European Shag Phalacrocorax aristotelis

Status;  International importance;  Population estimates;  Distribution;  Annual abundance/ productivity; Phenology/diet/survival

 

<<<Back to Species list

 

Description

European Shag vignette

The following has been adapted from original text by Sarah Wanless and Mike P. Harris in Seabird Populations of Britain and Ireland (with permission from A&C Black, London).

 

The European shag is endemic to the northeast Atlantic and the Mediterranean. An inshore species that is almost never observed out of sight of land, it takes a wide range of small fish that it catches on or near the seabed over both sandy and rocky substrates. The species nests on offshore islands or on cliffs and colonies range in size from a few to several thousand pairs.


Conservation status

 

European shag is currently identified as a conservation priority in the following:

EC Birds Directive - migratory species

Red listed in Birds of Conservation Concern 4 (2015 update)

(further information on Conservation Designations for UK Taxa)

Amber listed in  Birds of Conservation Concern in Ireland 2014-2019 (2014 update)


International importance

 

UK Population % Biogeographic Population % World Population
26,600 AON* 38.3 (ssp. aristotelis) 34.1

 

*AON = Apparently Occupied Nests

The UK population figure (rounded to the nearest hundred) was derived from data in Mitchell, P.I., Newton, S.F., Ratcliffe, N. and Dunn, T.E. (eds.) 2004. Seabird Populations of Britain and Ireland. Poyser, London. This was also the source of figures for the Biogeographic and World populations.

 


UK population estimates and change 1969-2002 (census data)

 

European shags build large conspicuous nests which superficially appear straightforward to count, but there are, however, several major problems associated with a widespread survey of this species: i) detection of nests - these can be hidden among boulders and in caves, or can easily be overlooked when present at low densities among other species in large cliff nesting colonies; ii) a prolonged and variable breeding season - in Britain eggs have been laid in every month of the year except September and October; iii) occasional years when many adults do not breed - however, such events tend to be localised and did not appear to be a problem during census years. Seabird 2000 aimed to overcome the second problem by conducting a single count in the period of maximum nest occupancy (1 May-25 June). Previous censuses probably suffered from similar problems, so these will all have led to the underestimation of the absolute size of the breeding population.

 

 

Operation Seafarer

(1969-70)

Seabird Colony Register

(1985-88)

Seabird 2000

(1998-2002)

UK Population estimate (AON*) 29,956 36,276 26,565
% change since previous census N/a +21 -27

 

*AON = Apparently Occupied Nests

For census results for individual countries and Ireland, the Channel Islands and the Isle of Man see under relevant sections below.

 


Distribution/abundance

 

The Seabird 2000 census provides the most comprehensive recent assessment of the distribution and abundance of breeding seabirds. Numbers of European shag found in different regions, and a map showing the location and size of colonies, is provided in the Seabird 2000 European shag results page (PDF, 2.0 mb).

 

An interactive map is available on the NBN Gateway, where you can filter to display only the Seabird 2000 data.  For more recent, but less comprehensive, coverage view the distribution on the NBN with all available contributing datasets.

 

The locations sampled during the annual Seabird Monitoring Programme provide some information on distribution and are accessible via the Seabird Monitoring Programme online database.

 


Annual abundance and productivity by geographical area

 

With reference to the regional accounts below please note the following.

Breeding abundance: graphs of abundance index with 95% CLs are only shown for a region where the trend produced has been deemed accurate (see methods of analysis). Where a trend was thought to be inaccurate, graphs of abundance at major colonies in a region may be shown instead, particularly if such colonies hold greater than 10% of the regional population, are monitored frequently and may thus help illustrate regional population fluctuations outwith national censuses. Occasionally, too few data have been collected regionally to produce either of these.

Productivity: graphs of productivity are only shown if analysis of breeding success data produced a significant result for regional and/or year effects (again see methods of analysis). If results were not significant, then a regional mean productivity value is given. However, on some occasions, too few data are available from which to provide a meaningful average. Furthermore, for 11 species where the quality of monitoring data available was considered high, population viability analysis was undertaken at the UK level and the results of this are also reported.   

 



 

Breeding abundance

 

UK shag abundance trend

Figure 1: Trend in UK abundance index (solid line) of European shag 1986-2015 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

The UK shag population increased slightly from 1969-70 (30,000 pairs) to 1985-88 (36,000 pairs), possibly as a result of better coverage of previously inaccessible coastlines through the use of inflatable boats, increased legal protection (e.g. under the Wildlife and Countryside Act 1981) and reduced persecution. However, numbers had fallen by 27% by the time of Seabird 2000. Data collected by the SMP show an initial steep rise in the index up to 1987 probably due to many adults choosing not to breed in 1986 (e.g. on Canna (Lochaber) and the Isle of May (North-east Fife)); thus, counts at many colonies were low that year. The trend also shows how breeding abundance is heavily affected by the incidence of mass mortality events – or ‘wrecks’  – which occur during prolonged periods of onshore gales, when species such as shag find it hard to forage. Severe events, such as those in the winter of 1993/19941 (also preceded by a year when many pairs skipped breeding) and 2004/2005, considerably affected populations on the east coast of the UK. Subsequent recovery from the 1994 'wreck' was slow with breeding numbers not fully restored before the next 'wreck' occurred in 2005. There appears to have been no recovery before the 'wreck' during winter 2012/3, when over 650 corpses were recovered between Orkney and Suffolk. It is therefore not surprising that index values for 2013 and 2014 (approximately 50% below the baseline) were the lowest yet recorded. In 2015, the index increased slightly to 45% below the baseline. Measures of the return rates of shags to the Isle of May show the impact of such 'wrecks' (see Figure 4 below). Predictions of increased storminess due to climate change suggest such mortality events may become more frequent and have significant effects on population size2.  Predation of eggs and chicks by invasive mammalian predators is known to have localised effects on shag population size.

 

Table 1 shows how numbers have changed at some of the most important UK colonies (those in the SPA network) in the period since they were surveyed for Seabird 2000. Numbers have fallen in most SPAs, except on Mingulay and Berneray (both Western Isles), with some particularly large declines recorded on Foula and Fair Isle (both Shetland) and the Farne Islands (Northumberland).  

 

Area SPA Name Seabird 2000 Count (Year) Change (%) % per annum
Shetland Foula 2,300 2000 48+ 2015 -98 -22.7
Shetland Fair Isle 663 2001 204 2013 -69 -9.4
East Coast Buchan Ness to Collieston Coast 408 2001 363 2013 -11 -1.0
East Coast Firth of Forth Islands 1,289 2001 839 2015 -35 -3.0
East Coast St Abb's Head NNR 233 2000 133 2015 -43 -3.7
East Coast Farne Islands 1,287 2000 720 2015 -64 -6.6
West Coast Canna and Sanday 740 1999 336 2015 -55 -4.8
West Coast Mingulay and Berneray 281 1998 294 2014 +5

+0.3

West Coast Isles of Scilly 1,092 1999 998 2015 -9 -0.6

Table 1: Recent counts of the number of European shag (AON) recorded in SPAs in the UK compared to the number recorded in them during Seabird 2000. The percentage that each colony has changed by, and the per annum change, is also provided. (Note: data for St Abb's Head relates to only part of the SPA).

 

Productivity

 

UK shag productivity trend

Figure 2: Trend in UK productivity (no. of chicks fledged per pair) of European shag 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Shags are heavily reliant on sandeels3 and food shortages are likely to have contributed to periodic low productivity and population decline. Sandeel abundance is thought to have declined as a result of increases in sea surface temperature that changed the abundance and composition of plankton and reduced sandeel recruitment. Shag productivity in 2009, 2010, 2014 and 2015, which were comparatively better years for sandeels than many during the last decade, was high, in common with other sandeel specialists. In addition to the steep increase in 2014 (1.50 chick fledged per pair) productivity increased slightly further in 2015  to 1.55

 

Analysis of the SMP dataset found mean breeding success of European shags at monitored colonies was 1.21 chicks per nest per year between 1986 and 2008, and was relatively stable throughout the period4. The quality of the dataset meant a decline in breeding success of 5% or greater could be detected with confidence. Were existing levels of breeding success to be maintained, population viability analysis (using available life history information such as population size, clutch size, age at first breeding and survival rates of different age classes) suggests the national population may decline by a modest 9% over 25 years. Breeding success would need to fall to 1.10 for a decline in abundance of 25% over 25 years to occur. For the UK population to decline by over 50% in 25 years success would need to fall to 0.90.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 27,077 31,560 21,487
% change since previous census    N/a +17 -32

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

Scotland shag abundance trend

Figure 1: Trend in abundance index (solid line) of European shag in Scotland, 1986-2015 with 95% confidence limits (dotted lines). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Long-term census data show an increase of 17% between Operation Seafarer and the Seabird Colony Register after which numbers fell by nearly one-third by Seabird 2000. Scotland holds about 80% of the European shags found nesting in the UK so it is not surprising that the post-1986 trends shown for both are similar. Common features are the initial steep rise to 1987 due to many adults choosing not to breed in 1986 (e.g. on Canna and the Isle of May) and the dips in abundance in 1994 and 2005 due to 'wrecks' along the east coast in the preceding winters, from which numbers have been slow to recover (1994-2004) or appear to have not recovered at all (2005 to present). Yet another 'wreck' during the 2012-13 winter, centred on Aberdeenshire and the Firth of Forth, further lowered numbers breeding in east coast colonies. Fourteen east coast colonies between Orkney and Berwickshire counted in 2013 held 1,872 AON compared to 3,433 AON during 2004, when abundance was at its highest during the last 20 years (Figure 1), representing a fall of 45% over the last decade. As a result of these 'wrecks', non-breeding events and general declines, the abundance index value for Scotland in 2014 was almost 68% lower than the peak in 1987.

By 2015, population sizes at three major colonies in Shetland (Fair, Foula and Sumburgh Head) declined by c. -87% since Seabird 2000. A study of European shag population and breeding dynamics at these colonies suggested that the majority of the decline could be accounted for by high mortality associated with prolonged gales in the late winter of 2003, 2011 and 20145. European shag has recently been added to the UK Red list6 because of severe population decline, and continued (indeed enhanced) monitoring and ringing of the species is to be encouraged.

 

Productivity

 

Scotland shag productivity trend

Figure 2: Trend in productivity (no. of chicks fledged per pair) of European shag in Scotland, 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Data from Scottish shag colonies have generally shown productivity to be high; usually, more than one chick is fledged per pair each year, although productivity is fluctuating and considerably lower than in England and Wales. In addition to the steep increase in 2014 (1.53 chick fledged per pair) productivity increased slightly further in 2015 to 1.55. At Fair Isle, Sumburgh Head and Foula (all Shetland), breeding was markedly early in 2014 and 2015, the percentage of incubation, percentage of hatching and overall breeding success were all high (1.41 chicks fledged per pair) and well above the 1988-2014 mean of 1.12 chicks fledged per pair7. Productivity can be affected by a variety of factors. For example, detailed monitoring on the Isle of May in 1990 revealed that over-winter survival (from 1989) was low, the breeding season was very late, many shags did not breed or even construct nests and that only two-thirds of nesters laid eggs. In addition, adults also appeared to have trouble finding sufficient quality food during the season with few sandeels of any size available. Various combinations of these factors, together with inclement summer weather, have reduced overall productivity in other years.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 2,111 3,491 3,863
% change since previous census    N/a +65 +11

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

England shag abundance trend

Figure 1: Trend in abundance index (solid line) of European shag in England, 1986-2015 with 95% confidence limits (dotted lines; drawing of upper limit restricted to preserve detail in the abundance index). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

National census data indicate a large increase in numbers had occurred between 1969-70 and 1985-88 but numbers were then stable between 1985-88 and 1998-2002. Detail provided by data collection for the SMP show the affects of winter 'wrecks' in the abundance indices for England; 'wrecks' occurred in the winter preceding the 1994, 2005 and in 2013 breeding seasons. Prior to the first two events indices had been rising but were stable in the years immediately before the most recent 'wreck'. However, recovery from the 1994 crash was slow and still incomplete before the next decrease in 2005. For example, on the Farne Islands (Northumberland), the largest and most frequently monitored colony in England, numbers fell from 1,948 to 771 AON between 1993 and 1994. Partial recovery then saw numbers increase to 1,678 AON in 2003 before another sharp decline to 937 AON by 2005. There had been no further recovery up to 2012 when 965 AON were recorded which then fell steeply to 582 AON in 2013. Numbers partially recovered in 2014 to 795 AON but fell to 720 AON in 2015. The net result is that the national index value for 2015 lies 48% below that for 1986.

Productivity

 

England shag productivity trend

Figure 2: Trend in productivity (no. of chicks fledged per pair) of European shag in England, 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

Productivity of European shags at English colonies had generally been falling since recording began but reached a low point in 2004 when an average of only 0.41 chicks were fledged per pair. Since 2004, productivity has increased in most years; 2010 and 2014 were particularly successful breeding seasons. Almost all data has been collected on the Farne Islands with few contributions from other colonies. European shags breeding at this colony have been very successful with mean breeding success measuring 1.12 chicks fledged per nest between 1987-2015. Presumably, this is due to good feeding conditions in most years although details provided are scant. Low levels of breeding success on the Farne Islands in 2004 (the effects of which are evident in the graph above) was in response to poor weather and low food availability during the chick rearing stage, combined with predation of nest contents by gulls early in the season, after which few replacement clutches were laid. In 2015, European shags again had a successful breeding season on the Farne Islands fledging 1.67 chicks per nest.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 550 785 914
% change since previous census    N/a +43 +16

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

Wales shag abundance trend

Figure 1: Trend in abundance index (solid line) of European shag in Wales, 1986-2015 with 95% confidence limits (dotted lines; drawing of upper limit restricted to preserve detail in the abundance index). Based on SMP data; view the methods of analysis (PDF 158 kb).

 

European shag populations in Wales, in common with those in England and Scotland, increased between the first two national censuses. In contrast to that seen in England, the abundance index of the Welsh population underwent a steady decline until the early 1990s, as was the case in Scotland. Thereafter, the trend was generally upward until 2001, coinciding with increased numbers recorded by Seabird 2000. Since the last census the trend has been relatively stable until 2012. The 'wrecks' preceding the 1994 and 2005 breeding seasons, which reduced breeding numbers and are common to both the Scottish and English indices, do not feature in the Welsh index. However, the 'wreck' during the 2012-2013 winter may have had an effect on numbers, as the index value has now fallen to its lowest point since 1993.

 

Productivity

 

Wales shag productivity trend

Figure 2: Trend in productivity (no. of chicks fledged per pair) of European shag in Wales, 1986-2015. Based on SMP data; view the methods of analysis (PDF 158 kb).

 

The productivity of European shags breeding in Wales has consistently been high; there are few years when productivity falls below 1.30 chicks fledged per AON. Several colonies have been monitored over the years although the most frequently monitored colonies are Middleholm (Dyfed), Bardsey and Puffin Island (both Gwynedd). In 2015, breeding success on Bardsey (1.28 chicks fledged per nest) and at Puffin Island (1.41 chicks fledged per nest) were well below average at both colonies (Bardsey, 2.07 between 1987-2015 and Puffin Island 1.66 between 2010-2015); no data was available for Middleholm.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 218 440 301
% change since previous census    N/a +102 -32

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

Census data show numbers of European shags in Northern Ireland doubled between Operation Seafarer and the Seabird Colony Register but then declined by one-third by Seabird 2000. Very few sites holding European shags in Northern Ireland are monitored regularly so it is difficult to assess the species' current status. Counts at Gun's Island (Co. Down), near Strangford Lough, the most frequently monitored colony, ranged from eight to 22 between the Seabird Colony Register and Seabird 2000. However, recently this colony has been deserted with none breeding from 2007 onward. On Rathlin Island (Co. Antrim), only 46 and 42 pairs were found in 2007 and 2015, respectively, compared with over 100 during the Seabird Colony Register. In 2014, 43 AON were counted at four colonies (two of which are newly established) which held only 18 AON during Seabird 2000. Based on this limited and conflicting information, it is difficult to predict whether the national population has increased or declined since the previous census.    

 

Productivity

 

Data submitted to the SMP on the productivity of European shags in Northern Ireland are sparse; thus, no meaningful average productivity value can be given.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 2,783 4,676 3,426
% change since previous census    N/a +68 -36

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

Over the long-term, national census data collected during Seabird 2000 indicated numbers had increased since Operation Seafarer in 1969-70, although fewer were found compared to that recorded during the Seabird Colony Register in 1985-88. Few colonies are monitored frequently, or in any one year; however, a census at 26 European shag colonies was carried out in 20158. At these a total of 1,268 AON were recorded compared to 1,661 AON during Seabird 2000, a decrease of 24%. The majority of AON (37%) were recorded on Lambay Island (Co. Dublin), one of the largest colonies in the country where the population has declined by 58% since Seabird 2000 (1,122 AON). 

 

Productivity

 

Few data on the productivity of European shags at colonies in the Republic of Ireland have been collected as part of the SMP. On average, productivity measured 1.17 chicks per pair per year between 1996 and 2011 (the most recent year when breeding success data have been collected).

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 3,001 5,116 3,727
% change since previous census    N/a +70 -37

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

Over the longer-term, national census data collected during Seabird 2000 indicated numbers had increased since Operation Seafarer in 1969-70, although a decline was recorded since the Seabird Colony Register in 1985-88. Very few sites holding European shags throughout Ireland are monitored regularly. Low numbers of shags breed in Northern Ireland compared to the Republic of Ireland, which holds c. 90% of the All Ireland population.  Based on limited and conflicting information from Northern Ireland, it is difficult to predict whether the national population has increased or declined since the previous census. However, a census at 26 European shag colonies was carried out in 20158. At these a total of 1,268 AON were recorded compared to 1,661 AON during Seabird 2000, a decrease of 24%. Numbers in Ireland seem to be falling but without more extensive data from Northern Ireland and some of the remaining colonies in the Republic of Ireland it is difficult to assess the current status.

 

Productivity

 

No data on the productivity of European shags have been collected at colonies in Northern Ireland and few data have been submitted from colonies in the Republic of Ireland as part of the SMP. On average, monitored colonies fledged 1.20 chicks per pair per year between 1996 and 2014.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 567 575 912
% change since previous census    N/a +1 +59

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

Shag Calf of Man abundance 2015

Figure 1: Abundance of European shag on the Calf of Man (Isle of Man), 1986-2015.

 

Long-term national census data show little change in numbers of European shags on the Isle of Man between 1969-70 and 1985-88 but an increase of 60% thereafter by the time of Seabird 2000 during 1998-2002. Few colonies are monitored regularly as part of the SMP. The most frequently counted and largest colony is on the Calf of Man where close to 40% of the European shag population was recorded during the SCR (225 AON in 1986). Numbers increased up to 1994 when 337 AON were counted. During Seabird 2000, 218 AON were recorded although counts were only done from land (land and sea during 1986 and 1994). Other land-based counts immediately after Seabird 2000 suggested numbers were stable at this colony; 220 AON counted in 2000, 208 AON in 2001 and 205 AON in 2002. Similar numbers to this were recorded between 2011 and 2013 but either side of this period counts in 2005, 2010 and 2014 are far lower in the range of 115-135 AON (with the lowest count recorded since 1986, 58 AON in 2015). Clearly, such fluctuations mask any apparent trend at the colony. No data have been collected from any other Manx colonies since Seabird 2000 so the species overall status in the Isle of Man is unknown.    

 

Productivity

 

Data submitted to the SMP on the productivity of European shags on the Isle of Man are sparse; thus, no meaningful average productivity value can be given.

 

 

Population estimates and change 1969-2002 (census data)

 

 

Operation Seafarer    

(1969-70)

Seabird Colony Register    

(1985-88)

Seabird 2000    

(1998-2002)

Population estimate (AON*) 570 1,443 1,403
% change since previous census    N/a +153 -3

 

* AON = Apparently Occupied Nests

 

Breeding Abundance

 

National census data show that the numbers of European shags in the Channel Islands recorded during 1998-2002 had more than doubled since 1969-70 from 570 pairs to 1,403, although there had been almost no change over the latter half of this period since the Seabird Colony Register in 1985-88. Few colonies have been visited since Seabird 2000. On Alderney and Burhou, which held 207 AON during Seabird 2000 (c.15% of the population), surveys in 2014 recorded only 65 AON representing a decline close to 70%. Two further colonies, on Little Burhou and Coque Lihou, held totals of 112 and 102 AON in 2013 and 2014 respectively compared to 88 AON in 1987. With no data available from other colonies since Seabird 2000 the current status of European shag on the Channel Islands is somewhat unknown.

 

Productivity

 

Data submitted to the SMP on the productivity of European shags in the Channel Islands are sparse; thus, no meaningful average productivity value can be given.

 

 


UK phenology, diet, survival rates

 

Phenology

 

No systematic data on phenology (timing of life-cycle events) have been collected as part of the SMP.

 

Diet

 

Figure 3 shows a marked decline in the proportion by weight of sandeels in the diet of European shag chicks on the Isle of May, especially between 2004 and 2009, although during most of this period this had no clear negative effect on productivity; in fact, 2008 and 2009 were two of the most productive breeding seasons on record. In these years, although sandeels still comprised the greatest single constituent, the remainder comprised gadids (members of the cod family, including whiting and rockling), clupeids (such as sprat), goby, butterfish and wrasse. In 2013, the most frequent prey (by occurrence in a regurgitate) in the 13 regurgitations collected on the Isle of May was butterfish, which occurred in 53.8% of samples. Sandeels constituted only 28.2% of the biomass, the joint lowest percentage on record. In contrast, sandeels were the most frequent prey recorded (by occurrence in a regurgitate) in 2014 and 2015, occurring in 85.7% and 94.2% of samples respectively (from 21and 54 regurgitates) and contributed 80.3% and 86.6% of the biomass, a proportion that was close to and above the long-term average, respectively.  The remains of other items found were Gadidae (both 4 samples), Cottidae (both 3), crustacea (2 and 4), flatfish (2 and 3), Clupeid (1), dragonet (1) and Gobiidae (1) were found in 2014 and Callionymidae (7), Pholidae (3), Cottidae (3), Labridae (3), mollusc (2) and polychaete worm (1) were found in 2015. Similar to 2104, productivity in 2015 was well above average at 1.91 chicks per nest built (1986-2014 average = 1.09; CI=0.90-1.28) 8

 

Shag Isle of Man sandeel diet

Figure 3: Percentage of sandeels (by weight) in the diet of young shags at the Isle of May (North-east Fife), 1987-2015.

 

Return rates and survival rates

Important notes on interpretation:

Estimation of European shag adult return rate is currently only undertaken at one site within the Seabird Monitoring Programme - the Isle of May (North-east Fife). Return rates are based on sightings of individually colour-ringed birds and are calculated as the proportion of marked birds present in year one that is seen in the following year. Because not every adult alive is seen each year, return rates for 2015 presented need to be treated as a minimum estimate of survival of birds seen alive in 2014. In contrast, survival estimates (not currently undertaken for shag) do take into account birds that are not seen one year but which re-appear in following years.

 

No clear trend in return rate is evident from the Isle of May data. In 2015, the return rate was estimated to be 87.9%, above the long-term average at the colony (78.1%, 95% CI = 71.2-85.6). A notable feature of European shag biology is their susceptibility to die from periods of low food availability caused by unusually prolonged periods of strong onshore winds, which makes foraging difficult. Such 'wrecks' occurred around eastern Britain during the winters of 1994/95, 2004/05 and again in 2012/13 and are reflected in the very low return rates in the following breeding seasons (Figure 4). The 'wreck' during winter 2012/13 resulted in over 650 corpses being recovered from beaches between Orkney and Suffolk. It is therefore not surprising that the return rate for the Isle of May fell in 2013, although not all corpses recovered would have been from this colony. Measurements of shag return rates on the Isle of May show the impact of such 'wrecks' (Figure 4) 8.

 

Predictions of increased storminess due to climate change suggest such mortality events may become more frequent and have important impacts on population size3. In addition, European shags often take a year off breeding, which is manifested in return rates because, although they have not died, they are not seen at the breeding colony in the second year of a pair.

 

shag Isle of May return rate 2015

Figure 4: Annual return rate of European shag breeding on the Isle of May (North-east Fife), 1987-2015.

 


References

1 Harris, M.P. and Wanless, S. 1996. Differential responses of guillemot Uria aalge and shag Phalacrocorax aristotelis to a late winter wreck. Bird Study 43: 220-230.

2 Frederiksen, M., Daunt, F., Harris, M.P. and Wanless, S. 2008. The demographic impact of extreme events: stochastic weather drives survival and population dynamics in a long-lived seabird. Journal of Animal Ecology 77 (5): 1020-1029.

3 Harris, M.P. and Wanless, S. 1991. The importance of the lesser sandeel Ammodytes marinus in the diet of the shag Phalacrocorax aristotelis. Ornis Scand. 22: 375-82.

4 Cook, A.S.C.P. and Robinson, R.A. 2010. How representative is the current monitoring of breeding success in the UK? BTO Research Report No. 573, BTO, Thetford.

5 Heubeck, M., Mellor, R.M., Gear, S and Miles, W.T.S. 2015. Population and breeding dynamics of European Shags Phalacrocorax aristotelis at three major colonies in Shetland, 2001-15. Seabird 28: 55-77.

6 Eaton, M.A., Aebischer, N.J., Brown, A.F., Hearn, R.D., Lock, L., Musgrove, A.J., Noble, D.G., Stroud, D.A. & Gregory, R.D. 2015. Birds of Conservation Concern 4: the population status of birds in the United Kingdom, Channel Islands and Isle of Man. British Birds, 108, 708–746. Available online: http://britishbirds.co.uk/wp-content/uploads/2014/07/BoCC4.pdf [Accessed 29 September 2016]

7 Heubeck, M. and Mellor, M. 2015. SOTEAG Ornithological Monitoring Programme: 2015 Progress Report. Aberdeen Institute of Coastal Science and Management, University of Aberdeen.

8 Newton, S., Lewis, L. and Trewby, M. 2015. Results of a Breeding Survey of Important

Cliff‐Nesting Seabird Colonies in Ireland 2015. National Parks and Wildlife Service, Ireland.

9 Newell, M., Harris, M.P., Gunn, C.M., Burthe, S., Wanless, S. and Daunt, F.  2014. Isle of May seabird studies in 2014. JNCC Report No. 475j, Peterborough.

8 Newell, M., Harris, M.P., Gunn, C.M., Burthe, S., Wanless, S. and Daunt, F. 2016. Isle of May seabird studies in 2015. JNCC Report No. 475k, Peterborough.

 


Partners

Data have been provided to the SMP by the generous contributions of its partners, other organisations and volunteers throughout Britain and Ireland. Partners to the SMP are: BirdWatch Ireland; The British Trust for Ornithology; Centre for Ecology and Hydrology; Natural Resources Wales; Department of Environment, Food and Agriculture (Isle of Man); Department of Environment, Heritage and Local Government (Republic of Ireland); States of Guernsey Government; JNCC; Manx Birdlife; Manx National Heritage; The National Trust; National Trust for Scotland; Natural England; Northern Ireland Environment Agency; The Royal Society for the Protection of Birds; Scottish Natural Heritage; Seabird Group; Shetland Oil Terminal Environmental Advisory Group; Scottish Wildlife Trust.  More about the SMP partners >>

 
European shag image appears courtesy of Ian Rendall ©, is subject to international copyright law and may not be reproduced in any form whatsoever.