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Studies of Red-throated Diver and Merlin in 2005at the proposed Viking Windfarm siteMainland, ShetlandDigger Jackson and Lauren BeasleyFebruary 2006 Non-confidential version
Summary• Detailed studies were made in 2005 on merlin and red-throated divers breeding on the proposed Viking windfarm site in the central part of Mainland island, Shetland. This work was complimentary to the programme of general bird surveys and vantage point watches also undertaken by Natural Research in 2005. Merlin and red-throated diver were singled out for detailed study because they are nationally rare species that will necessarily require detailed consideration in any impact assessment of the proposed windfarm. These species are both vulnerable to the activities of illegal egg collecting and disturbance by photographers. For this reason the locations of nesting sites are considered strictly confidential and, therefore, are not provided in this version of the report. • The primary aims of the study were to determine the number and distribution of breeding pairs and monitor their breeding success. For red-throated divers a further aim was to describe and map the flight lines of birds at specific nest sites in accordance with the guidelines set out by SNH. • Of eleven traditional merlin territories checked on the Viking Site and buffer, six pairs were found and these produced 18 fledged young. This represents about 30% and 0.5% of the Shetland and UK populations respectively. A single bird was seen at a seventh territory. The reasons why several traditional territories were vacant in 2005 are unknown. A moderate rate of territory vacancy has been normal in recent years in Shetland. • Forty territory-holding pairs of red-throated divers were found on the Viking Site or within the 2 km buffer zone in 2005, of which 37 were proven breeding (eggs or chicks seen). This is very similar to the number found in 2004 by a NR study. It represents about 9% of the Shetland population and 5% of the UK population. Therefore, the site has very high conservation importance for this species. • In addition to the breeding pairs there were a large number, probably at least 20 individuals, of non-breeding birds summering on the Viking Site. These mainly occurred as loose gatherings on the few larger lochs. • Six (15%) breeding sites used in 2004 were vacant in 2005 and a similar number of ‘new’ sites occupied. This shows there is a moderate amount of year-to-year change in breeding site occupancy on the Viking Site, a pattern that is normal for this species. • Red-throated diver breeding success in 2005 was high. Of 37 proven breeding pairs, 70% were successful and reared a total of 37 chicks. Breeding success was much higher than 2004 when only 41% of the proven pairs reared young. • Red-throated diver egg laying dates in 2005 showed a bimodal distribution with the main peak occurring from mid May to mid June, and a small peak in late June and early July. • Over 400 hours of flight watches were conducted at 21 breeding lochs/lochans. This resulted in 872 flights being described and mapped. The SNH guideline number of flights (at least 15 incoming flights) was broadly achieved or exceeded at 17 breeding sites and approximately half achieved at four others. • Approximately two-thirds of flights were by breeding adults, mainly inbound or outbound feeding flights. The remaining third of flights were by non-breeding birds but because they were of longer average duration, these made up approximately half of all diver flight activity (flying time) observed. The flight paths of non-breeders were much less predictable than those of breeding birds because they ranged widely over breeding areas often taking in a succession of lochs of various status, rather than feeding flights to/from the sea. • About a quarter of the breeding pairs studied in detail made extensive use of a loafing area at a nearby (within 1 km) loch or marine voe. The flights of these pairs showed a more complex spatial pattern because their activity had two foci; the flights could be between any combination of breeding site, loafing area and marine feeding area. • The adequacy and completeness of the data collected so far on these species is considered and recommendations made for further data collection.
IntroductionThis report concerns detailed studies on red-throated diver and merlin undertaken in spring/summer 2005 on the proposed Viking Windfarm site in Shetland, Scotland. The site covers an area of slightly over 100 km2 in the central part of Mainland Shetland (Map 1). These two species were singled out for detailed study because they are both listed on Annex 1 of the European Bird Directive, were known to occur in good numbers on the site and, because of their rarity and potential vulnerability, will necessarily require detailed consideration in any impact assessment of the proposed windfarm. Although several other Annex 1 species were known to breed on the Viking site (e.g. dunlin, golden plover and arctic tern) these were not the subject of specific study in 2005 because it was felt that the significance of the numbers present first needed to be evaluated through generic breeding bird survey work undertaken on the site in 2005. In 2005 Natural Research employed two teams of ornithologists on the Viking site. In this report, these teams are referred to as the Diver Team and the Bird Survey Team respectively. The Diver Team consisted of Digger Jackson and Lauren Beasley and was solely employed to collect data on merlin and red-throated divers. The Bird Survey Team consisted of Steve Minton, Mark Chapman and Dave Hall and was primarily employed to undertake generic upland bird surveys and vantage point watches for all species covering the entire site. They also assisted in collecting diver flight data later in the season. This report summarises the methods employed and data collected during the detailed studies on merlin and red-throated diver in 2005. It also attempts to put these data into a wider context, for example in terms of regional and national populations, historical breeding success and the recommended data requirements for the windfarm impact assessment. It is beyond the scope of this report to make any comments of the likely impact of the proposed Viking development on these species, though attention is drawn to certain findings that have particular relevance to making such assessment. Red-throated Diver and merlin are rare breeding birds in the UK and are both targeted by illegal egg-collectors. Disturbance of these species at their breeding sites can be also be detrimental, for example by birdwatchers or photographers. Therefore, for the welfare of the birds, it is important that exact breeding sites remain confidential. All visits to merlin and red-throated diver breeding sites were carried out under licence from SNH. This report is a slightly shortened version of a more detailed report provided to the client. For confidentiality reasons, information pertaining to specific breeding sites has been excluded and the full details of the recommended programme for further work have been removed.
Abbreviations and definitions used in reportSNH - Scottish Natural Heritage RSPB - Royal Society for the Protection of Birds NR - Natural Research EIA -Ecological Impact Assessment VP - Vantage Point
Large loch – a freshwater body over 5 ha in area (large within context of Viking Site). Small loch – a freshwater body between 0.5 and 5 ha in area (small within context of Viking Site). Lochan - freshwater body on blanket peatland between 0.025 and 0.5 ha in area . Pool- a peatland pool less than 0.025 ha in area (these are generally too small for divers). Satellite loch - a second loch in a diver breeding territory that is additional to the nesting loch and is used for loafing by adults and, in some cases, chick rearing. Typically, satellite lochs are considerably larger than the nesting loch and within 0.5 km of it. Adult diver - a bird in full summer plumage (includes breeding and non-breeding birds).
Merlin Study
BackgroundThe merlin Falco columbarius is a small falcon that breeds on moorland and hunts mainly small songbirds such as meadow pipit and wheatear (Ellis and Okill 1990). This is currently the only regularly breeding bird of prey in Shetland. In recent years the Shetland breeding population has been less than 20 pairs, though it used to be somewhat greater (Pennington et al 2004). It generally nests on heather clad slopes, sometimes in an old crow nest. AimThe aim of the work on merlin was to establish the number of occupied breeding territories on and close to (within 2 km) the Viking Site and determine breeding success. MethodLocal ornithologists Pete Ellis (coincidentally Conservation Officer for RSPB) and Dave Okill provided NR with a list of all locations where merlins were known to have bred on or close to the Viking Site in the past 10 years. When these historical nest sites for all years were mapped there was a strong tendency for nest sites to form loose clusters. These clusters were in most cases at least 2 km apart and within a cluster nest positions were typically 0.5 – 1.5 km apart. Each cluster was interpreted as a traditional territory. All the known recent (past 10 years) historical locations, together with several other locations with apparently suitable habitat (slopes with extensive areas of deep heather) were searched by the Diver Team for signs of occupation between 14 and 28 May, the time when most pairs are in the first half of the incubation stage. In addition the Bird Survey Team (working on the generic breeding bird survey) were requested to be vigilant for signs of merlin and report any information to the Diver Team. Initial searching consisted of systematically walking through suitable nesting areas (mainly heathery slopes and gullies) and watching out for merlin and signs of their activity. As areas of dense heather were approached observers clapped occasionally to encourage sitting birds to flush. Systematic searches were made of grassy knolls and potential lookout rocks etc for prey remains and plucked feathers, signs that indicate the presence of breeding birds. Merlin have a tendency to return to the same general area to nest, therefore if a search of a historical location was not initially successful, the search was extended to include all apparently suitable habitat within 1 km of that site. Although some pairs are demonstrative and easily located, others can be secretive and hard to detect. Therefore, all traditional territories where birds were not located on the first visit (4 of 10) were visited again a few days later, and the vicinity around old nest locations watched from vantage points for two hours. Once occupancy of a territory was established no further searches of that territory were made. Where nests were found, follow-up visits were made in June by the NR staff or Pete Ellis to determine breeding success and ring chicks. All visits to merlin sites were carried out under licence from SNH. Number of territories and breeding successThe recent historical data (provided by Pete Ellis and Dave Okill) indicated that the Viking Site has about eleven merlin territories, though not all these are occupied each year. The exact number of territories is a matter of interpretation and judgement particularly in the northern part of the site, as pairs switch between often widely spaced nest sites within a territory between years. Occupancy of many territories was not proven in some years either because birds were absent or the territory was not surveyed. Six territories had proven breeding attempts in 2005 and a seventh held at least one adult bird though breeding was not proven. This represents about one third of the current Shetland population and 0.5% of the UK population (Pennington et al 2004, Rebecca and Bainbridge 1998). One area that was interpreted previously as a single territory (G) was occupied by two pairs in 2005 (territories G & L). There were no signs of occupancy at four territories (this assumes that the territories referred to as J and K are different). The distribution of occupied territories showed a distinct northern bias with five of the six territories with confirmed breeding being in the northern half of the Viking Site. Four of the six nests found were in old hooded crow nests. The average clutch size was 4.6 (n=5). Four pairs reared 18 young, all these were ringed by Pete Ellis. This level of breeding success was quite high compared to recent years (Pete Ellis personal communication). A juvenile merlin from territory G was killed by vehicle collision soon after fledging. Of about 30 prey remains found, the vast majority were of meadow pipit (c 70%) and wheatear (c 25 %). The only other prey items found were one skylark and one snipe. The total of
6-7 occupied territories on or within 2 km the site was in line with the
expectations of local ornithologists Pete Ellis and Dave Okill. Merlin
numbers have declined and stabilized in Shetland over the past 20 years
(Pennington et al. 2004). The reasons for this are unclear; habitat
quality and prey density appear to be broadly satisfactory, though concerns
have been expressed over whether heather quality (height) may have declined
due to overgrazing. Information gaps and recommendations for further studySome pairs of breeding merlin can be remarkably secretive and hard to detect and this raises the question of whether any pairs might have been overlooked in 2005. On and within 1 km of the site it is unlikely that any pairs were missed because of the high level of dedicated search effort for merlins in May and the relatively restricted areas with suitably nesting habitat. Furthermore, five experienced ornithologists were employed full time on fieldwork on the site throughout the breeding season and reported no records or signs of merlins away from the territories where occupancy was proven. However, it is possible that the single bird seen at territory B was actually one of a pair. None of the traditional sites were in the outer (1-2 km) buffer zone, nor were any particularly suitable looking areas of nesting habitat noted there. This meant that search effort for merlins in the outer buffer zone was not as great as in the inner buffer zone or core area. In addition, the outer buffer area was not covered by the generic bird surveys. Therefore, it is possible that pairs went undetected in the outer buffer area. It is recommended that the outer buffer be searched more thoroughly in future years to be certain there are no additional territories there. It is also recommended that the traditional territories that were not proven in 2005 are surveyed again as the non-occupancy of these territories may be only temporary. Red-throated Diver Study
BackgroundThe red-throated diver Gavia stellata is a goose-sized seabird with somewhat unusual breeding habits. Unlike other seabirds, it chooses to nest inland on small freshwater bodies, typically small lochs and lochans on open moorland (Bundy 1976, Cramp and Simmons 1978). The breeding lochans usually support a single pair only. They lay a clutch of two eggs in a simple nest built on the shore or an islet close to the water’s edge. The chicks are fed small whole fish, such as sand eels and gadoids that adults catch in the sea nearby (typically within 5 km) and then fly back with one at a time to the breeding loch. Chicks grow slowly and fledge at 6-8 weeks old, when they leave the breeding grounds and move to the sea (Okill and Wanless 1990). Red-throated divers are mainly present on Shetland from April to September, after this they migrate approximately south to coastal areas all around mainland Britain and the near continent (Wernham et al 2002). There is general agreement that several aspects of their biology make red-throated divers particularly vulnerable to collision risk with wind turbines. Firstly, their habit of nesting inland and feeding at sea results in many flights each day back and forth across the moorland breeding grounds. Feeding flights continue in all weather conditions including fog and certainly in twilight, if not darkness, when they are likely to be much less aware of obstacles (divers have not been reported to have any special adaptations for vision in low light intensity). Secondly, although they fly strongly and quickly, divers have a high wing loading and are amongst the least agile of flying birds; they can only change direction or gain height relatively slowly. Thirdly, divers are long-lived birds with low annual reproductive rates and do not breed until several years old (Okill 1992, Okill 1994). This means that a considerable part of the population consists of non-breeding immature birds and young adults. Many of these non-breeding birds also spend the summer on the breeding grounds where they loaf around on the larger lochs and fly around visiting and bothering the established breeding birds. This behaviour is presumably part of the process through which these birds ultimately establish their own breeding territory. This behaviour appears to be particularly prevalent on Shetland (say compared to Lewis) where diver population densities are high. The relevance here though is that it means that non-breeders account for a high proportion of all diver flights over the breeding grounds. As non-breeders are the future breeding population it is important that data are collected on their use of the site also so that this aspect can be properly considered in any impact assessment.
AimsThe primary aims of the 2005 red-throated diver fieldwork were to determine the number of pairs breeding on or close to (within 2 km) the Viking Site, monitor breeding success and map flight lines of breeding and non-breeding birds. For the flight lines, the aim was to map at least 15 incoming (from the sea) feeding flights for each pair that had chicks; this is the minimum data requirement suggested by SNH in their guidelines for wind farm EIAs (SNH 2005). In practice, as flights of all types were logged during watches, this also meant that normally at least as many out going flights and flights by visiting non-breeders were logged and mapped. Secondary aims were to collect data on loch use and flight behaviour of non-breeding birds, loch attendance and diet. Data on these were obtained opportunistically whilst undertaking work to meet the primary aims. MethodsThe Viking Site and surrounding 2-km buffer zone has over 120 lochs and lochans, most of which are situated on upland peatlands well away from roads. Monitoring this many lochs is a major task and was undertaken by both teams. The Bird Survey team visited all lochs within the core area twice during their general bird survey work covering the whole site, once during May to mid June and again between mid June and late July. This team was also able to make additional checks at some lochs when walking to their VP sites. The Diver Team made additional checks between mid June and early August at occupied lochs in the core area to monitor breeding success and also checked the lochs, in most cases at least twice, in the 2km-buffer zone around the site. Visit frequency was kept to the minimum required to obtain basic information on breeding success, in particular to determine if eggs hatched and if chicks survived to fledge. On most visits observers attempted to disturb nesting birds as little as possible by viewing sites from afar. A final check was made when chicks were between 4 and 6 weeks old. Two small islands, Linga and Papa Little, lying in the buffer zone on the western edge of the site were not surveyed. Data on diver flight lines were collected by undertaking prolonged focal point watches of all breeding lochs with chicks except some that hatched late in the season (August) or were close to the edge of the 2-km buffer boundary. Some additional diver flight-line data were collected by the Bird Survey Team during their general VP watches and, after the Diver Team had departed, at a few lochs with late-season broods. In total, flight-line watches were made at 21 lochs with chicks. Most focal point watches were made from a vantage point overlooking the breeding loch, typically from 200-400 m away. Vantage points were chosen on the basis of having an adequate view of the loch and surrounding areas, not being so close as to disturb the birds (judged from their behaviour) yet not so far that flights, particularly incoming flights, were difficult to detect. In most case the topography (mainly rather flat-topped hills) meant there were few if any choices in viewpoint location. At some lochs it was impossible to find a vantage point that met the above requirements, in which case one was chosen where although the loch itself was not visible flights in and out generally were. Watches were made at all times of day, but especially in the five-hour periods after dawn and before dusk. For about half the watches the Diver Team erected a small tent at the VP to act as a hide, provide shelter and to sleep in over night; this proved to very effective and greatly facilitated early morning sessions. Watches typically lasted between three and eight hours, though a few were shorter. They were conducted in all weather conditions though some had to be abandoned due to fog. Several watches were made at each loch, mostly in the period when chicks were between 10 and 30 days old. During a watch observers recorded all diver flights observed to pass within 1 km of the breeding loch and mapped the flight lines onto 1:25,000 scale OS maps. For each flight the following data were recorded: the number of birds, type of flight, height above ground (every 15 seconds) and whether the birds vocalised or carried food. At the time of recording, six flight types were recognised: inbound flights by residents, outbound flights by residents, other flights by residents, flights by intruders (non-breeders), flights by neighbouring pairs and flights by birds of unknown status. During the focal point watches of breeding lochs non-breeding birds were usually easily distinguishable from the resident and any neighbouring breeding birds by differences in their behaviour. In particular, breeding birds were those that tended eggs and chicks and were typically present on their territory most of the time, whereas non-breeding birds only visited breeding territories for short periods and evoked territorial behaviour from the residents. Away from a breeding territory, unless carrying food, breeding birds could not be distinguished with certainty from non-breeding birds. All visits to red-throated diver
breeding sites were carried out under licence from SNH.
Number of divers and diver lochsA total of 40 occupied red-throated diver-breeding territories were found in 2005 on the Viking Site (29 pairs) and within the 2km-buffer (11 pairs, including three pairs just beyond the 2km buffer) Table 1. Breeding was proven for 37 (93%) of these pairs. Although the other three pairs were present at their breeding lochs on most occasions they were visited, there was no evidence that they attempted to breed; no nests were found or chicks seen though it is possible that they had attempts that failed early in the season. All confirmed nesting sites were either a lochan or a small loch (see Definitions). However, for many breeding pairs the territory comprised more than just the nesting loch and included other water bodies in the same lochan /pool complex or a small loch nearby. Eleven lochs/lochans that were considered suitable for breeding and in seven cases are known to have held breeding pairs in the past three years, were apparently not occupied 2005 (Table 2). All of these were likely visited by non-breeders and will likely hold breeding birds in the future. In addition to the 40 territory-holding pairs there were many non-breeding birds present in 2005. These spent most of their time on the few relatively large lochs that were close to concentrations of breeding pairs. It was not unusual for 5-10 bird to be present on these lochs. Smaller numbers, usually singles or pairs, were often seen at the other large lochs, though it was usually not clear if these were non-breeders or off-duty birds from nearby breeding lochans. Either way, it was clear that all the larger lochs on the Viking Site were used to a greater or lesser extent by divers. As non-breeders are not attached to a single site it was not possible to determine exactly how many there were. Nevertheless, the average number seen on the larger lochs where non-breeders spend much of their time suggests that there were approximately 20 non-breeders summering on the Viking Site. Therefore, the total number of summering sub-adults and adults (non-breeders and breeders) was probably slightly over 100 birds. Over forty small lochs, lochans and lochan complexes were visited that appeared to be largely unsuitable for nesting divers, either because they were too small, too shallow, too weedy or the surrounding topography rose too steeply (Table 2). This preliminary assessment was undertaken by DBJ based on twelve years experience of diver nest site selection. Some sites were considered borderline cases where breeding in the future cannot be ruled out. The number of
breeding pairs on the Viking Site represents about 9% of the Shetland
breeding population and 5% of the UK population (Gibbons et al
1997). DistributionThe Viking Site is considered as five sections, two sections in the northern half are referred to as the Upper-west and Upper-east sections, and three sections in the southern half are referred to as the Lower-west, Lower-east and Lower-south sections (Map 1). The breeding lochs and other lochs used by divers were not evenly distributed across the site (Tables 1 and 2). The distribution pattern largely reflects the distribution of suitable lochs and lochans. The five sections had between five and 13 pairs each. The greatest concentrations in the north occurred in the eastern part of the Upper-east section, in particular around Muckla Moor and Logie Hill, and as a band running along the upland ground in the eastern part of the Upper-west section. In the southern half of the site the main concentrations were in the eastern half of the Lower-east section, the north-west half of the Lower-west section and a small cluster of sites in the south of the Lower-South section. The larger
lochs on the site, i.e. those used by non-breeding birds and off-duty
breeding adults were spread across the site, especially in the central
western valleys of the southern half of the site (Table 2). Those that were
most favoured by divers were the lochs closest to the main concentrations of
breeding lochs, in particular Gossa Water, Smerla Water, Loch of Gonfirth,
Laxo Water and Marrofield Water. The other relatively large lochs namely,
Lamba Water, Petta Water, Loch of Voe, Sand Water (Petta Dale), Mill Loch (Westersta)
and Maa Water, appeared to be of less importance but all were regularly used
by small numbers of divers. Timing of breedingFor 34 of the 38 nesting attempts monitored the timing of egg laying could be calculated to within a few days accuracy, in most cases by assuming a 30-day period between the first egg and hatching (Figure 1). The distribution of first-clutch laying dates shows a bimodal distribution. Nearly two thirds of pairs (62%) laid between mid May and mid June and two pairs (6%) laid a little before mid May. The remaining ten pairs (29%) started clutches between mid June and mid July, with a peak in late June/early July. The single replacement clutch was laid in late June. The causes of the bimodal distribution are not known. It is possible that some of the second peak comprised replacement clutches of pairs whose first nest was not detected. However, this does not explain most cases since the lochs had been thoroughly checked for nests several times during the main part of the laying season. Indeed, in many cases the adults did not appear to take up residence until well into June at these late-laying breeding lochs. One possible explanation for the pattern observed is that most of the late-laying pairs were inexperienced young birds, some probably breeding for the first time. It is also likely that the relative lack of clutches laid in the mid June was caused by some environmental effect, such as weather or food supply, that resulted in delayed laying for pairs that would otherwise have laid earlier. Breeding successBreeding success by red-throated divers in 2005 on the Viking Site was high relative to average breeding in Shetland and Orkney over the past two decades (Okill and Wanless 1990, Booth 1999, Okill 2005). Of the 37 proven breeding pairs, 26 (70%) successfully reared 37 young. All of the 27 first clutches examined were of two eggs. The only replacement clutch found had a single egg. Many of the first clutches either lost an egg or one egg failed to hatch. Of the 37 proven pairs, eight (22%) failed at the egg stage and three (8%) at the chick stage; two when chicks were very small (they may not even have left the nest) and one when the brood was about four weeks old (Table 2). The causes of these failures were generally unknown. The 4-week-old brood that failed was predated (D Okill personal communication). The most likely causes of egg loss were predation by birds (e.g. great skua, great black-backed gull) or accidental dislodgement into the water when an incubating adult leaves the nest. One full-term egg failed to hatch, due to shell damage possibly caused by a predation attempt, and a nest with a single full-term egg was abandoned, perhaps because the egg was infertile. The hatching rate of the clutches laid after mid-June was somewhat lower than for those laid earlier, 27% (n=11) vs 19% (n=26) respectively. Chick survival through the fledging period was high. Of 34 young that were first seen when <5 days old, 31 survived to fledge (i.e. at least 5 weeks old when last seen). The survival of late season broods (i.e. those from clutches laid after mid June) was lower than for early season broods; 63% (n=8) of the late season broods reared at least one chick compared with 100% (n=20) of early season broods. Furthermore, the final mean brood size of successful broods was slightly lower for the late season broods; 1.20 chicks (n=5) vs 1.43 chicks (n=21) respectively. One pair that successfully fledged two chicks in mid July apparently lost a juvenile shortly after it fledged but before it was independent. This pair was seen to fly out from the breeding loch with the juveniles to sea and return to roost on the breeding loch, but on the last two occasions they were seen there was only one juvenile with them. This observation indicates that newly fledged juveniles are vulnerable and that this period of the life cycle merits greater study (See ‘Information gaps and recommendations for further study’). One adult diver
was found floating dead at its breeding site (lochan AZ) on 20 July. The
bird carried a ring and had originally been caught as a breeding male on the
same lochan in 2004 by Dave Okill. At the time of finding the bird appeared
to have been dead 2-3 weeks and, although slightly decomposed, there was no
evidence of major physical trauma or broken bones. Also at the time of
finding, there was a pair of divers on the loch. Although it cannot be
proven, it is strongly suspected that the dead male was mortality wounded in
a territorial fight with another male. Comparison with 2004Red-throated diver numbers and breeding success were also monitored on the Viking site in 2004 (Stirling and Hulka 2004). The numbers of proven breeding pairs in 2004 (39 pairs) was slightly greater than in 2005 (38 pairs). [Note. The 2005 total included at least one pair from a site not checked in 2004]. Data are not available on the number of additional territory holding pairs present in 2004 but for which breeding was not proven. Breeding success on the Viking Site in 2005 was about 60% higher than in 2004. In 2004 only 16 (41%) of the 39 proven pairs reared young. The causes of the lower breeding success on the Viking Site in 2004 are unknown but appear to be mainly connected to higher losses at the egg stage. Indeed, the mean size of successful broods in 2004 (1.50 chicks) was slightly greater than in 2005 (1.42 chicks). The between year difference in breeding success on the Viking Site is broadly in line with wider results obtained from a large sample of sites monitored across Shetland, though in the wider sample mortality of well grown chicks was more common in 2004 (Okill 2005, D Okill personal communication) Six of the
sites with proven breeding in 2004 were classed as unoccupied in 2005. This
represents 15% of the 2004 proven breeding sites. Between 2004 and 2005 a
total of 46 occupied territories were identified on the Viking Site, with
attempted breeding proven at 44 of these. Flight dataFlight watches totalling just over 400 hours were carried out at 21 diver breeding lochs that had chicks and at two strategic locations identified during survey work as having particularly heavy diver flight traffic. These watches resulted in 872 logged and mapped diver flights. Approximately two thirds of the flights were by breeding birds, comprising 293 inbound flights (from the sea), 277 outbound flights (to the sea) and 20 flights of other types. The remaining third of flights were by birds classified as non-breeders. There was a strong pattern in the frequency of flights recorded through the day (Figure 2). Flight activity of all types peaked around dawn and dusk and was at a minimum around noon and the early afternoon, when it was approximately three times less than the peak. For seventeen of the 21 breeding lochs targeted the amount of flight data collected are sufficient to meet the SNH guidelines for EIA. The other four have about half the data required. It is beyond the scope of this report to present full details and analyses of these flight data, (this is being done as a separate exercise). However, some general comments are made below on the patterns observed. It should also be noted that the 2005 watches revealed a number of aspects of red-throated diver flight behaviour that are not generally appreciated and are likely to influence the collection of further data and the conclusions of the eventual EIA.
Key aspects of diver flight activity patterns on Shetland are: 1) Approximately 50% of flight activity (total flying time) observed on the breeding grounds was by non-breeding birds. Although some of their flights were to and from the sea (to feed), most of those observed were birds visiting (flying over or circling around and at times alighting on) the breeding sites of established breeding pairs or flying to/from the larger lochs where non-breeders gather. The flights of non-breeders were much less predictable than those of breeding birds and were often very protracted, covered large areas and involved several birds. 2) The vast majority of flights of the resident pairs investigated were restricted to the areas between the breeding lochan and nearby coasts and the immediate vicinity (within approximately 400 m) of the breeding loch itself. Many of the inbound and outbound flights of breeding divers were not direct but follow circuitous routes (i.e. arcs rather than straight lines). It appeared that birds usually selected the most energetically economical flight path. To achieve this they often chose routes that minimised the need to gain altitude, for example to fly over high ground, by following valleys and flying through gaps between hills. Much of the Viking site has a complex three-dimensional geography, therefore the optimum outbound route was not necessarily the same as the optimum inbound route. At some sites, there was also evidence that birds were selecting the safest route with respect to the risk of kleptoparasitism by skuas. Flying over water (sea or large lochs) appeared to provide some safety from skuas (as divers retreat to the water and dive) and there was evidence that some pairs appeared to maximise the time inbound flights with fish were over water, even if this meant a taking a slightly longer route. Birds that nest at moderate altitude (>150 m) but close to the sea (<2 km) did not make direct flights from the nearest sea to the nesting loch. This was presumably because red-throated divers are able to gain height only slowly (high wing loading) and therefore, it takes birds several kilometres of flying to reach the height of the nesting loch. Typically, the inbound flights of these pairs (about 25% of all pairs on the Viking site) consisted of flying a series of large circles over the sea and adjacent low ground of about a kilometre in diameter in order to gain height. Only when sufficient altitude was obtained (typically after several minutes of circling) did the bird make a beeline for its nesting lochan. 3) The breeding territory of at least a quarter of the pairs included an exclusive loafing area on the nearest freshwater loch or marine voe, normally within one kilometre of the nesting lochan where the adult birds spent much of their time. In three cases the adults moved their chicks overland from the nesting lochan to the loafing loch (but never the sea) when about two-three weeks old. The flights of the pairs with these loafing areas showed a more complex pattern because their activity had two foci. The flights were between various combinations of breeding site, loafing area and marine feeding area. Unlike other areas studied in detail red-throated diver flights in Shetland are highly complex. For example, Stirling and Hulka showed that breeding red-throated divers in Lewis made more or less direct flights to and from the nearest area of sea (Lewis Wind Power ES 2005). This does not appear to be the case on Shetland, presumably because the coastline is highly indented and therefore breeding lochs are often equidistant from two or more areas of sea. The low predominantly topographical relief of Lewis also makes it less likely that divers would deviate from direct flight lines in order to optimise energy efficiency (see below). Another important distinction between diver behaviour on Lewis and Shetland is the higher proportion of activity by non-breeding birds. This is perhaps unsurprising in view of the large diver population present in Shetland and consequent competition for suitable breeding sites and other resources.
DietFish prey
brought in by divers was identified where possible and in many cases
photographed. In total 212 fish were seen and about half these
photographed. These data currently await analysis and are only considered
here briefly. The majority of fish identified appeared to be small gadoids,
in particular silock (saithe) Pollachius virens . About 15% of items
appeared to be greater sand eel Hyperoplus lanceolatus and
about 15% lesser sand eel Ammodytes marinus. Information gaps and recommendations for further studyThe Viking Site is clearly of great importance for red-throated divers. Therefore, it is important that the EIA is based on adequate data and considers all possible aspects of the bird’s life cycle and ecology that may be affected. Although the information collected in 2005 goes along way towards satisfying the data requirements there are a number of issues that require further study. Areas of work requiring further study have been identified and it is hoped these will proceed in 2006. The proposed work programme includes additional monitoring on the distribution and abundance of divers and further collection of flight behaviour data. The 2005 work highlighted the importance of the site to non-breeding divers. Therefore, the recommended work programme includes dedicated studies on the flight behaviour and site use of non-breeding birds.
AcknowledgementsWe would like to thank Mark Chapman, Steve Minton and Dave Hall, (the Bird Survey Team) for their help with collecting field data and Mike Madders, Andrew Thorpe, Barbara Cooke and Stephanie Provan for their wider behind the scenes support and guidance. Dave Okill and Pete Ellis, the resident diver and merlin enthusiasts, are thanked for their expert advice, providing data, and their interest in the study. ReferencesBooth C.J. 1999. Breeding success of Red-throated Divers on Orkney Mainland 1973-1998. Scottish Birds 20, 94-97. Bundy, G. 1976. Breeding biology of the Red-throated Diver. Bird Study 23, 249-256 Cramp, S, and Simmons, K.E.L. (eds) 1977. Handbook of birds of the Western Palearctic. Vol I. Oxford University Press. Ellis, P.M. and Okill, D.J. 1990. Breeding Ecology of Merlin Falco coloumbaris in Shetland. Bird Study 37, 101-111. Gibbons, D.W., Bainbridge, I.P., Mudge G.P., Tharme, A.P. and Ellis, P.M. 1997. The status and distribution of the Red-throated Diver Gavia stellata in Britain in 1994. Bird Study 44, 194-205. Jackson, D.B. 2005 Environmental correlates of lake occupancy and chick survival of Black-throated Divers Gavia arctica in Scotland, Bird Study 52, 225-236. Lewis Wind Power 2005 Environmental Statement. LWP. Okill, J.D. and Wanless, S. 1990. Breeding success and chick growth of Red-throated Divers Gavia stellata in Shetland 1979-88. Ringing & Migration 11, 65-72. Okill, J.D. 1992. Natal dispersal and breeding site fidelity of Red-throated Divers Gavia stellata in Shetland. Ringing & Migration 13, 57-58. Okill, J.D. 1994. Ringing recoveries of Red-throated Divers Gavia stellata in Britain and Ireland. Ringing & Migration 15, 107-118. Okill, J.D. 2005. Report to S.O.T.E.A.G. on Red-throated Divers in Shetland 2005. Unpublished report. 2004. Birds of Shetland. Christopher Helm. Rebecca, G.W., and Bainbridge, I.P. 1998. The breeding status of the Merlin Falco columbarius in Britain in 1993-94. Bird Study 45, 172-187 SNH 2005. Survey methods for use in assessment of the impacts of proposed onshore wind farms on bird communities. Version 6.5. Scottish Natural Heritage. Stirling, J and Hulka, S. 2004. Summary of breeding diver research central Mainland Shetland 2004. Unpublished Natural Research technical report. Wernham, C.V., Toms, M.P., Marchant, J.H., Clark, J.A., Siriwardena, G.M. and Baillie, S.R. (eds) 2002. The Migration Atlas: movements of the birds of Britain and Ireland. T & A.D. Poyser, London. Map 1: Map showing the boundary of the proposed Viking windfarm site, one and two kilometre buffer zones and regional sections referred to in report. Figure 1: The frequency distribution of red-throated diver laying dates on the Viking Site in 2005. The weeks shown are nominal weeks based on periods of seven or eight days. Based on 34 clutches for which laying date could be estimated to an accuracy of a few days.
* includes 3 pairs just outside buffer zone.
Table2. Number of lochs on the Viking Site that were not occupied breeding sites in 2005 but were regularly visited by red-throated divers.
3 Part or all these lochs were used as a ‘satellite loch’ by one or two (1 case) breeding pairs.
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