water to cover hisv territory, but this open water was not considered to be a part of the territory unless it had vegetation growing in it. Indeed, Cetti's Warblers favoured fairly extensive areas of scrub or willow carr and moved into sites including spits and Cetti's Warblers in east Kent 93 islands with limited areas of scrub only when the larger areas were not available or were already occupied. Dampness, however, is a relative term and 1975 was a particularly dry summer. An attempt was made to assess sites in relation to knowledge of them over the previous 18 years, but, obviously, many were in a state of change and external factors altered the amount of visible water. When present, water was usually less than 1 m deep and often full of growing plants. It seemed that the presence of surface water was not an essential element in the habitat, but the nature of the dominant vegetation was. Dry sites included scrub on coal slag up to 15 m above the highest water levels, carr and scrub where the ground had been silted up by gravel washing processes and, in one case, a strip of oak Quercus and hawthorn extending from a large block of deciduous wood­ land and bordering hayfields. The three dominant plants were bramble Rubus fruticosus, found in all sites, common reed Phragmites australis, which occurred in 58 sites (95%), and scrub willow, which occurred in 51 sites (83%). Brambles can tolerate ground which is under water for part of the year and common reeds will grow, though more sparsely, on completely dry ground. This combination of dominants, however, does imply that even the wettest sites have some ground which is dry for at least part of the year. Mature trees were present in a number of sites, but probably had little significance to the Cetti's Warb­ lers. Thus, mature willows were found in 17 sites (28%) and mature alder Alnusgtutinosa/incana and oak in ten sites each (16%). Hawthorn scrub and trees up to 6 m high occurred in 17 sites (28%) and this was usually the main large plant where willow scrub was absent. Blackthorn Primus spinosa, exotic pines Firms, poplars Populus and gorse Ulex were all recorded in more than one site. Apart from brambles and common reeds, the following plants were common elements in the ground cover: bulrushes Typha latifolia/angustifolia, netdes Urtica dioicajurens, sedges Carex, rushes Juncus, willowherbs Epilobium, meadowsweet Filipendula ulmaria and raspberry Rubus idaeus. The dominant plants recorded have two common features. They tend to be between low and middle level in height and form a dense fades. They also tend to be among the first plants to colonise new ground and, because of their adaptability, may be deliberately planted by man to cover un­ sightly areas, such as coal slag heaps and waste ground. As an early stage in the succession, they may be superseded by taller trees as the ground dries out. Fortunately for Cetti's Warblers, not only are human activities in southern England ensuring that there is a constant supply of new ground for these plants to colonise, but also the Cetti's Warblers themselves have proved to be able to adapt to a relatively wide range of growths; so long as scrub exists they appear not to reject areas including tall trees, even when the latter form a closed canopy. This suggests that their acceptance of an area need not be restricted to a limited stage in the succession, but that they can occupy their niche through several stages^1 Further, they are not unduly disturbed by the presence of human activity (including noise from gravel washing and burning plants and heavy traffic) so long as this activity is not directed towards them. 94 Cetti's Warblers in east Kent Breeding Five pairs were proved to breed in 1975, four of them within the main area east of Canterbury. I recorded two of these myself. The first pair was watched feeding young in a nest in a bramble, close to the edge of a shallow reed-fringed dyke on several dates in early June. The young were being fed on grubs extracted by both parents from a rotten willow branch lying on the ground, by clasping the rotten wood with their large, powerful toes and tugging the grubs out with considerable effort. On some occasions, they attacked the branch in a manner reminiscent of woodpeckers (Picidae). The second pair had young just out of the nest in an area of mixed brambles, reeds and willow scrub in the corner of a flooded gravel pit much frequented by fishermen. The late date of this observation, 12th August, implies a second or even third brood. Although Witherby et al. (1940) and, for Portugal, Ferguson-Lees (1964) both mentioned the likelihood of more than one brood being raised, recent literature (e.g. Harrison 1975) has recorded Cetti's Warblers as single-brooded. The only evidence of more than one brood is from the first breeding pair in West Germany, which produced two broods in 1975 (Bonham and Robertson 1975). Although it may be that these new colonists are more prolific than more stable popu­ lations, it would be unusual for a small resident passerine, which exhibits territorial behaviour in breeding habitat from at least early March to at least mid August, to be restricted to one brood when it has ample time for at least three. It seems likely that more than one brood is normal. The other three pairs each had young in the nest in June.

Discussion The 1975 census indicated that Cetti's Warblers were consolidating their position in a very limited area of east Kent and, in order to do so, had moved into apparently marginal habitats in this area. This consolidation followed the pattern of Savi's Warblers Locustella luscinioides (Pitt 1967), which recolonised England along the same valley in i960 and reached a peak (which has not been surpassed to date) in 1965. The Cetti's Warbler, however, has the advantage of being a resident species, and can, therefore, establish territories before migrant warbler species arrive; it has bene­ fited from a remarkable run of mild winters since 1963. It is also more catholic in its choice of habitats and has, perhaps, like the Collared Dove Streptopelia decaocto before it, found a niche unoccupied by another species. Thus, it has now built up substantial numbers in the Stour Valley, where it is now probably the third commonest breeding warbler and among the ten commonest small passerines in some parts of the valley between Chilham and Grove Ferry. The widespread existence of suitable habitats throughout lowland and southern Britain favours its continued spread and increase. It has few competitors in this favoured habitat. Holiyer (1975) sug­ gested that the Wren Troglodytes troglodytes might be one, but the structural differences make this unlikely. It is more likely that Wrens increased in numbers so considerably after 1963 that they moved into every available Cetti's Warblers in east Kent 95 habitat, including those, such as swampy scrub, which are apparently not attractive as breeding habitats to most passerines. Their abundance in this habitat means that every Cetti's Warbler can hardly avoid having a Wren within earshot. A decline in the number of Wrens, following a cold spell in early 1976 in east Kent (P. J. Grant in litt.), was not followed by an ob­ served increase in Cetti's Warblers in the 1976 breeding season. This might suggest that the cold spell also had a detrimental effect on the resident Cetti's Warblers, but it similarly implies that, if immigration took place in spring 1976, the vacant Wren niches were not particularly attractive. Unfortunately, no census was attempted in 1976, although records sub­ mitted to the Kent Ornithological Society (and personal observations by DBR, JNH and CB) suggest numbers in the main areas similar to those in 1975. There may have been some withdrawal from marginal habitats, but this was probably compensated for by some increase in the optimum areas. There was still very little evidence of spread outside the Stour Valley in east Kent. CB recorded night singing during early May 1976 and this appears to be the first recorded instance. I suggest that the Sedge Warbler Acrocephalus schoenobaenus is the closest competitor, particularly as recent years have seen that species moving into drier habitats. Sedge Warblers are perhaps, however, at a disadvantage, being migrants, and have under­ gone some decline recently, along with other trans-Saharan migrants. It may not be coincidence that, in 1975, Sedge Warblers were unusually scarce in the Stour Valley.

Acknowledgements Without the assistance and constructive criticism, both in the field and by the pen, of C. Bignal, M. Davenport, P. J. Grant, J. N. Hollyer and D. B. Rosair this paper could not have been written: I am grateful to them for their co-operation, and their companionship in the field. I am particularly grateful to P. J. Grant for encouraging me to write this paper, to Asria Kouttjie and Marjorie Wanstall for typing the manuscripts and to Dr J. T. R. Sharrockfor his constructive criticism of the first drafts. Summary The history of Cetti's Warblers Cettia cetti in east Kent from 1971 to 1974 is summarised. The methods and results of a census carried out in the summer of 1975 are presented. A total of 61 singing birds was recorded and is discussed in terms of the distribution of sites and their nature in respect of the presence of surface water, the dominant plants and their position in relation to the Stour Valley. Two of the five confirmed breeding records are detailed and the prospects for the species in east Kent are discussed. Cetti's Warblers were found to be increasing and spreading in east Kent, but to favour consolidation in the initial area of colonisation along the Stour Valley near Canterbury. They were found to be able to adapt to apparently marginal habitats in this area of concentration. References BONHAM, P. F., and ROBERTSON, J. C. M. 1975. The spread of Cetti's Warbler in north­ west Europe. Brit. Birds 68: 393-408. FERGUSON-LEES, I.J. 1964. Studies of less familiar birds. 129. Cetti's Warbler. Brit. Birds 57:357-359- "'-■■-?' HARRISON, C. 1975. A Field Guide to the Nests, Eggs and Nestlings of British and European Birds. London. HOLLYER, J. N. 1975. The Cetti's Warbler in Kent. Kent Bird Rep 22: 84-95. PITT, R. G. 1967. Savi's Warblers breeding in Kent. Brit. Birds 60: 349-355. 96 Cetti's Warblers in east Kent

WILLIAMSON, K. 1963. Identification for Ringers. 1. TAe Genera Cettia,Locustella,Acrocephalus and Hippolais, Tring. WITHERBY, H. F., JOURDAIN, F. G. R., TICEHURST, N. F., and TUCKER, B. W. 1940. The Handbook of British Birds. London, vol. 2.

W. G. Harvey, 2 Little Bredlands, Bredlands Lane, Westbere, Canterbury, Kent CT2 OHD

[Brit. Birds, 70: 96-107, March 1977]