Eurasian Bittern distribution and abundance in the UK during the 2009/10 winter Simon Wotton, Mark Grantham, Nick Moran and Gillian Gilbert Abstract By collating records from a number of sources, primarily via BirdGuides, BirdTrack and county bird recorders, it was found that there were a minimum of 600 wintering Eurasian Bitterns Botaurus stellaris in the UK during the 2009/10 winter. There were records from nearly 400 sites, the majority in England. It was estimated that the winter population included 208 resident UK Bitterns. rom late summer onwards there is a mainly from the Nordic and Baltic countries, regular influx of wintering Eurasian where the Eurasian Bittern (hereafter FBitterns Botaurus stellaris into the UK. ‘Bittern’) populations are entirely migratory It is thought that these birds migrate in a (Wernham et al. 2002). Although there have generally southwesterly direction, probably been a number of foreign-ringed Bitterns Robin Chittenden 342. Eurasian Bittern Botaurus stellaris, Strumpshaw Fen RSPB reserve, Norfolk, February 2010. 636 © British Birds 104 • November 2011 • 636–641 Eurasian Bitterns in the UK in winter 2009/10 recovered in the UK, none of those ringed in apparent increase in wintering records in Britain & Ireland have been recovered over- recent years, we attempted to collate records seas (Wernham et al. 2002). from the 2009/10 winter period in order to In recent years, increasing numbers of obtain a current snapshot of the numbers wintering Bitterns have been recorded at and distribution in the UK. many sites in the UK, most of which do not currently support booming (singing) males. Collating records Bitterns are not necessarily restricted to Bitterns were more evident in the UK during extensive reedbeds in winter and can often be the winters of 2009/10 and 2010/11, probably found in small wetlands with only small because weather conditions forced them to patches of Phragmites. Bitterns have long forage more widely when their traditional been known to be susceptible to severe wetland habitats became frozen. Our first winter weather (e.g. Rivière 1930). Bibby attempt to build up a picture of the numbers (1981) showed that wintering records in the and distribution of wintering birds was made UK between 1960/61 and 1978/79 peaked in using records that were submitted to January and that, during this period, there BirdGuides and BirdTrack. were more Bittern records during severe BirdGuides (www.birdguides.com) pro- winters, particularly during 1962/63 and vides up-to-the-minute news on rare and 1978/79. scarce birds across Britain & Ireland. The review of Special Protection Areas Archived news and photographs can be (SPAs) in the UK by Stroud et al. (2001) used searched easily and quickly. BirdTrack is a a figure of 100 for the Bittern population in partnership between the BTO, the RSPB, the non-breeding season. Ten sites were iden- BirdWatch Ireland and the Scottish Ornithol- tified as SPAs for non-breeding Bitterns, and ogists’ Club that looks at migration, move- at that time those sites held 50% of the esti- ments and the distribution of birds mated non-breeding population. Given the throughout Britain & Ireland. BirdTrack increase in the UK breeding population and (www.birdtrack.net) provides facilities for Robin Chittenden 343. Eurasian Bittern Botaurus stellaris, Norwich, Norfolk, December 2010. British Birds 104 • November 2011 • 636–641 637 Wotton et al. observers to store and manage their own records as well as using these to support species conservation at local, national and international scales. In addition, county recorders were contacted and asked to confirm the sightings from their recording area, especially the maximum numbers at particular sites and loca- tion details to minimise double- counting. Where possible, records were used from January 2010 only, since January is the peak month for winter records. Using just one single calendar month also helped to avoid potential double-counting through birds moving between sites, although there is little evidence for this from sites where wintering numbers are monitored regularly. Fig. 1. The distribution of the 376 sites in the UK where Estimating the resident UK Eurasian Bitterns Botaurus stellaris were reported during the 2009/10 winter, and the 20 sites in England holding population booming males in 2010 but where there were no winter In addition to winter immigrants, reports. There were no reports from Orkney and Shetland. we needed to account for the largely resident breeding population. Adult males and most adult females are considered to stay mainly at or near their breeding sites during winter, while immatures are likely to dis- perse from their natal sites, and this can occur soon after fledging (Gilbert et al. 2005). The UK’s breeding Bittern population has been monitored annually since 1990, and it is thus possible to estimate the minimum number of UK-breeding Bitterns remaining during the 2009/10 winter. In 2009, there were a minimum of 82 booming males and 39 con- firmed nests that reached the chick stage (Wotton et al. 2009). The number of breeding adults in 2009 was thus assumed to be 164 (the number of singing males is the best estimate we have of the breeding Fig. 2. Records of Eurasian Bitterns Botaurus stellaris in population and, although the the UK during the 2009/10 winter, by county/recording breeding system of Bitterns is pol - area. The shaded counties, from palest to darkest, represent 1–4 birds; 5–9 birds; 10–19 birds; 20–49 birds; 50+ birds. ygynous, we know that the propor- Government Office Regions (see table 1) are outlined tion of male and female chicks more heavily than recording areas. hatched is 50:50 (Gilbert et al. 2007) 638 British Birds 104 • November 2011 • 636–641 Eurasian Bitterns in the UK in winter 2009/10 and there is no evidence of an imbalance in resident UK Bitterns during the winter of the sex ratio). Of the 39 confirmed nests, 18 2009/10 to have been in the region of 208. were probably successful, seven were possibly successful, 12 probably failed and for two the Distribution during the 2009/10 outcome was unknown. The number of winter fledged young in 2009 was estimated from During the 2009/10 winter, Bitterns were the 25 possibly and probably successful nests. reported at 376 sites across the UK: 342 in An average of four eggs per nest is typical England, one in Northern Ireland, seven in and the average chick survival from hatching Scotland and 26 in Wales (fig. 1). In addition, was found to be 39.1% (Gilbert et al. 2007). between October 2009 and March 2010, there Those measures suggest that approximately were no reports from 20 of the 58 sites 39 chicks fledged in 2009, which we believe is holding booming males in 2010 (Wotton et the best and most accurate estimate of pro- al. 2010); these 20 sites held a minimum of ductivity available. 27 booming males in the spring of that year. In addition, there are likely to be a Radio-tracking studies have shown that number of immature non-breeders (i.e. first- males in particular remain predominantly on year birds) in the population. Bitterns can their breeding territory through the winter. boom and breed in their first year, but the There was no evidence that these 20 breeding evidence from following radio-tagged birds sites that produced no winter sightings had into their first breeding season suggests that become unsuitable – only one of the 20 was it is equally likely that males would not boom frozen for a long period, although even there and that females would not nest (Gilbert suitable foraging areas could be found in the pers. obs.). The number of fledged young immediate vicinity. As a consequence, we from 2008 was estimated to be 27, which, added the 27 males onto the figure for the with a 61% mortality rate (Gilbert et al. reports of birds seen. 2010), suggests that ten of these birds were In all, 571 Bitterns were reported during alive during the 2009 breeding season. the 2009/10 winter, in addition to the 27 resi- Assuming that only half of these Bitterns dent males not reported but probably bred in 2009, we thus have five immature present. Table 1 shows the number of sites non-breeding residents in the Bittern popu- and the number of Bitterns by country and lation during the 2009/10 winter. It is not region. East Anglia and Southeast England, possible to estimate how many non-breeding particularly Kent, held the most Bitterns, as adults there may be in the UK population. highlighted in fig. 2. It is clearly apparent Consequently, we estimated the number of from table 1 and fig. 2 that Bitterns were Table 1. The number of sites and the minimum number of wintering Eurasian Bitterns Botaurus stellaris in the UK during 2009/10, by country and English Government Office Region (London is included in Southeast England). Country Government Office Region Minimum Minimum number of sites winter total East England 105 166 East Midlands 40 55 Northeast England 12 14 Northwest England 18 27 Southeast England 91 137 Southwest England 51 77 West Midlands 17 26 Yorkshire & the Humber 28 47 England 362 549 Northern Ireland 11 Scotland 77 Wales 26 41 UK 396 598 British Birds 104 • November 2011 • 636–641 639 Wotton et al. widely distributed across the Midlands and example, between November and March, in southern counties of England. 2009/10 there were 47.5 frost days in England, compared with 81.3 in 1962/63 and Discussion 63.4 in 1978/79; http://www.metoffice.gov.uk/ During the 2009/10 winter, there were a climate/uk/datasets/).