Little Bitterns Breeding in South Yorkshire
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Little Bitterns breeding in South Yorkshire Andrew M. Allport and David Carroll he Little Bittern Ixobrychus minutus occurs in Africa, Europe, Asia and TAustralia. It is a summer visitor to Europe, where it breeds locally throughout central, southern and eastern Europe, but is absent from Scandinavia and Britain and Ireland (Cramp & Simmons 1977). It is an accidental of almost annual occurrence in Britain and Ireland, with 140 records in the 27 years 1958-84, mainly in the south of England and East Anglia, with 70% of all spring vagrants being south of a line from The Wash to the mouth of the Severn. Individuals (including 'barking' males) and apparent pairs also summered during that time. Pairs were present in Somerset during June-August 1958 (Pyman 1960), in Huntingdonshire in June-July 1960 (Pyman 1961), and elsewhere in 1979 and 1980 (Sharrock et al. 1981, 1982), and males stayed for six weeks in Somerset in June-July 1970 (Smith et al. 1971) and perhaps from June to August in West Glamorgan in 1976 (O'Sullivan et al. 1977). Occasional breeding was strongly suspected in East Anglia in the nineteenth century and in Kent in 1947, and doubtfully occurred in Surrey in 1956 (Parslow 1973), but, until 442 [Brit. Birds 82:442-446, October 1989] Little Bitterns breeding in South Yorkshire 443 the events described in this paper, there was no definite record of breeding in Britain and Ireland. By 1977, there were only six records of Little Bitterns in Yorkshire this century. Four of these occurrences were in autumn, and one in winter; the sixth record concerned a male at Hornsea Mere from 29th May to 11th June 1977, and this was the last Yorkshire record prior to 1984 (Mather 1986). In view of past records, therefore, it was most unexpected that this species should first be proved breeding in northern England. In 1984, a pair of Little Bitterns arrived at Potteric Carr, a 140-ha nature reserve in South Yorkshire. From 11th June, when the male was discovered by R. P. Lambe and later seen by S. Boyes, the site was closely watched. By late July, successful breeding was confirmed. Study method Observations made by AMA, Yorkshire Wildlife Trust warden at the time, varied from three-quarters of an hour to six hours per day. This took place on 27 out of the 67 days on which the Little Bitterns were known to be present. The total of about 80 hours of watching provided most of the observations described in this paper. A number of other observers contributed details on the frequency of sightings, and records for those days when the warden was not in attendance. Nest site The nest site was well hidden, in the middle of a small, secluded area of common reed Phragmites australis standing in water approximately 30 cm deep. On one occasion, the male was seen carrying a reed stem. The nest was some 12 m from the edge of Low Ellers Marsh, which covers an area of 13 ha. Habitat surrounding the site includes mixed reed-fen, open water and willow-carr Salix. This is criss-crossed by drier areas, with some scrub- invaded railway embankments and areas of woodland, predominantly birch Betula. Observations The male was seen frequently in flight, often every half hour or so for the first few days. At this time, he inhabited many different areas of the reed- fen around the nest site, and occasionally visited adjoining areas of water and reedbed. He soon established a routine. From about 20th June onwards, he would most often be seen in the morning, flying 370 m from his regular feeding area alongside a ditch, to the reedbed nest site. Observations indicated that there was a fairly large population of common frogs Rana temporaria and common toads Bufo bufo in the surrounding marshes. The male Little Bittern was seen to take amphibians on several occasions, but smaller food items could not be identified. The female may have arrived with the male, but she was not seen until 15th June, and was much less frequently observed. Moreover, throughout late June and early July, the female was not seen for several days at a time. She was most often seen in the early morning and evening, but generally did not fly far from the nest site. Feeding often took place some 20-30 m 444 Little Bitterns breeding in South Yorkshire away from the nest. She was markedly more crepuscular and secretive in behaviour than the male. Both adult Little Bitterns were normally very quiet, and only on very few occasions were calls heard from either individual. On 17th June, the male was heard during late evening, uttering a bark-like croaking. This was heard intermittently for a period of 30 minutes while the bird remained concealed. The only other call made by the adults was when signalling the juveniles to be fed, and is described later. While it was difficult to establish unquestionable conclusions from the discontinuous observations and limited data collected, the following impression was formed from the frequency and timing of nest-site visits. It was evident that the majority of the incubation was carried out by the female. The male seemingly incubated the eggs only for relatively short periods; the activity of both parents indicated probable male incubating times from 30 minutes to three hours, and appeared to occur in the middle of the day or in the evening. From 9th July onwards, there was a considerable increase in sightings and nest-site visits by both adults. It is likely that this was the period when the juveniles were becoming increasingly demanding of the parents, as they rapidly developed. Indeed, juvenile begging calls could be heard on 18th July, and subsequently on most occasions when a parent bird approached. The male appeared to carry out most of the feeding of the young. Observations indicated that he fed the juveniles as many as ten times more frequently than the female in the early part of their development. The juveniles seemed not to foray from the nest for some time, and in fact were not seen near the edge of the nest-site reedbed until 26th July. At this stage, they were evidently still quite dependent on their parents, and retained varying amounts of down on crown, nape and mantle. By 26th July, the feeding rate had increased, with intervals between feeds varying from five to 30 minutes. Lulls occurred around midday/early afternoon, when two hours could elapse without any sign of feeding. Over the two weeks or so prior to their dispersal, feeding increasingly followed a predictable pattern. An adult would often settle on a dead branch out in the open, a few metres away from a juvenile. It would call, uttering a soft 'quer . quer', whereupon one or two of the young birds would fly to the adult while giving a raucous 'kraar . kraar', recalling a crow Corvus. Upon alighting, they flapped their wings vigorously, with underwing- coverts flashing white. One young would then grab hold of the adult's bill at its base, and pull downwards until food was regurgitated directly into the juvenile's throat. A number of interspecific encounters were noted during the Little Bitterns' stay. Virtually all involved the resident colony of Black-headed Gulls Lams ridibundus. Adult encounters were generally of an aggressive nature. When approached by a Black-headed Gull, and on one occasion by a Carrion Crow Corvus corone, the male Little Bittern's crown and mantle feathers were raised in threatening posture. When mobbed, he would strike upwards with his bill partially open. In comparison, on the Little Bitterns breeding in South Yorkshire 445 two known occasions when juveniles received this mobbing, they adopted a typical 'bittern-stance', with chest feathers ruffled and bill pointing skywards. In one instance, while exhibiting this 'concealment' posture, the juvenile was perched conspicuously on a dead branch in open water. As no specific observations supplied egg-laying or hatching dates, the incubation and fledging periods are conjectural. If we assume that the advertising calls of the male on 17th June were prior to the full clutch being laid, and that the juveniles fledged fully around 5th August, the periods of incubation and fledging would be consistent with recent literature. Cramp & Simmons (1977), for example, gave incubation at 17- 19 days, and fledging at 25-30 days. They also suggested that juveniles become independent at or soon after fledging. Observations at Potteric Carr showed, however, that the young were certainly dependent on the adults for several days after fledging. They continued to be fed by the parents until at least two days before their apparent dispersal from the area. At least three juveniles fledged, one of which was, when last seen, still significantly smaller than the other two. It had paler plumage and was less developed, with considerable amounts of down across the crown and nape. Some difference in rates of development are to be expected, of course, owing to asynchronous hatching. This smaller juvenile was last seen on 11th August, the day after the others had apparently dispersed. The last Little Bittern to be seen was the male, on 16th August. Possible difficulties in separating female and juvenile plumages may have contributed towards uncertainty about the exact number of young reared. This is obvious from records already published. Mather (1986) stated that three fledged juveniles were present, and this is the considered opinion of the finders and of AMA. Four young are claimed by some observers whose records appear in the 1984 ornithological reports of the Yorkshire Naturalists' Union and the Doncaster and District Ornithologi cal Society; Rhodes (1988) accepted the latter records of four young birds.