The seabird monitoring programme surveys seabird populations across Shetland and has been carried out since 1978. Factors such as the innate mobile nature of birds, migration patterns and their vulnerability to floating oil slicks and illegal discharge from passing tankers have all been taken into account when designing the programme.
Surveying of cliff-nesting seabird populations – Northern Fulmar, European Shag, Black-legged Kittiwake, Common Guillemot, Razorbill and Black Guillemot (also known as Tystie),
Pre-breeding season population counts of black guillemots
Monitoring of breeding red throated divers
Population counts of moulting common eiders
Winter counts of diving seabirds and seaducks.
Beached bird surveys – monthly surveys of beached seabirds on selected beaches. If any oil is present on the birds, then it is analysed to identify the origin and possible source of pollution.
Seabird ringing scheme.
Supplementary monitoring – contribution to surveys carried out on the Island of Foula relevant to SOTEAG’s needs.
Monitoring of cliff-nesting seabirds:
Northern Fulmar Fulmarus glacialis. The numbers of Fulmars at the four monitored sties in 2024 had decreased when compared to numbers in 2023 with more than 5% change at all sites.
The mean count of apparently occupied nest sites (AOS) had decreased particularly severely at Burravoe, Troswick Ness and Sumburgh Head in 2024 by 16.1%, 12% and 13.8% respectively. Such simultaneous decreases across all sites in one year is extremely unusual. The size of the monitored population at Burravoe, Esha Ness and Sumburgh Head has gradually increased during the last 15 years, although at Troswick numbers have slightly decreased during this period.
The reason for the decrease in 2024 is not known, but one possibility is that a relatively high proportion of adults were in low reproductive condition following three consecutive winters of severe weather and sea conditions, and although still alive they simply did not attempt to breed this year. It is also possible that some breeders may have died during the 2023-24 winter, since the Beached Birds Survey indicated unusually high mortality during the preceding autumn and winter period, although it is unknown whether corpses were Shetland breeders.
In comparison with other monitored cliff-nesting species, numbers of Fulmars are high at all the monitored sites (mean AOS >180 at all sites).
Since 1976, there has been a broadly similar pattern of long-term population change across the four sites, comprising population increase until 2000, decrease from 2000 to 2006, and population increase thereafter.
Breeding success in 2024 was similar to 2023 at all monitored sites. The mean breeding success across the four sites in 2024 (0.32 chicks fledged per AOS) was the same as in 2023. This was slightly unexpected, given that population size had decreased at all four sites in 2024 and the possibility that adults were not in good breeding condition in 2024.
European Shag Phalacrocorax aristotelis.
In 2024, the number of Shag nests counted from land at No Ness was 75, a decrease of 54% since 2023, and the third-lowest count on record. The number of nests at Sumburgh Head was 67 in 2024, the lowest count on record and a decrease of 49% since 2023. It is possible that breeders may have died during the 2023-24 winter, since the Beached Bir Surveys indicated unusually high Shag mortality in January, February and March, although it is not certain whether corpses found were Shetland breeders.
It was an extremely poor nesting year for Shags, with very low numbers of nests recorded at the monitored sites and transect. Currently, breeding numbers at Sumburgh are 75% lower than in the late 1980s. This is likely due to decreases in adult survival and recruitment of new breeders, possibly caused by decreases in food availability around Shetland and decreases in winter survival due to increasingly severe winter weather and sea conditions.
In 2024, Shag breeding success was low at both monitoring sites. At Burravoe, 0.94 chicks fledged per incubated nest, down by 30.4% compared with 2023 and the fourth-lowest measurement on record. At Sumburgh Head, 0.35 chicks fledged per incubated nest, down by 68.2% since 2023 and the third-lowest measurement on record.
Breeding success has fluctuated over the years, but the 2024 data are notably low compared with most previous years and indicate a widespread poor breeding season.
Black-legged Kittiwake Rissa tridactyla.
The population count at Compass Head was 21 nests. This was the lowest total on record, a decrease of 22.2% compared with the 2023 count, and a continuation of the general pattern at this site since 2016 of an extremely low and slowly decreasing number of nests.
In 2024, mean breeding success had decreased since 2023 at all five monitored sites and was distinctively low at four sites. At Burravoe in Yell, breeding success had decreased by comparatively little, down by 18.9%, from 0.74 chicks fledged per incubated nest in 2023 to 0.60 in 2024. However, breeding success had decreased by 75.3% at Esha Ness (0.81 in 2023 cf. 0.20 in 2024), by 95.6% at Sumburgh Head (1.13 in 2023 cf. 0.05 in 2024), and by 100% at Ramna Geo and Dalsetter (0.56 and 0.62 in 2023, respectively, but zero at both sites in 2024).
At Sumburgh Head, Ravens were frequently present and observed harassing Razorbills in 2024, and although egg/chick predation was never observed, Ravens may have been a contributing factor to the high Kittiwake breeding failure in 2024, as occurred in 2023.
Since 1986, mean kittiwake breeding success has been highly variable, with no obvious trend.
Common Guillemot Uria aalge.
In 2024, the mean number of Guillemots had decreased by more than 10% at all four monitoring sites in comparison with 2023. Simultaneous decrease across all sites in one year is very unusual.
Reasons for the decreases in 2024 are uncertain, but it is possible that following three consecutive winters of severe weather and sea conditions, a relatively high proportion of adults were in low reproductive condition and simply did not breed in 2024.
At Sumburgh Head adult Guillemot attendance was sporadic but generally remained quite high whereas breeding attempts were unusually few, implying that at this plot, winter mortality of adults had not been severe, but environmental conditions were still such as to prohibit many pairs from laying eggs.
Numbers of Guillemots at Burravoe were extremely low this year, with a mean count of just five individuals (nine in 2023, three in 2022 and seven in 2021). Here, but not elsewhere, a pair of Ravens nest on the cliff very close to the Guillemot breeding ledges and it is possible that the unusual drop in Guillemot numbers at this site since 2019 (and potential extinction of the colony) could be due to sustained harassment of adults and predation of eggs by Ravens, which becomes significant once the colony drops below a certain size threshold.
Since 1976, mean population counts from the monitoring sites have been variable, with no consistent patterns shown across all sites.
At Esha Ness there has been a near-continuous decrease in numbers since 1976, although with a slight general increase through the last ten years, also apparent at Troswick Ness and Sumburgh Head.
Guillemot breeding success at the Sumburgh Head monitoring plot in 2024 was 0.10 chicks fledged per apparently incubating pair, a decrease of 79.2% since 2023 and the fourth-lowest measurement on record.
Breeding attempts were unusually few this year, at just 60 laying pairs, compared with most previous years when there have been well over one hundred.
In total, just six chicks fledged, with the first departing on the evening of 9th July and the last on the evening of 20th July.
Razorbill Alca torda.
Mean population counts in 2024 had decreased in comparison with 2023 at Burravoe, Troswick Ness and Sumburgh Head but increased, although only very slightly, at Esha Ness.
The decrease at Troswick Ness was unusual since it was at odds to the recent general pattern of population increase there since 2013.
There are now comparatively few Razorbills at Burravoe and Esha Ness compared with peak numbers in the late 1990s.
When the number of birds at a monitoring site becomes very low (e.g. less than 10 individuals, such as at Burravoe) then changes in population size calculated as a percentage are comparatively large and appear far larger than changes at sites where there are higher numbers of birds.
The reason(s) for the recent decline at Burravoe site are unclear but a possible factor is harassment of adults and predation of eggs by a Raven pair that nest nearby on the cliff.
Razorbill breeding success in 2024 was zero, which has only previously occurred once, in 2011, the year that monitoring began at this site. Eggs were seen on only two occasions in 2024, and the total number of apparently breeding adults was just 31, a decrease of 36.5% from 2023 and the lowest total on record.
No chicks were seen this year. The most likely explanation for this is perhaps that, after three consecutive winters of severe weather and sea conditions, many adults were in poor breeding condition and simply took a year off from breeding in 2024.
Additional monitoring
Two coastline transects for Shag population monitoring were added to the monitoring programme.
Two coastline transects including more nests for Kittiwake population monitoring were added to the monitoring programme.
Pre-breeding season population counts of Black Guillemots Cepphus grille revealed: Monitoring period: Each site is counted once or twice per year, prior to 9am between late March to late April.
Population counts of Black Guillemots in breeding plumage are made during the birds’ displaying and courtship period in early spring, prior to egg-laying.
Counts are made in dry conditions with little or no sea swell and little or no wind (at most an offshore wind of Force 4), when displaying individuals gather together close inshore on the sea and can be readily observed.
In 2024, the weather in late March and throughout April was extremely unsettled and mostly very windy, with few days suitable for surveying Black Guillemots. Nonetheless, monitoring was completed at the majority of sites. However, unfortunately, the strong wind and sea conditions prevented monitoring at Sullom Voe Oil Terminal, also at Levaneap, Burra, Mu Ness and Mousa this year.
Counts this year were variable in comparison with 2023, with an increase in numbers recorded at Hillswick Ness (156 to 157), Wats Ness (57 to 72) and Virkie-Boddam (81 to 84), but a decrease at Ronas Voe (75 to 70), Kirkabister (141 to 125), Aithsetter (89 to 84) and Kettla Ness (86 to 74).
Monitoring of breeding Red-throated Divers Gavia stellata.
Red-throated Diver population monitoring at the Northmavine survey area (72 lochs) is done every year except for those when the full Shetland-wide Eider census is carried out by SOTEAG, which currently happens once every 3 to 4 years. Monitoring at the Tingon survey area (29 lochs) is done every year by staff of the Shetland Amenity Trust.
Red-throated Diver pairs usually hold a territory comprising one entire loch. Sometimes on larger lochs, however, there may be more than one pair, each with a territory of part of the loch. Each pair makes a shallow nest scrape (or several scrapes) at the water’s edge, can lay a clutch of one or two eggs, and can fledge up to two chicks in a season.
Population monitoring at each study area comprises walking the perimeter of every loch and counting all proven breeding pairs (incubating adult, egg(s) and/or chick(s) seen), all additional active territories where breeding not proven (fresh empty nest scrape(s) found but no other evidence of breeding), and all lochs where adults were seen but no evidence of breeding was found.
In 2024, the first round of monitoring visits to the Northmavine survey area was completed between 3rd and 17th June, when 11 proven breeding attempts were found. The second round of visits was completed between the 4th and the 18th of July, when a further 12 proven breeding attempts were found. In total, 23 breeding attempts were confirmed for the year (25 in 2023), an additional seven active territories where breeding was not proven were confirmed (five in 2023), giving a total of 30 apparently occupied territories, plus there were 21 lochs where adults were seen but no evidence of breeding was found.
From 1981 to 1994, the number of breeding pairs decreased at Northmavine, but thereafter numbers slowly increased, back up to a relatively stable population in the last four years of 22-25 pairs.
Population counts of moulting Common Eiders Somateria mollissima
Annual monitoring of 3(+) large survey areas, started in 1982. Each survey area is monitored once per year. Monitoring period: 15th July to 10th September. Every three to four years, the monitoring is expanded to cover 30 large survey areas – the full Shetland-wide census.
Monitoring of the north Yell Sound, Sullom Voe and south Yell Sound survey areas is the core aim each year.
The Eider moult period is in early autumn, from mid-July to early September, during which adults congregate into flocks, become temporarily flightless whilst completing their primary and secondary moult, and are generally less mobile, more visible, and therefore easier to count than in most other months.
The most recent full census was in 2023 and the next is scheduled for 2026. In 2024, the normal monitoring of the north Yell Sound, Sullom Voe and south Yell Sound survey areas was fully completed.
Weather and sea conditions during the monitoring period were unusually rough though, and only a few days widely spaced apart were suitable for boat surveys.
In total, 96 birds were counted in south Yell Sound, 250 birds in Sullom Voe, and 10 birds in north Yell Sound.
The grand total of 356 birds in these survey areas in 2024 was 30.4% higher than in 2023 (273 birds), but close to the most recent 10-year average of 383 birds.
Winter counts of seaduck and diving seabirds
Annual monitoring at 6 large survey areas (sea state permitting), started in 1979, 1979, 1992, 2000, 2000 and 2007. The aim is to count each area at least once per year.
The winter of 2023/24 was extremely unsettled in Shetland, with storms, high winds and heavy sea swells virtually continuous throughout. There were just three days when conditions were calm enough for monitoring, meaning that only three surveys were completed this year.
Survey 1: The Sullom Voe and southern Yell Sound area
Sullom Voe In Sullom Voe, the 2023/24 count of Red-throated Divers was the highest on record (12) and the count of Great Northern Divers was the second ever highest (10). Numbers of Cormorants (10), Shags (166), Long-tailed Ducks (4), Red-breasted Mergansers (106) and Black Guillemots (101) using the Voe all remained comparatively stable.
However, the number of Slavonian Grebes (22) was low, having decreased by 35.3% since the 2022/23 count, and Goldeneye numbers were the lowest on record (9) since the 1982/83 count, having decreased by 52.6% since 2022/23. Counts of Common (2) and Velvet (2) Scoters, Common Guillemots (7) and Razorbills (12) were low as normal.
Yell Sound In Yell Sound survey area, the count of Shags was 1139, the second-highest count on record and an increase of 4.4% on the 2022/23 survey. Numbers of great Northern Divers (7), Slavonian Grebes (3), Goldeneyes (14), Red-breasted Mergansers (9) and Black Guillemots (391) this year generally remained at normal levels.
However, in comparison with the 2022/23 survey, there were notable decreases in the numbers of Red-throated Divers (6) – 70.0% and Cormorants (72) – 43.8%. The count of Long-tailed Ducks (23) remained comparatively low this year, while numbers of Eiders (1), Common Scoters (5) and Velvet Scoters (1) remained in single figures as normal. The number of Guillemots seen this year was again quite high (39), the highest count since 2003/04.
Survey 2: Whiteness Voe to Skelda Voe area, in the west mainland
During the 2023/24 survey, record high numbers of Red-throated Divers (23), Shags (205) and Common Scoters (15) were seen. The counts of Great Northern Divers (42), Guillemots (63) and Razorbills (18) were also high, all being the second highest on record. The Slavonian Grebe (27) count remained low, although had increased by 12.5% since last year, while numbers of Eiders (107) and Black Guillemots (99) remained at normal levels. The numbers of Long-tailed Ducks (8), Goldeneyes (3) and Red-breasted Mergansers (63) were all low this year, and had declined since the 22/23 count by 38.5%, 82.4% and 26.8%, respectively. Black-throated Diver is encountered very rarely, with only singles recorded in the winters of 2013/14, 2019/20, 2020/1, 2022/23 and this year.
Survey 3: Pool of Virkie to Bay of Quendale area, in the south mainland
The 2023/24 counts of Long-tailed Ducks (255) and Common Scoters (14) were the highest on record, the count of Goldeneyes (51) was the second highest ever recorded, while the count of Great Northern Divers (52) was the third highest on record. Numbers of Shags (114), Eiders (24) and Black Guillemots (43) were generally at normal levels in comparison with the last ten years. The number of Guillemots (25) seen during the 2023/24 survey was relatively high (25 this year, 15 last year), which was similar to the other areas surveyed in 2023/24. The counts of Cormorants (3), Red-throated Divers (2) and Red-breasted Mergansers (4) were low, and all decreased in comparison with the 2022/23 survey. However, counts of these three species have always been relatively low.
Beached Bird Surveys
72 beaches have been monitored monthly around Shetland since 1978. The Shetland-wide monthly beached birds survey is carried out by two SOTEAG staff and 16 local volunteers. All seabird corpses down to a single wing with all primary feathers present are identified to species (or the lowest possible taxon), aged externally as far as possible and examined for oil contamination. Samples of oiled plumage or oil residues on beaches are collected for analysis with the aim being to determine the type, origin and source of the oil.
In 2024, the beached bird survey was carried out monthly. To avoid double-counting, corpses were marked with a spot of paint when they had been counted. The Shetland Beached survey remains unique, as the only county-wide survey in the UK to systematically measure monthly seabird corpse levels around the coast. The number of seabird corpses found per km surveyed has generally decreased across the years (1979-2024), although with a slight, but fluctuating, general upturn in numbers since 2017.
The number of oiled seabird corpses found per km surveyed has generally decreased across the years (1979-2024), remaining comparatively very low (<0.1 oiled corpses/km) since 2008. The 2024 survey results continued the general recent pattern of very low oiled seabird occurrence per year. The exceptionally high number of seabird corpses found in 2022 was because of the HPAIV pandemic. This led to relatively extreme mortality in Eiders, Gannets, Great Skuas and Common Guillemots. In 2024, there were 2.839 corpses/km and 0.031 oiled corpses/km.
In 2024, 1003 seabird corpses were found, and of these, 11 were found to have oil on them (2.839 corpses/km and 0.031 oiled corpses/km). There were no incidences of oiled seabirds between September and December.
