Bird Study (1999) 46, 48–61 Estimating the population size and rate of decline of Kittiwakes Rissa tridactyla breeding in Shetland, 1981–97 MARTIN HEUBECK1*, R. MICHAEL MELLOR2, PAUL V. HARVEY3, ANTHONY R. MAINWOOD4 and ROGER RIDDINGTON5 1Aberdeen University Research and Industrial Services Ltd, c/o Sumburgh Lighthouse, Virkie, Shetland ZE3 9JN, UK, 2Fairview, Quendale, Shetland ZE2 9JB, UK, 3Scottish Natural Heritage, Stewart Building, The Esplanade, Lerwick, Shetland ZE1 OLL, UK, 413 Ben Bhraggie Drive, Golspie, Sutherland KW10 6SX, UK, 5Fair Isle Bird Observatory, Fair Isle, Shetland ZE2 9JU, UK In the early 1980s, Shetland’s Kittiwake population totalled 54 000 pairs in 55 breeding stations of greatly varying size. Since 1985, periodic counts of nests have been made at all stations, mostly from an inflatable boat and in the same manner as a comprehensive survey in 1981. Different stations were surveyed in different years and at varying intervals. Changes in numbers at individual stations varied in size and direction. To estimate the total breeding population, and variation in the overall rate of change between 1981 and 1997, ‘missing’ nest counts at each station were calculated from mean annual rates of change between actual counts; the overall estimated population was expressed as the annual sum of actual and calculated nest counts. Shetland’s Kittiwake population declined by 50% during 1981–97. Although the decline was progressive, the rate increased markedly during 1989–93, then decreased between 1993–97. The main causes of the decline are considered to have been successive years of low breeding success, due to low availability of sandeels, increased rates of predation of nests by Great Skuas and increased predation of adults by skuas. n 1981, the first comprehensive survey of repeated counts at fixed study plots, it was Inesting Kittiwakes Rissa tridactyla in the more appropriate to survey all known colonies Shetland Islands found 54 300 pairs. These (and suitable habitat where new ones might were dispersed among colonies of greatly form) at least once every three to five years.2 varying size, with 90% of colonies consisting of By the early 1990s, overall breeding numbers fewer than 250 apparently occupied nests had declined further, although only slightly on (AONs).1 Fair Isle and Foula, which together held 60% of When colonies were resurveyed for the Sea- the population in 1991–94.3 All colonies have bird Colony Register in 1985, it was apparent been resurveyed at least once since 1994: those that, although there had been an overall on Fair Isle by Fair Isle Bird Observatory, on decrease in numbers of AONs, rates of change Noss by Scottish Natural Heritage (SNH), had varied greatly, both between and within on Foula by A.R.M, and elsewhere by M.H. and different colonies. It was decided that, instead R.M.M, with assistance from S.N.H. This paper of attempting to infer population change from presents these latest counts and it estimates the average rates at which the overall Shetland *Correspondence author. population, and the largest colonies, have Email: [email protected] declined since 1981. © 1999 British Trust for Ornithology Shetland Kittiwake decline 49 METHODS Aggregations of breeding Kittiwakes are most frequently referred to as ‘colonies’, and Boyd’s4 definition of groups of nests separated by at least 50 m of unoccupied cliff (or sea) was used in analyses of the 1981 survey.1 However, the term ‘colony’ has social as well as geographic connotations and here we have used the term ‘breeding station’,5 defined as colonies or colony groups less than a mile apart. Colonies on Foula were regarded as a single breeding station, because of their remoteness from other colonies and because earlier surveys were recorded as whole-island counts. The locations of all known breeding stations during 1981–97 are shown in Fig. 1. Surveys were carried out as detailed previously,3 i.e. from an inflatable boat in suitably calm weather during June, although sections of Eshaness, Noss and Fair Isle were counted from land, while the 1996 and 1997 surveys of Foula were made from a larger, hard-hulled boat. Although the basic count unit was an AON (defined as a well-built nest capable of holding eggs), trace nests (partially built or disinte- 25 km grated) were also counted where it was possible to distinguish them and the following analyses are of the total number of nests at each breeding station. Some nest counts here there- fore differ slightly from previously published figures, which referred to AONs only. To estimate the total breeding population in any year, the number of nests at each breeding Figure 1. The location of all known Kittiwake breeding stations in Shetland during 1981–97. The station in years between surveys was calcu- numbers refer to stations named in the Appendix. lated from average annual rates of change Stations referred to in the text are, in alphabetical between actual sets of counts (Appendix 1). At order: 48 Clett Head, Whalsay; 22 Eshaness; 57 Fair Noss, Foula and Fair Isle (the only stations not Isle; 56 Foula; 34 Hermaness; 8 Kettlaness; 53 Noness; surveyed in 1981), values for the early 1980s 49 Noss; 17 Papa Stour; 27 Ramna Stacks; 21 Skerry of were calculated from the mean annual percent- Eshaness; 41 Strandburgh Ness; 1 Sumburgh Head; age change between 1980–85, 1976–87, and 54 Troswick Ness; 9 West Burra; 12 Westerwick. 1975–86, respectively. To calculate the rate of decline at a breeding station found to have 1994 and 1995 which would have projected to been abandoned, it was presumed to have an improbable 426 nests by 1997. A second consisted of a single nest the previous year. The population estimate was therefore calculated, total Shetland breeding population was then based on the annual total of the last actual assumed to have been the annual sum of actu- counts at breeding stations. al, and calculated, numbers of nests. At In analyses, breeding stations are categorized breeding stations not surveyed since 1995/96, according to their size in 1981: small (fewer the number of nests in 1996/97 was calculated than 250 nests), medium (250–749 nests), large from the last known average annual rate of (750–1749 nests) and major (more than 1750 change. However, one recently established nests). station increased from 40 to 88 nests between Kittiwake breeding success was monitored in © 1999 British Trust for Ornithology, Bird Study, 46, 48–61 50 M. Heubeck et al. study plots at 11 breeding stations between 50.5%, from 54 600 in 1981 (slightly higher than 1986 and 1997, using methods developed by Richardson’s figure1) to 27 000 in 1997 (Fig. 2, Harris.6 The number of study plots and the Appendix), an overall average annual decline proportion of the total of nests that each of 4.2%. However, the rate of decline was far breeding station contained varied, and in most from uniform, either over time or between cases they were not selected randomly from a breeding stations. pool of potential plots. Plots were visited every During 1981–85 an increase (+1.0% p.a.) in four to six days throughout the breeding numbers at Fair Isle, by far the largest breeding season, until the last remaining chicks were station, offset a steeper decrease (–3.1% p.a.) within four to six days of fledging. Breeding elsewhere in Shetland (Fig. 3). Then, between success was defined as the number of young 1985 and 1988 there was an increase of 7.3% fledged per nest at which an adult was seen to (+1.8% p.a.) in the total number of nests at be, or appeared to be, incubating. small, medium and large breeding stations (Fig. 2). Other than Fair Isle, which continued to increase during this period, none of the RESULTS major stations was counted twice during Had all extant breeding stations (i.e. disregard- 1985–88, so we do not know whether any ing the years before stations were known to concurrent increases occurred at Noss, Foula, have formed, and after they were known Hermaness or Sumburgh Head. The annual to have disappeared) been surveyed annually percentage change in the estimated total popu- during 1981–97 there would have been a total lation (Fig. 4) suggests that a steep decline in of 908 counts; the total achieved was 340 overall numbers began in 1988 (after which (37.4%). However, sampling of the population numbers also began to decrease on Fair Isle) varied considerably between years, with no that averaged –4.8% p.a. between 1987 and counts made during 1982–84 and in some other 1991 (–7.7% p.a. for Shetland excluding Fair years predominantly the smaller breeding Isle), but which halted abruptly in 1992. After a stations being surveyed. Nevertheless, the pro- further large decrease in 1993 the rate of decline portion of the estimated population actually began to slow, to –7.4% p.a. between 1996 and sampled exceeded 20% in all but one year 1997 (Fig. 4, Appendix). (1993) during 1985–97 (Table 1). Apart from Fair Isle, numbers at the four The estimated number of nests at Shetland’s other major breeding stations all declined Kittiwake breeding stations decreased by between 1981–97 (Fig. 5). The rates of decline at Table 1. The number of extant Kittiwake breeding stations in Shetland and the number surveyed annually, the number of nests counted, and the percentage of the estimated population counted each year. No. of breeding No. of No. of nests Proportion of estimated Year stations stations surveyed counted population sampled (%) 1981 55 52 20 783 38.1 1982–84 55 0 0 0 1985 55 23 20 494 38.1 1986 56 30 2 889 43.4 1987 54 15 10 923 21.7 1988 54 18 3 681 46.4 1989 55 30 10 139 21.2 1990 54 12 9 153 20.3 1991 53 35 11 010 26.7 1992 51 15 8 506 66.0 1993 52 29 5 057 13.7 1994 51 20 10 107 29.9 1995 51 24 8 076 26.1 1996 51 17 5 891 20.5 1997 51 21 5 111 64.2 © 1999 British Trust for Ornithology, Bird Study, 46, 48–61 Shetland Kittiwake decline 51 Fair Isle Noss Foula Hermaness Sumburgh 750–1749 250–749 <250 55 000 50 000 45 000 40 000 35 000 30 000 Number of nests 25 000 20 000 15 000 10 000 5000 0 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 Year Figure 2.