Merlin

Overall vulnerability

Feature assessed:

• Merlin (Falco columbarius)

Special qualities:

• Internationally important and locally distinctive wildlife and habitats

Feature description:

The merlin is the UK’s smallest bird of prey, not much bigger than a blackbird. Merlin mainly feed on small birds such as meadow pipit, and usually take their prey on the wing. Similar in appearance to the kestrel, they are distinguishable by their smaller size and stockier build, as well as their darker back and white stripe above the eye. Merlin are a sparsely distributed breeder across PDNP moorland, often migrating to coastal areas in the winter. A true moorland specialist, they have not recovered well from historical population crashes. The PDNP represents the most southerly edge of the Pennine breeding population, and the most southerly English breeding population.

How vulnerable are merlin?

Merlin in the PDNP have been rated ‘very high’ on our vulnerability scale. This score is due to high sensitivity and exposure to climate change variables, with a poor current condition, and a low adaptive capacity score.

Merlin are a scarce species in the PDNP, with a small population that has not rebounded as well as some other raptors in recent years. Climate change poses a significant threat to merlin, as the PDNP and much of the UK will become unsuitable as temperatures rise. Merlin are very likely to be extinct from the PDNP by the end of the 21st century. Changes to merlin nesting habitat, especially mature heather stands may also put pressure on populations. Despite being a mobile species and able to change nesting habits, merlin are unlikely to be able to adapt enough to offset climate effects. This is due in part to insufficient support and management, but is largely due to the extent of climate impact.

Current condition:

Merlin have a small population in the PDNP, being sparsely distributed across moorland in the Dark and South West Peak areas. Populations do not seem to be recovering as well from historical persecution and pesticide use compared to some other raptor species, possibly because of their more specialised lifestyle. The 2004 Moorland Breeding Bird Survey found merlin numbers to be increasing, though they are still a rare sight in the PDNP. 24 pairs were estimated to have nested in the PDNP in 2018. It is unclear whether merlin were ever very populous in the PDNP, with some historical sources describing their rarity.

Human behavioural and land use changes were probably the main causes of historical merlin decline, and still affect merlin populations today. High levels of organochlorines and other pesticides contributed to large-scale decline of invertebrates, depleting food resources for prey birds. Bioaccumulation of pesticides through the food chain also directly caused some merlin fatality. The most damaging pesticides are now banned, and bird of prey numbers have increased, but pesticide effects will still be present.

Illegal persecution is also a major threat to merlin, which are too small and quick to be shot easily but can still be affected by poisoned bait and other traps. Legal predator control may also affect merlin by decreasing crow numbers. As well as nesting on the ground, merlin can use abandoned crow nests in trees where they often nest in association with crows as an early warning against larger raptors. As much of PDNP land is open access, ground-nesting merlin can easily be disturbed and nests trampled. However, there is weak evidence for this being a significant effect.

Merlin are unlikely to be significantly impacted by predation, despite the high predator densities in parts of the PDNP. Studies in the Shetland Isles have shown no significant effect of nest predation on population decline.

Wildfire in the PDNP has probably had an impact on merlin numbers, destroying potential nesting and feeding habitat such as heathland and grassland. 552 wildfires occurred within the PDNP boundary between 1976 and 2018.

What are the potential impacts of climate change?

Overall potential impact rating

Direct impacts of climate change

As average annual temperatures increase, the merlin climatic envelope is very likely to contract northwards. UK merlin are at the southernmost edge of their European breeding range, so northward range contraction would result in the loss of breeding populations in the PDNP. Multiple models show climate suitability for merlin being lost from much of the UK over the 21st century, with only the Scottish Highlands remaining a refuge. The PDNP will be one of the first places to experience this loss. European winter migrants may still visit the PDNP after this loss, but the breeding population is unlikely to remain. Data Certainty: Very High

Wetter winters and a greater frequency of summer storms may lead to higher episodic nitrogen deposition, especially in the uplands. Excess nitrogen availability causes increased sensitivity of heather to drought and frost, meaning some mature stands may be lost. This would represent a loss of nesting habitat for merlin, decreasing their breeding opportunities. Data Certainty: Moderate

Drier summers may result in increased productivity for merlin. Wet summers decrease breeding success in aerial predators, as the activity and visibility of their avian prey is reduced. Less summer precipitation may therefore increase food availability and breeding success in merlin. However, due to density dependent effects in population change, it is unclear whether this will lead to an increase in population size. Data Certainty: Moderate An increase in the frequency and intensity of spring and summer storms may counter this effect, as heavy rain events are particularly detrimental to foraging success. Data Certainty: Moderate

Human behaviour change

An increase in visitor numbers due to hotter drier summers may cause greater disturbance of nesting merlin. It is unclear at what distance disturbance is significant, with estimates ranging from less than ten to 500 metres. This variation may be due to differences in ground and tree nesting merlin, or nesting stage, with merlin at a later incubation stage being less willing to abandon eggs. If the disturbance distance is at the higher end of this range, visitors may disturb merlin without even realising they are present. Decreased nesting success would result from this.

Climate change may cause declines in red grouse numbers, and this could result in an increase in illegal raptor persecution. Despite not being a main target for persecution, merlin will still be affected by poisoned bait and other traps. Merlin fatalities due to persecution could have a significant effect on the already small population. Other agricultural landowners may also react to changing conditions by intensifying management. This may decrease farmland bird populations both in the PDNP and at coastal wintering grounds. A reduction in food resource would decrease merlin fitness and reduce overwintering survival. Data Certainty: Moderate

As sea levels rise, mitigation efforts may prove detrimental. Hard flood defences and squeezing of the remaining wintering habitat would increase competition for resources and could reduce abundance of small avian prey such as meadow pipit and dunlin, resulting in reduced wintering survival for merlin. Data Certainty: Low

An increase in upland afforestation as a carbon capture method may have mixed effects on merlin populations. Birds of open moorland merlin are usually negatively associated with woodland, as they are replaced by generalists and woodland species. Conversion of heathland and grassland to woodland would therefore decrease habitat suitability for merlin and their avian prey. However, trees on moorland edges can provide nesting opportunities for merlin. European merlin populations are generally tree nesting and UK merlin are increasingly using trees as nesting habitat, particularly on woodland edges where they provide a good vantage point, so newly created forests may represent increased nesting resource for merlin. Data Certainty: Moderate

An increase in the frequency and severity of flooding events may lead to increased natural flood management efforts. In the PDNP, this is likely to be through moorland rewetting and restoration of natural hydrological processes. Increased heterogeneity of habitat may benefit merlin by increasing nesting and feeding opportunities, with the habitat mosaic meaning nesting and feeding areas are less separated. Clough woodland establishment may also increase nesting habitat on moorland edges. Data Certainty: Low

Invasive or other species interactions

Warmer wetter winter conditions may increase heather beetle numbers, meaning that outbreaks would be more likely. Mature stands of heather may be lost, removing some merlin nesting habitat. Heather may be replaced by fast growing colonisers like bracken, which could take advantage of the warmer climate to spread. It is unknown whether merlin will take advantage of bracken stands or move elsewhere. Heather moorland may also be lost to grassland, increasing available feeding habitat but removing breeding habitat. Data Certainty: High

Increased annual average temperatures may lead to an increase in generalist predators due to higher winter survival rates. Nest predation does not appear to have a significant impact on merlin, but at very high predator densities, animals such as foxes could conceivably take merlin young. Higher temperatures may also increase disease and parasite prevalence. It is unclear how this would affect merlin, but the greatest effect may be on their avian prey. Fewer and poorer condition prey would lead to poorer condition of merlin, lowering fitness and decreasing breeding success. Merlin wintering grounds may also be affected by high temperatures, with high spring temperatures being correlated with algal blooms in estuaries. These blooms could decrease food resources for the merlin’s avian prey, leading to reduced fitness before migration. Increased migration fatalities and decreased breeding success upon their return may follow. Data Certainty: Low

Sedimentation or erosion

Increased erosion due to sea level rise will cause changes at coastal wintering grounds. Erosion may remove some estuarine habitat and change the sediment composition in other areas, reducing invertebrate abundance in the substrate and removing feeding habitat for avian prey. A reduction in small wintering birds would cause reduced wintering survival in merlin, resulting in fewer and poorer condition, birds returning to the PDNP. Data Certainty: High

An increase in winter rainfall and summer storm events may lead to increased erosion, damaging merlin nesting habitat. Heather moorland in the PDNP is often on thin peat soils, making it highly susceptible to erosion. Erosion exposes heather roots, making the plants more susceptible to drought and winter desiccation. As a result mature stands of heather may be lost. As these stands are essential for ground nesting merlin, this would cause displacement of some birds and potentially drive a move towards tree nesting habits. Data Certainty: High

Other indirect climate change impacts

Greater occurrence of winter flood and waterlogged conditions are likely to reduce below ground biomass allocation in heather and other plants in affected areas, causing root dieback. Heather with lower underground biomass allocation is more susceptible to stressors such as drought and frost. As a result, some heather may be lost and community composition altered, resulting in less suitable nesting habitat. Data Certainty: High

Hotter, drier conditions in spring and summer may lead to an increase in wildfire in the PDNP. This would be especially damaging during the nesting season, as nests could be destroyed. Burns covering large areas could also prevent re-nesting opportunities, as merlin tend to be faithful to an area. Data Certainty: Low

Higher summer temperatures and an increase in drought may cause excess stress on heather. Heather stands, particularly those on south facing slopes, may die off due to heat stress. Older heather stands suitable for nesting may be more vulnerable to this. Heather that remains may no longer grow tall enough for merlin nesting. As a result, nesting opportunities may be reduced. Data Certainty: Low

Nutrient changes or environmental contamination

Increased atmospheric carbon dioxide and nutrient availability may cause an increased plant growth rate, affecting merlin feeding grounds. An increased growth rate, coupled with a longer growing season due to increased temperature, may cause grass growth and dominance to increase, as faster growing grasses will be better able to make use of available resources. This may encourage higher livestock levels on acid grasslands, leading to reduced populations of merlin prey. However, stress tolerant grasses on moorland are likely to be limited by other factors such as nutrient availability, so this effect may be small. Data Certainty: Low

What is the adaptive capacity of merlin?

Overall adaptive capacity rating

As migratory birds, merlin have few barriers to movement and may be able to move with changing conditions. However, the lack of habitat further north in the PDNP is expected to cause their loss from the area. Merlin in the South West Peak may be able to move to Dark Peak nesting sites, but as their climate envelope continues to move they too will likely move northwards out of the PDNP boundary. Data Certainty: High

Merlin have shown some evidence of adaptability in the face of change. Site faithfulness due to territory holding is a significant barrier to adaptation in individual birds, but would not prevent the population shifting, albeit slowly. Despite their current inability to adapt to urban spaces in the UK, this does occur in North America. This may be related to the ground nesting habits of most UK merlin. However, they have shown some movement towards tree nesting habits, although this has been in conifer plantations. Data Certainty: Moderate

Some economic support for adaptation is available in the form of environmental land management schemes. These may be effective in preserving some merlin feeding habitat, as acid grassland on moorland edges is at risk of conversion to more productive ‘improved’ grassland. However, in many cases such schemes are unlikely to be effectively implemented, and their future is very much uncertain. Data Certainty: Low

Several institutions that could make adaptations exist within the PDNP, and in the case of the National Trust own much of merlin nesting habitat. Merlin are also specially protected by Schedule 1 of the Wildlife and Countryside Act 1981, are listed in annex 1 of the European Commission Directive on the Conservation of Wild Birds. They are a red listed bird species in the UK. The South Pennines Special Protection Area (SPA) is also designated for birds and specifically mentions merlin in its citation. However, current SPA provisions are not thought to be adequate for merlin conservation, and additional provisions may be required. Data Certainty: High

Compared to other raptor species, there is a lack of information on merlin. This means that management interventions are unlikely to be as effective, as specific knowledge of species ecology is needed to inform decisions. Even if effective, management is unlikely to be able to offset climate change effects on merlin. Data Certainty: Low

Key adaptation recommendations for merlin:

Improve current condition to increase resilience

The current condition of a feature is an important factor alongside its sensitivity and exposure, in determining its vulnerability to climate change. These recommendations are aimed at improving the condition of the feature at present, therefore making it better able to withstand future changes to climate.

• Greater focus on preventing bird of prey persecution in the PDNP is needed, as well as prosecution of offenders.

• Lower grazing intensity is beneficial to both merlin and their avian prey.

• Further research is required to understand drivers of population changes. Breeding success has been relatively good so it is important to uncover the reasons why Merlin are not doing well in the PDNP.

• Undertake research or collaboration with organisations linked to wintering grounds.

• Predator control could be a useful tool in high predator density areas, but may inadvertently increase predator populations and disrupt other species interactions. Research is needed to determine if a more natural system would be a better option for the future.

• Develop fire contingency plans, and ensure management of habitats reduces fire risk e.g. rewetting and increasing species or structural diversity. Influence visitor and behaviour management plans and practices to minimise ignition risk.

• Encourage further uptake of environmental land management schemes by farmers within the PDNP.

Improve current condition to increase resilience: Increase structural diversity to improve resilience at a landscape scale

The current condition of a feature is an important factor alongside its sensitivity and exposure, in determining its vulnerability to climate change. These recommendations focus on increasing the structural diversity of the area or habitat in which the feature is found. This can help to offset the effects of climate change on the feature, as well as to allow it to be in a better position to recover from future climate changes.

• Increasing the heterogeneity of moorland vegetation has been suggested to be beneficial to merlin.
• Woodland creation on moorland edges may provide nesting habitat and allow merlin to be more adaptable.