Citation: Babcock and Booth (2020) Habitat: Vegetation Management. Tern Conservation Best Practice. Produced for “Improving the conservation prospects of the priority species roseate tern throughout its range in the UK and Ireland” LIFE14 NAT/UK/000394’
Last updated: October 2020
This is a live document we update regularly. If you have comments and suggestions, please email Chantal.Macleod-Nolan@rspb.org.uk
Last updated: October 2020
This is a live document we update regularly. If you have comments and suggestions, please email Chantal.Macleod-Nolan@rspb.org.uk
| Babcock and Booth (2020) Habitat: Vegetation Management. Tern Conservation Best Practice. |
Key Messages
- Tern colony management often involves holding a habitat in early succession, e.g. managing developing vegetation or creating new areas of shingle.
- A variety of vegetation management techniques are available and the most appropriate will depend on species present and resources available.
- Annual vegetation management work will often be required and should be factored into management planning including attempts to establish new colony sites.
- More drastic management options are likely to cost more but also have a longer-lasting effect.
Information
Most tern species prefer to nest in open areas with sparse vegetation cover. There is however, some variation in tern species preferences; while most prefer an open aspect, roseate terns often select sites with shelter either from vegetation or burrows (Ramos & del Nevo 1995). Preferences can also be affected by local circumstances such as the presence of predators (Saliva & Burger 1989).
The presence of seabirds can over time enrich soils and promote the development of coarse grasses and taller ruderal vegetation, making the site unsuitable for these same species. In a natural system, if a colony site became unsuitable breeding terns would relocate to nearby sites with suitable open habitat. However, habitat loss means viable alternatives are less likely to be available.
Vegetation management at seabird colonies presents challenges in terms of timing, as tern nesting coincides with the growing season, meaning that mowing, grazing and most herbicides cannot be applied during plant growth when they are most effective. Additionally, the inaccessibility and ecological sensitivity of seabird colonies often prevent the use of vegetation management techniques that require heavy equipment, bulky materials, or dangerous chemicals.
If a UK site is designated (e.g. SSSI or ASSI) consent for habitat management must be sought from the relevant statutory body, ideally as part of a management plan. This must also be agreed between the landowner and land manager (if different) and other stakeholders.
Most tern species prefer to nest in open areas with sparse vegetation cover. There is however, some variation in tern species preferences; while most prefer an open aspect, roseate terns often select sites with shelter either from vegetation or burrows (Ramos & del Nevo 1995). Preferences can also be affected by local circumstances such as the presence of predators (Saliva & Burger 1989).
The presence of seabirds can over time enrich soils and promote the development of coarse grasses and taller ruderal vegetation, making the site unsuitable for these same species. In a natural system, if a colony site became unsuitable breeding terns would relocate to nearby sites with suitable open habitat. However, habitat loss means viable alternatives are less likely to be available.
Vegetation management at seabird colonies presents challenges in terms of timing, as tern nesting coincides with the growing season, meaning that mowing, grazing and most herbicides cannot be applied during plant growth when they are most effective. Additionally, the inaccessibility and ecological sensitivity of seabird colonies often prevent the use of vegetation management techniques that require heavy equipment, bulky materials, or dangerous chemicals.
If a UK site is designated (e.g. SSSI or ASSI) consent for habitat management must be sought from the relevant statutory body, ideally as part of a management plan. This must also be agreed between the landowner and land manager (if different) and other stakeholders.
Published Research
Lamb (2015) reviewed vegetation management in North Atlantic tern colonies and identified 12 techniques falling into three general categories:
Lamb et. al. (2014) tested two vegetation management techniques, burning and synthetic weed barrier treatments, to assess their ability to create and maintain common tern Sterna hirundo nesting habitat on two nearshore islands in Maine, USA. Treatments were applied to overgrown habitat before nesting started. Regrowth and common tern nesting and fledging success were monitored in treated plots and controls. Burned areas did not remain open for the full nesting season, but regrew shortly after egg-laying, creating unsuitable habitat and leading to near-complete nest failure in these plots. Weed barrier treatments created conditions similar to pre-existing common tern nesting habitat throughout the season, and with near identical fledging success.
Two studies from Canada (Morris et al. 1980; Morris et al. 1992) found that the fledging success of common terns on a man-made breakwater in Lake Ontario, Canada was significantly higher when vegetation was manually removed from the site and gull numbers were controlled, but that a suite of interventions including vegetation management and habitat creation could not prevent a decline in breeding numbers at a mainland site which was subject to chronic disturbance, particularly from dogs, from the nearby urban centre.
Saliva & Burger (1989) conducted a controlled trial over two seasons at two sooty tern Onychoprion fuscatus breeding colonies in grasslands in the Culebra Archipelago of eastern Puerto Rico. They found that terns did not nest in experimental plots that had been entirely cleared of vegetation but did nest in three plots in which partial removal of vegetation had resulted in 25%, 50% or 75% vegetation cover. Only at the end of the laying period, and after some vegetation had colonized part of the plots, did a few terns nest at the edge of one completely cleared experimental plot. This contrasted with most sooty terns in the tropics, which usually nest on areas of bare ground with no vegetation cover. The researchers considered that in this study area the protection from predation offered by a degree of vegetation cover outweighed the preference for open areas.
Lamb (2015) reviewed vegetation management in North Atlantic tern colonies and identified 12 techniques falling into three general categories:
- Pre-emergent techniques (used prior to the period of plant growth), including soil removal, solarization (a clear barrier over soil to increase temperature beneath to a level intended to kill buried seeds), application of salt or seawater and mulching. Used at 34% of colonies surveyed, and 16% found applications were effective for the full season. These methods often have longer lasting effects, but higher costs and in some cases require work with heavy equipment.
- Post-emergent techniques (used during the period of plant growth), including manual removal, herbicide treatment, burning and grazing, were used in 83% of the colonies surveyed and were effective for the full season in 46% of applications. Regarded by colony managers as cost effective and implemented with readily available equipment, but usually requiring annual repetition and usually additional spot treatment in season.
- Habitat construction, including weed barriers, gravel substrate application, dumping dredged materials (silt or sand) and re-planting was used at 63% of colonies surveyed, with 70% effectiveness. The combination of weed barriers and new substrate provided immediate habitat with long duration. Simply spreading gravel over previously unsuitable habitat without creating a barrier beneath was the least effective of the techniques.
Lamb et. al. (2014) tested two vegetation management techniques, burning and synthetic weed barrier treatments, to assess their ability to create and maintain common tern Sterna hirundo nesting habitat on two nearshore islands in Maine, USA. Treatments were applied to overgrown habitat before nesting started. Regrowth and common tern nesting and fledging success were monitored in treated plots and controls. Burned areas did not remain open for the full nesting season, but regrew shortly after egg-laying, creating unsuitable habitat and leading to near-complete nest failure in these plots. Weed barrier treatments created conditions similar to pre-existing common tern nesting habitat throughout the season, and with near identical fledging success.
Two studies from Canada (Morris et al. 1980; Morris et al. 1992) found that the fledging success of common terns on a man-made breakwater in Lake Ontario, Canada was significantly higher when vegetation was manually removed from the site and gull numbers were controlled, but that a suite of interventions including vegetation management and habitat creation could not prevent a decline in breeding numbers at a mainland site which was subject to chronic disturbance, particularly from dogs, from the nearby urban centre.
Saliva & Burger (1989) conducted a controlled trial over two seasons at two sooty tern Onychoprion fuscatus breeding colonies in grasslands in the Culebra Archipelago of eastern Puerto Rico. They found that terns did not nest in experimental plots that had been entirely cleared of vegetation but did nest in three plots in which partial removal of vegetation had resulted in 25%, 50% or 75% vegetation cover. Only at the end of the laying period, and after some vegetation had colonized part of the plots, did a few terns nest at the edge of one completely cleared experimental plot. This contrasted with most sooty terns in the tropics, which usually nest on areas of bare ground with no vegetation cover. The researchers considered that in this study area the protection from predation offered by a degree of vegetation cover outweighed the preference for open areas.
Case Studies
RSPB Larne Lough, Northern Ireland: combining vegetation control methods
At Blue Circle Island and Swan Island in Larne Lough, Northern Ireland, a new vegetation management programme combining strimming and chemical control has been implemented since autumn 2018. A trial creating plots for Sandwich terns Thalasseus sandvicensis by strimming alone was unsuccessful since the vegetation grew back too quickly. Sandwich terns on Blue Circle Island seem to prefer to nest close to the edge of open areas, next to vegetation, and so cutting scalloped edges creates suitable conditions, with plenty of edge and the open areas not too big. In the autumn of 2019, restoration works on Blue Circle Island included the creation of new common tern habitat with a geotextile membrane to supress vegetation covered with nesting substrate. Any emerging vegetation will be treated with herbicide before the breeding season.
RSPB Larne Lough, Northern Ireland: combining vegetation control methods
At Blue Circle Island and Swan Island in Larne Lough, Northern Ireland, a new vegetation management programme combining strimming and chemical control has been implemented since autumn 2018. A trial creating plots for Sandwich terns Thalasseus sandvicensis by strimming alone was unsuccessful since the vegetation grew back too quickly. Sandwich terns on Blue Circle Island seem to prefer to nest close to the edge of open areas, next to vegetation, and so cutting scalloped edges creates suitable conditions, with plenty of edge and the open areas not too big. In the autumn of 2019, restoration works on Blue Circle Island included the creation of new common tern habitat with a geotextile membrane to supress vegetation covered with nesting substrate. Any emerging vegetation will be treated with herbicide before the breeding season.
Rockabill, Ireland: long-term management of tree mallow
On Rockabill the principal vegetation issue for BirdWatch Ireland is the growth of tree mallow Lavatera arborea. This species is well established on the main island, having originally been introduced to provide cover for terns, and there is usually considerable growth between the wardens leaving in August and coming back in May. On arrival, any nesting areas with soil are covered by dense growth. The wardens aim to remove 95% of this before the terns return in early May. Tree mallow is frost sensitive and will die back in winter if it is cold enough, so firstly any dead material is removed. Dense stands of live mallow are clipped at ground level, using garden loppers, leaving roots and soil intact to prevent erosion (loose soil especially on the sloping west side of the island increases the likelihood of tern eggs rolling away). No herbicide is used but Birdwatch Ireland are considering using pumped seawater on an experimental basis, to try to kill off tree mallow in the pre-nesting season.
On Rockabill the principal vegetation issue for BirdWatch Ireland is the growth of tree mallow Lavatera arborea. This species is well established on the main island, having originally been introduced to provide cover for terns, and there is usually considerable growth between the wardens leaving in August and coming back in May. On arrival, any nesting areas with soil are covered by dense growth. The wardens aim to remove 95% of this before the terns return in early May. Tree mallow is frost sensitive and will die back in winter if it is cold enough, so firstly any dead material is removed. Dense stands of live mallow are clipped at ground level, using garden loppers, leaving roots and soil intact to prevent erosion (loose soil especially on the sloping west side of the island increases the likelihood of tern eggs rolling away). No herbicide is used but Birdwatch Ireland are considering using pumped seawater on an experimental basis, to try to kill off tree mallow in the pre-nesting season.
Figure 1. Tree mallow on Rockabill at the start of the season (Birdwatch Ireland)
Other vegetation, on the island such as Atriplex species and scurvy grass Cochlearia officinalis is thinned or removed in those smaller areas where it occurs. The latter is entirely removed from nesting areas as it tends to hold a lot of moisture and will cause any newly hatched chicks in the vicinity to get wet and chill (usually resulting in death). Hottentot Fig Carpobrotus edulis was successfully eradicated in 2008.
All removed vegetation is placed on rocks below the high tide mark or in designated compost heaps in areas where there is a low occupancy by terns. It should not be left lying in pools used by terns for bathing as rotting vegetation in water pools can encourage the development of botulism bacteria.
The Skerries, Wales: spraying scurvy grass and nettles along slope contours
The Skerries (RSPB) are a group of sparsely vegetated rocky islets, with a total area of about 17 hectares (42 acres) lying 3 kilometres (1.9 mi) off the northwest corner of Anglesey, Wales. They are particularly important for the Arctic terns, numbers of which are nationally important; the colony includes common terns and roseate terns Sterna dougallii breed occasionally in very small numbers. The islands are designated SSSI for seabirds, in particular the terns, which form part of the larger Irish Sea population, and as part of the Ynys Feurig, Cemlyn Bay and The Skerries SPA.
Tree mallow was introduced to a walled lighthouse garden area around the mid-1990s and quickly took over the entire area. By the late 1990s the tree mallow was managed by hand pulling around 50% of the area of the garden, but more recently it has died out naturally. By the 2010s the only vegetation management was hand-pulling to maintain access paths, but the increasing size of the seabird colony continues to add nutrients resulting in thicker vegetation, mainly scurvy grass, which gradually began to swamp tern nests and chicks. In 2017, with SSSI permissions from Natural Resources Wales, the densest areas of scurvy grass and thick patches of nettles Urtica dioica, were sprayed with herbicide early in the season before the terns arrived.
The spraying in the first year successfully restricted the growth of lush vegetation but a slope that was sprayed stayed quite bare for a month and some nests were washed out by runoff in heavy rain; while the lack of cover meant that chicks from nests in the bare area ran into the longer vegetation for cover and so could still become wet and chilled In 2018, the programme of spraying was changed to spray more smaller areas along the contour of the slope and retain some vegetated areas for shelter for chicks. No spraying was done in 2019 or 2020, but it is intended to continue spraying patchily to create a network of open areas.
The Skerries (RSPB) are a group of sparsely vegetated rocky islets, with a total area of about 17 hectares (42 acres) lying 3 kilometres (1.9 mi) off the northwest corner of Anglesey, Wales. They are particularly important for the Arctic terns, numbers of which are nationally important; the colony includes common terns and roseate terns Sterna dougallii breed occasionally in very small numbers. The islands are designated SSSI for seabirds, in particular the terns, which form part of the larger Irish Sea population, and as part of the Ynys Feurig, Cemlyn Bay and The Skerries SPA.
Tree mallow was introduced to a walled lighthouse garden area around the mid-1990s and quickly took over the entire area. By the late 1990s the tree mallow was managed by hand pulling around 50% of the area of the garden, but more recently it has died out naturally. By the 2010s the only vegetation management was hand-pulling to maintain access paths, but the increasing size of the seabird colony continues to add nutrients resulting in thicker vegetation, mainly scurvy grass, which gradually began to swamp tern nests and chicks. In 2017, with SSSI permissions from Natural Resources Wales, the densest areas of scurvy grass and thick patches of nettles Urtica dioica, were sprayed with herbicide early in the season before the terns arrived.
The spraying in the first year successfully restricted the growth of lush vegetation but a slope that was sprayed stayed quite bare for a month and some nests were washed out by runoff in heavy rain; while the lack of cover meant that chicks from nests in the bare area ran into the longer vegetation for cover and so could still become wet and chilled In 2018, the programme of spraying was changed to spray more smaller areas along the contour of the slope and retain some vegetated areas for shelter for chicks. No spraying was done in 2019 or 2020, but it is intended to continue spraying patchily to create a network of open areas.
Coquet Island, England: managing coarse grasses and nettles to keep tern plots open
On RSPB Coquet Island the ‘plateau’ of the island is well fertilised by the breeding seabird assemblage and is dominated by rank grasses, mainly Yorkshire fog Holcus lanatus and false oat grass Arrhenatherum elatius with patches of nettles, particularly where lighthouse keepers historically fertilised vegetable beds. Tree mallow was planted on Coquet in the late 1990s in the mistaken belief that roseate terns used it preferentially; but after a visit to Rockabill in 2000 the process of removing the tree mallow by hand pulling was begun, and as of 2020 it is almost completely extirpated from the island.
The area of the island occupied by terns is divided into plots, largely within the nettlebeds with vegetation left in between to provide shelter for chicks. The plots are different sizes and shapes, a layout that evolved over time around the physical features of the island. There were originally over 50 plots but those to the east were taken over by black-headed gulls so maintenance of these was discontinued. Twenty-five plots are retained (see Figure 4) and are strimmed at the end of the breeding season and then cut 3 or 4 times with mowers over the course of the winter as required, including a cut as close as possible to the start of the seabird breeding season. Patches of nettles, dock Rumex sp. and thistles Cirsium sp. are treated with topical herbicide (Grazon) at the start of the season once there is enough growth for the herbicide to be effective. Vipers bugloss Echium vulgare is pulled by hand.
On RSPB Coquet Island the ‘plateau’ of the island is well fertilised by the breeding seabird assemblage and is dominated by rank grasses, mainly Yorkshire fog Holcus lanatus and false oat grass Arrhenatherum elatius with patches of nettles, particularly where lighthouse keepers historically fertilised vegetable beds. Tree mallow was planted on Coquet in the late 1990s in the mistaken belief that roseate terns used it preferentially; but after a visit to Rockabill in 2000 the process of removing the tree mallow by hand pulling was begun, and as of 2020 it is almost completely extirpated from the island.
The area of the island occupied by terns is divided into plots, largely within the nettlebeds with vegetation left in between to provide shelter for chicks. The plots are different sizes and shapes, a layout that evolved over time around the physical features of the island. There were originally over 50 plots but those to the east were taken over by black-headed gulls so maintenance of these was discontinued. Twenty-five plots are retained (see Figure 4) and are strimmed at the end of the breeding season and then cut 3 or 4 times with mowers over the course of the winter as required, including a cut as close as possible to the start of the seabird breeding season. Patches of nettles, dock Rumex sp. and thistles Cirsium sp. are treated with topical herbicide (Grazon) at the start of the season once there is enough growth for the herbicide to be effective. Vipers bugloss Echium vulgare is pulled by hand.
Herbicide (Glyphosate) is also used to keep the helicopter landing area to the east of the lighthouse clear (a Trinity House requirement) and pathways clear if required during the season. The Sandwich terns maintain nearly bare ground through the breeding season in the plots they occupy. In 2020, plot 52 (the heligarden), was sown with yellow rattle Rhinanthus minor seed in the hope that over time this semi-parasitic species will naturally suppress the coarse grasses. On Coquet Island large chick shelters designed to have the additional effect of supressing vegetation are also in use.
Shotton Works, England: spraying and strimming artificial islands
Between 1987 and 2000 three artificial islands were created from bunds in lagoons at the Shotton Steelworks in Northwest England. Herbicide treatment was used but this reduced during a period when terns were not nesting due to predation pressure, and in this time perennial weeds became established.
Netting used to prevent other species colonising before the terns arrive, accelerates early-season weed growth. This means that spraying immediately after the removal of the netting kills off weed growth more effectively than would be expected for the time of year, reducing weed cover until much later in the season.
Between 1987 and 2000 three artificial islands were created from bunds in lagoons at the Shotton Steelworks in Northwest England. Herbicide treatment was used but this reduced during a period when terns were not nesting due to predation pressure, and in this time perennial weeds became established.
Netting used to prevent other species colonising before the terns arrive, accelerates early-season weed growth. This means that spraying immediately after the removal of the netting kills off weed growth more effectively than would be expected for the time of year, reducing weed cover until much later in the season.
Figure 8. Artificial island at Shotton showing vegetation growth (Merseyside Ringing Group)
At the time of writing the vegetation management regime at Shotton is as follows:
- Remove anti-gull debris netting in late April when tern numbers have built up.
- Herbicide is applied on the same day as the netting is removed; vegetation dies back within 7-10 days and remains sparse until mid to late June.
- Perennial plants with long roots that have penetrated the ‘weed-resistant’ membrane grow strongly through July and August, however there are many chicks on the islands at this time so no attempts are made to remove vegetation.
- After the last chicks have fledged (usually the first or second week of September), the vegetation is sprayed with herbicide. Many plants will have dropped seeds before then.
- The treated vegetation is stimmed and removed in October (previously teams of volunteers raked the dead vegetation and removed it from the islands in February). Deep-rooted perennials need to be pulled out by hand.
RSPB Pagham Harbour, England: restoring islands by vegetation management and fencing
Tern Island is a small island of (0.24ha Above High Water) within Pagham Harbour on the south coast of England. It is protected by a dynamic sand and shingle spit which had accreted eastwards in the 10 years before 2015. Little terns have historically nested on both the spit and Tern Island, the latter also holding a population of the rare plant childing pink Petrorhagia nanteuilii.
The EU LIFE Little Tern Project (2013 – 2018) identified an opportunity to enhance and restore habitat for little terns on the spit and found funds (half from RSPB, half from EU LIFE) for works. However, due to local concerns around coastal shingle movements threatening homes in the area and calls for the spit to be breached there were delays. In the meantime, terns started to use Tern Island in preference to the spit.
In March 2018, work to remove vegetation and restore open habitat for breeding birds was undertaken on Tern Island, with SSSI consent. The shingle on parts of the island was mechanically ‘raked’ using a 360 digger and toothed landscaping bucket to reduce the dominance of coarse vegetation. Areas close to childing pink were weeded by hand. Parts of the cleared shingle were recharged with smaller shingle, more suitable for nesting little terns, and sand patches to reduce air flow through the stones and potential chilling of eggs. A predator-proof (combination stock and electric) fence was installed around the island.
Tern Island is a small island of (0.24ha Above High Water) within Pagham Harbour on the south coast of England. It is protected by a dynamic sand and shingle spit which had accreted eastwards in the 10 years before 2015. Little terns have historically nested on both the spit and Tern Island, the latter also holding a population of the rare plant childing pink Petrorhagia nanteuilii.
The EU LIFE Little Tern Project (2013 – 2018) identified an opportunity to enhance and restore habitat for little terns on the spit and found funds (half from RSPB, half from EU LIFE) for works. However, due to local concerns around coastal shingle movements threatening homes in the area and calls for the spit to be breached there were delays. In the meantime, terns started to use Tern Island in preference to the spit.
In March 2018, work to remove vegetation and restore open habitat for breeding birds was undertaken on Tern Island, with SSSI consent. The shingle on parts of the island was mechanically ‘raked’ using a 360 digger and toothed landscaping bucket to reduce the dominance of coarse vegetation. Areas close to childing pink were weeded by hand. Parts of the cleared shingle were recharged with smaller shingle, more suitable for nesting little terns, and sand patches to reduce air flow through the stones and potential chilling of eggs. A predator-proof (combination stock and electric) fence was installed around the island.
Figure 9. Cleared shingle on Tern Island, Pagham Harbour with the start of new predator fencing also visible (Ivan Lang; RSPB)
In 2018, more little, common and Sandwich terns fledged compared to before the project. Little terns nested on the cleared area outside the new predator fence, and a temporary perimeter fence was erected to protect them. This may have been because of competition for space or because the cleared habitat was most attractive to them. The improved fencing protected the enclosed area, but predation took place outside the fence.
In 2019, the little terns moved to New Island (about 200m to the north) probably due to the increase of black-headed gulls on Tern Island, which were also attracted to the new habitat and had increased from 18 to 523 pairs between 2013 and 2019.
References
Lamb J.S. (2015) Review of vegetation management in breeding colonies of North Atlantic terns.
Conservation Evidence, 12, 53-59 https://www.conservationevidence.com/individual-study/5557
Lamb, J. S, Hall, C. Scott, Kress, S. W, & Griffin, C. R. (2014) Comparison of Burning and Weed Barriers for Restoring Common Tern (Sterna hirundo) Nesting Habitat in the Gulf of Maine. Waterbirds, 37, 286-297. doi: 10.1675/063.037.0307
Morris R.D., Kirkham I.R. & Chardine J.W. (1980) Management of a declining common tern colony. The Journal of Wildlife Management, 44, 241-245
Morris R.D., Blokpoel H. & Tessier G.D. (1992) Management efforts for the conservation of common tern Sterna hirundo colonies in the Great Lakes: two case histories. Biological Conservation, 60, 7-14
Ramos, J.A., & del Nevo, A.J. (1995) Nest-Site Selection by Roseate Terns and Common Terns in the Azores, The Auk, Volume 112, Issue 3, 580–589, https://doi.org/10.1093/auk/112.3.580
Saliva J.E. & Burger J. (1989) Effect of experimental manipulation of vegetation density on nest-site selection in sooty terns. The Condor, 91, 689-698
Acknowledgements
Thanks to Steve Newton (Birdwatch Ireland), Monika Wojcieszek (RSPB Larne Lough), Ian Sims (RSPB The Skerries), Paul Morrison (RSPB Coquet Island) and Peter Coffey (Merseyside Ringing Group, Shotton) for valuable discussions of vegetation management.
Lamb J.S. (2015) Review of vegetation management in breeding colonies of North Atlantic terns.
Conservation Evidence, 12, 53-59 https://www.conservationevidence.com/individual-study/5557
Lamb, J. S, Hall, C. Scott, Kress, S. W, & Griffin, C. R. (2014) Comparison of Burning and Weed Barriers for Restoring Common Tern (Sterna hirundo) Nesting Habitat in the Gulf of Maine. Waterbirds, 37, 286-297. doi: 10.1675/063.037.0307
Morris R.D., Kirkham I.R. & Chardine J.W. (1980) Management of a declining common tern colony. The Journal of Wildlife Management, 44, 241-245
Morris R.D., Blokpoel H. & Tessier G.D. (1992) Management efforts for the conservation of common tern Sterna hirundo colonies in the Great Lakes: two case histories. Biological Conservation, 60, 7-14
Ramos, J.A., & del Nevo, A.J. (1995) Nest-Site Selection by Roseate Terns and Common Terns in the Azores, The Auk, Volume 112, Issue 3, 580–589, https://doi.org/10.1093/auk/112.3.580
Saliva J.E. & Burger J. (1989) Effect of experimental manipulation of vegetation density on nest-site selection in sooty terns. The Condor, 91, 689-698
Acknowledgements
Thanks to Steve Newton (Birdwatch Ireland), Monika Wojcieszek (RSPB Larne Lough), Ian Sims (RSPB The Skerries), Paul Morrison (RSPB Coquet Island) and Peter Coffey (Merseyside Ringing Group, Shotton) for valuable discussions of vegetation management.