The last published survey of the British breeding population of Bearded

Transcription

1 Colonisation by Bearded Tits of Leighton Moss, Lancashire John Wilson The last published survey of the British breeding population of Bearded Tits Panurus biarmicus (O'Sullivan 976) described the spread of the population up to 974 from its former restricted breeding area in East Anglia to areas along the South Coast and into Humberside. At mat time, a small population had been established for only two years in north Lancashire, and presented the opportunity to study the expansion of range to this isolated reedbed. This paper details the colonisation, and subsequent build-up and fluctuations of the breeding population of Bearded Tits, from 973 to 992. It also describes the breeding and eruptive behaviour observed in the 9 years since successful colonisation occurred. Study area The study area comprised the 34.5-ha RSPB reserve of Leighton Moss in north Lancashire. Bearded Tits were confined to the 79-ha reedbed. The reedbed is almost pure common reed Phragmiks australis, with fringing areas of yellow flag Iris pseudacorus and reedmace Typha latifolia, and small clumps of goat willow Salix caprea in the drier areas and along the dry edge. The reed areas are usually wet, with 0-20 cm of standing water in most parts in spring. Methods The Bearded Tit is a difficult species to census, and wet reedbeds also pose considerable difficulties: access may be tricky and the habitat can be physically damaged by trampling. Additionally, because of the risks of disturbing rare breeding species, such as Great Bitterns Botaurus stellaris, and, from 987, Marsh Harriers Circus aeruginosus, access had to be restricted mainly to paths. Bearded Tits do not sing, and are not territorial. Males often assemble in small groups in spring, and there are several records of three individuals feeding at one nest. Conventional census methods, using either the Common Birds Census or point counts, are therefore difficult to apply. At Leighton Moss, an 2 \Bril. Birds 86: 2-8, August 993]

2 Bearded Tits at Leighton Moss 3 annual census was carried out by mapping presumed pairs from sightings and observed nesting behaviour during the period from nest-building to feeding of young. For the five years , ringing of the population with BTO rings was carried out using mist-nets. Ringing was undertaken in 975 and 976 as part of a population study of reedbed birds, and was usually carried out, weather permitting, on three early mornings weekly from late May to late August and on two mornings weekly from September to November. Mist-netting was restricted to four separate locations within the 79-ha reedbed. These covered the main juvenile assembly areas, to which some adults, especially males, were also attracted. During , netting was carried out mainly to obtain faecal material for a study of the diet of Bearded Tits (Bibby 98). Catching was restricted in 977 and 978 mainly to two areas, both juvenile assembly areas; in 979, it was further restricted to one area only, and visits were usually only once or twice a week. Ringing was resumed in 992 as part of a national census of Bearded Tits, but was restricted to three mainly juvenile assembly areas. Observations on nesting behaviour were made during census work. Over 60 nest sites were located, mainly by watching from a distance the adults carrying food to the nest. Because of the risk of disturbance or destruction, only 5 sites were examined closely. Early-morning observations on eruptive behaviour were made over the years, mainly from the reserve's path network and hides. Observations were most intensive during the period of the ringing study, with much of the reedbed covered during this period. Results Bearded Tits were first recorded at Leighton Moss on 5th November 965, when a party of five appeared. They were recorded throughout that winter, but there were no sightings after 5th March 966. The next sighting was not until 3th November 97, when, once again, five were recorded; some of these remained throughout the winter, and two were present throughout the following spring and summer, but both were females, and no breeding took place that year. Four were seen on 24th October 972, and ten on 2th November, with four still present on 20th March 973. Despite intensive Table. Estimated breeding; population of Bearded Tits Panurus biarmkus at Leighton Moss, Lancashire, , and numbers of adults ringed or retrapped, and 992 M = male; F = female Year No. of pairs No. M ringed F Year No. of pairs No. ringed M F

3 4 Bearded Tits at Leighton Moss searching, only one pair could be located later. Breeding was proved when a recently fledged young was seen with one adult on 7th May 973. Two more successful nests were located later that season, the timing and location of the nests suggesting that all three broods were reared by one pair. Small parties of young were seen during the summer of 973, and the largest flock recorded that year was one of in mid October. Table details the recorded changes in population following the colonisation in 973. It took four years for the population to reach 30 pairs (an initial annual population increase of 233%), and the maximum recorded population of 40 pairs was reached seven years after die initial colonisation. The declines noted in 979, 982 and 986 all followed periods of cold weather in the preceding winter. Prolonged cold spells rarely produce heavy snowfall at Leighton, because of its proximity to the mild West Coast, die only heavy snowfall during the period being in December 98, when 2 cm fell; fortunately this soon melted. Details of the adults caught during the period of ringing studies are given in table. In 975, five males of an estimated population of six were caught, and 4 out of 6 males in 976. In all years, more adult males than females were retrapped. Observation suggested that males were drawn into die juvenile flocks, where the ringing effort was concentrated, much more than were the females. All the 60 nest sites were located in the thicker areas of pure reed, with the nest built into the top reed litter, usually less than 30 cm above the ground level or water level. As already noted, in normal springs, virtually all the reedbed is wet, with 0-20 cm of standing water. The densest reed litter occurred, however, in the sections with shallower water, and Bearded Tits showed a marked preference for such areas. Fledged juveniles were seen as early as 4th April. Incubation takes 2-3 days, and fledging only 9-2 days (Witherby et at. 94), suggesting that laying can start as early as 5th March. Most first-brood young appeared in May, with first eggs being laid in early to mid April. Table 2 shows the month of first ringing of juveniles from 975 to 978 and in 992. Those caught in May and June were from first broods, with the second brood appearing in late June and July. The low numbers caught in August and September suggest that very few third broods were produced at Leighton. Flocking of juveniles took place in late May in an early spring, or early June in a late spring. At Leighton, such flocks often gathered in the Table 2. Numbers of juvenile Bearded Tits Panurus biarmiens ringed in each month at Leighton Moss, Lancashire, and 992 Year May Jun MONTH OF RINGING Aug J"l Sep Oct Totals

4 Bearded Tits at Leighton Moss 5 Fig.. Locations of main sites mentioned in text Scale approximately cm = 25 km small areas of dead reed left after spraying with the herbicide Dahpon (as part of a programme to control reed encroachment). They also often frequented the willows along the central footpath or causeway which crosses die reserve. The largest numbers occurred on calm early mornings and usually dispersed by mid morning, often reassembling on still evenings. They usually called excitedly and, on occasions, made flights of up to m, just above the old reed tops. Only occasionally were adults seen in these groups. This behaviour lasted throughout June and July, with later broods joining the flocks. Ringing revealed that many young remained in the flock until the onset of moult. By early August, the groups had dispersed, and there was a quiet period when birds were moulting and became extremely difficult to locate or to mist-net. Some renewal of activity usually became evident at the end of August, with calling from the reed tops or willows, and flocking of those newly moulted, now in adult plumage. The activity and flock size increased throughout September. At first, behaviour then was very similar to the activity recorded in June, but by the second or third week in September they regularly started their 'high flying' activity (Pearson 975). Bearded Tits regularly took off in groups, rising to perhaps 200 m on occasions, gradually spiralling up and calling all the time. Many flocks rose to only about 30 m and then quickly plunged back to the reedbeds, calling excitedly and moving quickly through the reed tops before taking off again. Other flocks stayed in flight high above the reedbeds for up to four or five minutes. The high flying and associated activity reached a peak around to 0.00 BST, then gradually declined, and usually ceased by.00. Such activity was usually restricted to calm or nearly calm sunny mornings. This eruptive behaviour usually reached a peak in late September and continued into October. The greatest activity was recorded after a succession of calm, settled early mornings. Every flock observed has always remained above the Leighton reedbed and eventually returned to the reeds, but that they do leave in at least some years is confirmed by die recoveries of those ringed in spring or summer at Leighton and reported at Bolton, Lancashire, in October and March and (four) in Soudi

5 6 Bearded Tits at Laghtan Moss Yorkshire in January, all except one in the autumn or winter following ringing; the exception was in its fourth year. Eruptive behaviour as described above has been recorded in every year since colonisation, although there is proof from ringing of movement away from the reedbed in only two years. During the period from mid September to late November, Bearded Tits regularly fed in groups, usually of from five to 20 individuals, on the limestone paths at Leighton. They moved out of the reeds to alight on the path, starting close to the reed edge, but, if not disturbed, diey spread into the centre of the path up to 2 m from the reed edge. This activity appears to be associated with the eruptive behaviour described above; indeed, flocks which have been watched calling excitedly and taking part in high flying will gradually move from this activity to feeding on the paths. The path-feeding activity occurs during the same period as the eruptive behaviour, and, like the eruptive behaviour, gradually tails off in November, with smaller numbers as the month progresses. When on the paths, tire flocks feed avidly, pecking at something small. What they are taking has never been identified positively, but it has been assumed that they are taking reed seeds, which certainly fall during this period; it has also been suggested that they may be picking up grit, which they need at that time of year as they change from the soft insect food of summer to the hard reed-seed diet of autumn and winter. I can trace no published record of such behaviour, although it does occur also at Blacktoft Sands, Soudi Humberside, in some years (A. Grieve in litt). Discussion The census methods used were not ideal, the greatest difficulty occurring in the favoured, drier areas, where several pairs were nesting in close proximity and small groups of up to five adult birds could be seen together, especially in early ApriL The limits on access in some areas of the reedbed also undoubtedly led to some birds being missed, especially from 987 onwards, following the colonisation of the reedbed by Marsh Harriers. Thus, the numbers are considered to be accurate enough only to round down to the nearest five pairs in a year. The census methods were, however, applied consistently over the period, so should provide comparable results from year to year. The ringing gave some check on tire accuracy of the mapping census, but, partly because it was not primarily aimed at assessing population levels and was restricted to only two to four areas, was also certainly subject to some bias. The first two years, when the ringing effort was more widespread, produced a good fit with the census results, especially for males. Some adults in the areas of reed remote from the ringing sites were probably missed. The ringing effort was evidently extremely successful in catching juveniles, for no unringed adult birds were caught during the four years The origin of the Bearded Tits which colonised the Leighton reedbed is not known as none was ringed. Their arrival in die period late October to early November is, however, in keeping with the eruptive behaviour recorded elsewhere (Axell 966; Pearson 975; O'Sullivan 976). October is the major monur of movement by diis species both in East Anglia (Axell 966) and at Leighton. It could be assumed diat the most likely site of origin would be the

6 Bearded Tits at Leighton Moss 7 nearest breeding colonies at that time, which were 30 km southeast at Blacktoft Sands and the Humber reedbeds. Equally possible is that they came from either the East Anglian population or die Dutch population (see fig. ). The latter possibility is supported by a ringing recovery in January 966 at Marton Mere, near Blackpool (25 km south of Leighton), of a juvenile female ringed at Veluwemeer Nord IJsselmeer in die Netherlands five mondis earlier. The Leighton Moss population has fluctuated, widi declines occurring in 979, 982 and 986 following severe winters with some snow cover. The decline from 98 to 982 was about 37%, while the decline from 985 to 986 was 57%. Heavy snow, which lays the reed and prevents the birds gaining access to reed seeds, the main winter food, has caused severe declines in other areas (O'Sullivan 976). The successful colonisation of this isolated reedbed is a good example of the value to die species of its eruptive behaviour leading to dispersal and establishment in new areas. The habitat at Leighton is obviously suitable and the wetter, cooler weadier of the Nordiwest has not inhibited breeding. The highest population at Leighton Moss was the 40 pairs recorded in 980 and 98. This gave a density in the reedbed area of 0.5 pairs per ha. In the 08-ha tidal reedbed at Blacktoft Sands RSPB reserve, the density in years of high population was 0.9 pairs per ha (A. Grieve in litt.). At Titchwell RSPB reserve in North Norfolk, the density, again in a good year, in the freshwater reedbed was.5 pairs in 0 ha (N. Sills in litt). The Leighton reedbed is the wettest of the three reedbeds for which density estimates are available, suggesting that Bearded Tits do prefer the drier sites, a preference observed at Leighton, where the drier areas held die bulk of die population. This preference may be pardy because die diicker reed litter in such drier areas provides more nest sites tiian die wet areas. Anotiier factor is tiiat the Leighton population appears rarely to produce a third brood. Possibly the cooler, more maritime climate at Leighton, which allows an earlier start to die breeding season, produces a lower food supply later in the season when third broods would be in the nest. Third broods are regular at both Blacktoft and Titchwell, especially in warm dry summers (A. Grieve in litt.; N Sills in litt). The Leighton Moss reedbed remains the species' only breeding site in die Northwest, presumably because all other reedbeds in the area are too small. It has been surmised diat eruptive behaviour is a response to high population levels of this species (Axell 966) or to its dependence on the seed of a single plant species (see Bibby 983). At Leighton, the eruptive behaviour of excited activity and high flying has occurred each year. Even in the first year, when the population was under 20 individuals, such behaviour was noted, which does not appear to support the view that eruptive behaviour is a response to high population levels. It is difficult to compare years to test if die intensity and numbers involved in eruptive behaviour vary widi population levels, since, as detailed above, weather plays such an important part in the scale and intensity of eruptive behaviour. It should be remembered, however, that the Leighton population is presumed to have originated from birds which had dispersed by eruptive behaviour, and so would be more likely to exhibit such behaviour than does die long-established East Anglian population, an element of which may be sedentary.

7 8 Bearded Tits at Leighton Moss Acknowledgments I should like to thank Dr John Day, Glen Tyler and Norman Sills for commenting on earlier drafts of diis paper. Summary The reedbed at Leighton Moss, Lancashire, was colonised by Bearded Tits Pcmurus biarmkus in 973, and the population subsequently built up to a maximum of 40 pairs. The hypothesis that eruptive behaviour is a response to high population levels is questioned. References AXELL, H. E Eruptions of Bearded Tits during Brit. Birds 59: BlBBY, C.J. 98. Food supply and diet of the Bearded Tit. Bird Study 28: Studies of west Palearctic birds. 86 Bearded Tit. Brit. Birds 76: O'SULUVAN, J. M Bearded Tits in Britain and Ireland, Brit. Birds 69: PEARSON, D. J Moult and its relation to eruptive activity in the Bearded Reedling. Bird Study 22: WITHERBY, H, F., JOURDAIN, F. C. R., TiCEHURST, N. F., & TUCKER, B. W. 94. The Handbook of British Birds, vol.. London. John Wilson, Leighton Moss RSPB Reserve, Myers Farm, Siberdak, Carnforth, Lancashire LA5 OSW