Twite

Overall vulnerability

Feature assessed:

• Twite (Linaria flavirostris)

Special qualities:

• SQ2: Internationally important and locally distinctive wildlife and habitats

Feature description:

The twite is a small brown finch with streaked brown back and breast with a pale belly. It lacks the distinctive red breast of the related linnet, but the male has a pinkish rump during summer. Generally overwintering on coastal sites in Norfolk and the South East of England, twite return in spring for the breeding season. As seed foragers, they are generally dependent on proximity to plant rich meadows. Two distinct populations of twite are present in the PDNP. One population nests in limestone quarries in the White Peak, and the rest are part of the wider moorland nesting Pennine population. The PDNP represents the south-eastern edge of the UK breeding population. However, numbers are in sharp decline across the PDNP, presenting a real risk of loss in the near future.

How vulnerable are twite?

Twite 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 very poor current condition approaching complete loss, and a low adaptive capacity.

Twite are faring poorly in the PDNP, with moorland populations lost in many places and a small quarry dwelling population in the White Peak being the largest known colony. Climate change may cause twite to move out of the PDNP as they are on the south-eastern edge of their UK range here. Changes in flowering and seed setting timings of their plant food resources may cause a mismatch between twite nesting dates and seed abundance, and interrupt continuous food supply. Twite are a mobile species, but are unlikely to overcome future challenges presented to them. The greatest pressure will probably come from human land use change, so institutional and economic support will be needed to implement management interventions. Unfortunately, the resources currently available do not appear sufficient to retain twite in the PDNP.

Current condition:

The twite has suffered wholesale decline across the UK in recent years, and the PDNP has been no exception. Despite having been described as a stronghold in the past, the 2004 Moorland Breeding Bird Survey found heavy population losses, showing a 92% decline in numbers since 1990. It can be assumed that twite are functionally extinct in some areas of PDNP moorland, and close to being lost from the entire area. Trends for the White Peak are less well known, but a population of around 60 birds was estimated in 2017.

Twite decline is thought to be largely due to human land use changes leading to habitat loss. The move from hay meadows to silage making has led many grasslands in the PDNP to become less diverse and cut more often, drastically reducing seed availability. Similarly, agricultural ‘improvement’ of grasslands on the moorland edges has led to lower seed availability - the low floristic diversity also producing less variation in seed setting times, meaning food resources are not available year-round. Feeding sites need to be within two km of the moorland edge, so good feeding sites are often too far from nesting habitat to support twite populations. High stocking levels may also have contributed to this effect by reducing the suitability of grassland. Changes to moorland nesting habitat may have also contributed, with high stocking levels and inappropriate management reducing suitability. Twite only really established themselves in 1960s, even in 1930s with lots of available grassland they weren’t very successful. Their populations fluctuate largely.

What are the potential impacts of climate change?

Overall potential impact rating

Direct impacts of climate change

As average temperatures increase, suitable twite climate space is likely to contract northward. Twite are a cold adapted upland species, and the PDNP is at the south-eastern edge of their UK range. As their climate envelope shifts northwards and upwards, this is likely to result in twite being lost entirely from the area. Modelling suggests this range contraction will affect much of the UK, with the only suitable habitat remaining in the late 21st century being in northern Scotland. Data Certainty: Moderate Increased atmospheric carbon dioxide concentrations may change grassland community composition by favouring fast growing grass species. Compounded by a potential increase in winter nitrogen deposition due to increased rainfall, this may act to reduce the floristic diversity of grasslands, especially in uplands. This would reduce seed availability for twite by decreasing variability in flowering times, affecting breeding success. Data Certainty: High

Changes in annual precipitation cycles may affect twite feeding grounds. An increase in winter waterlogging and summer drought can act to change the community composition, in both cases potentially reducing floristic diversity. Waterlogging could move the community composition of lowland meadows towards a wet meadow species assemblage, increasing rush dominance and likely reducing seed availability. Waterlogged soils are also at greater risk of trampling and poaching causing soil compaction. Drought will favour tolerant species and reduce diversity, but may cause greater seed production in stressed plants. Both wetter and drier grasslands are likely less suitable for twite. Data Certainty: Low

Other indirect climate change impacts

Increased annual average temperatures may change natural growth cycles, potentially driving phenological mismatch in twite. The timing of seed setting in various meadow plants are important to ensure a continuous food supply during the breeding season. Seed setting times are likely to advance with warming temperatures, meaning that twite nesting dates may have to advance to match. As migratory birds, twite are less flexible in nesting dates and so a mismatch may occur. Different plants may also adapt at different rates, breaking the continuity of seed supply. Both these effects would lead to a reduction in twite numbers and breeding success. Data Certainty: High

Hotter summer conditions and more frequent drought will likely combine to increase the frequency and severity of wildfire, especially on heather moorland. Wildfire affects dense mature heather stands more than other vegetation types, and are the most likely to contain twite nests. This would represent both loss of preferred nesting habitat and potential nest destruction. Overall, twite nesting success will be reduced. Data Certainty: High

Human behaviour change

Hotter drier summers may result in increased visitor numbers in the PDNP. Higher footfall would result in increased disturbance to twite at any remaining moorland nesting sites, as well as increased trampling of vegetation at feeding sites. Twite breeding success could therefore be reduced. Combined climate changes may drive changes in PDNP agriculture. It is hard to predict exactly how land use will change, but changes could easily be detrimental to twite populations. As twite are dependent on managed grasslands in the PDNP, land abandonment can reduce food resource due to scrub encroachment, while increases in stocking can reduce food resource via overgrazing. Changes in management not directly aimed at increasing floristic diversity are unlikely to be beneficial. Data Certainty: Moderate

An increase in the frequency and severity of summer droughts and storms may have negative consequences for twite feeding resources. Greater unpredictability of weather may drive a move away from hay meadows as grassland management. If hay meadows are replaced by grassland types with lower floristic diversity, the suitability of in-bye land for twite feeding will be even further reduced, driving population decline. Data Certainty: Moderate

Sea level rise is likely to drive an increase in coastal re-alignment and flood barrier development, affecting twite at their wintering grounds. This may cause a reduction in area of the saltmarsh habitat preferred by twite, squeezing the remaining habitat as the sea level rises if new habitat is not created. Competition at the remaining habitat would be increased with wintering twite from other UK sites, birds arriving from northern Europe and other bird species. Twite wintering survival could therefore be reduced. Data Certainty: Moderate

Climate change mitigation efforts may be detrimental to twite populations. Afforestation for carbon capture and upland wind farm development for renewable energy can both reduce the area of suitable habitat for twite. Twite within the PDNP will almost certainly be protected from wind farm development, but construction along their migration routes and coastal wind farm installations at their wintering grounds could still affect these populations. Small birds are less at risk from collision, so migration fatalities are unlikely to increase significantly. Coastal wind farms could however reduce overwintering habitat, lowing winter survival. Data Certainty: Low

Sedimentation or erosion

Coastal wintering grounds are at high risk of erosion damage. An increase in the frequency and severity of storm events, and sea level rise may act to increase erosion at preferred coastal habitats, for example saltmarsh. Pioneer communities on newly created sandbanks will be at greatest risk from erosion. As these areas are lost, competition at the remaining sites will increase. This would result in greater winter fatality in twite, and poorer condition upon return to the PDNP, causing a reduction in breeding success. Data Certainty: High

Wetter winters and more frequent summer storms may cause an increase in soil erosion, especially on heathland where soil is already thin. This can increase root damage to heather, increasing susceptibility to winter desiccation. Loss of heather from some areas and stunted growth in others could further reduce the availability of preferred twite nesting habitat. Data Certainty: High

Invasive or other species interactions

Increases winter nitrogen deposition due to higher rainfall may damage twite nesting habitat through its effects on heather. Excess available nitrogen causes increased susceptibility to heather beetle outbreaks, stunting growth across large areas. This could reduce heather dominance and leave areas unsuitable for nesting, as dense stands of mature heather are preferred nesting habitat. Data Certainty: Moderate This process may be compounded if wetter conditions are beneficial to heather beetle. Twite can use other nesting habitat such as bracken beds, which may become more common as warmer conditions prevail. However, the overall effect would probably be a reduction in suitable nesting habitat and therefore breeding success. Data Certainty: High

Nutrient changes or environmental contamination

Warmer conditions may cause an increase in growing season in heathlands, reducing twite habitat suitability. A longer, warmer growing season would result in faster nutrient cycling, driving changes in plant community composition. Competitive species such as grasses may become more dominant, moving heather moorland towards an acid grassland assemblage. This grassland is relatively poor feeding habitat, and would replace preferred twite nesting habitat, leading to a reduction in twite breeding success. Data Certainty: Moderate

An increase in flooding may cause nutrient enrichment of meadows through runoff from improved grassland and arable fields. This nutrient loading of meadows would reduce floristic diversity. Less diverse meadows are poorer twite feeding habitat due to reduced seed availability, leading twite breeding success to decrease as a result. Data Certainty: Moderate

What is the adaptive capacity of twite?

Overall adaptive capacity rating

As migratory birds, twite can overcome some barriers to adaptation. Migration distances of up to 400 km have been recorded between wintering and breeding sites. However, most fledglings move less than five km from their natal sites, meaning their spread is relatively slow. This is exacerbated by the fragmentation of their feeding sites; hay meadows are increasingly rare both within and outside the PDNP, limiting twite site choice. Despite mixing flocks at wintering sites, twite population mixture is relatively rare. However, this does occur and can counter some of the effects of inbreeding and genetic drift in small populations. Data Certainty: High

Twite are relatively versatile in their habits, giving them some potential to adapt to changes to their habitat. Twite can feed on a wide variety of seed, using different methods including foraging from the ground, bending seed heads, and perching on plants. However, they are exclusive seedeaters so their diet is still restricted. Twite can also potentially utilise a variety of nesting habitats including both heathland, grassland, and crevices, as evidenced by the White Peak quarry population. Data Certainty: High

Environmental land management schemes could help improve the adaptive capacity of twite. Schemes aiming to increase floristic diversity in agricultural land adjacent to nesting areas could benefit twite populations by providing a food source to bolster the numbers of remaining birds. However, there is currently much uncertainty about the future of agricultural subsidies, and what change will mean for the future of farming in the UK. Data Certainty: Low

Some institutional support is currently available for twite, but there are limits in the practical application of beneficial management. Twite are protected under the European Union Birds Directive, and are a red listed species in the UK. They are also part of the designated upland bird assemblage for the South Pennines Special Protection Area (SPA), affording them some potential benefits. Projects such as the RSPB and Natural England Twite Recovery Project are also working to assist twite recovery. However, the scale of change that is needed, especially to agricultural land, means that the impact of these projects is limited. Effects relating to coastal wintering grounds will also be hard to address for organisations such as the PDNP. Data Certainty: Very High

Gaps in knowledge relating to twite requirements are likely to hamper efforts to make adaptations for the species. Due to a lack of research on habitat choice and causes of decline, interventions to assist twite populations may be less effective than they could be. The heavily reduced twite population currently present will also mean that future research is more difficult. Data Certainty: High

Key adaptation recommendations for twite:

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.

• Restoration of quarries where appropriate will provide habitat for the larger White Peak population to expand into.
• Integrated management of twite nesting and feeding habitat is essential to their preservation. Co-ordinated management of upland moorland and lowland meadows will give the best chance for twite survival.
• Lowering livestock densities on pastoral land can improve its suitability as feeding habitat.
• Twite are migratory birds, so management interventions in the PDNP will have a limited effect if not part of wider efforts in wintering grounds. Partnership with coastal conservation organisations in Norfolk and the south-east coast will be the most beneficial to PDNP twite.
• Encourage further uptake of environmental land management schemes by farmers within the PDNP.

Improve current condition to increase resilience: Targeted conservation efforts for important sites and at risk areas

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 conservation measures aimed at those sites that will have the biggest impact for this feature – either because they are particularly important for the feature or because they are most at risk from climate change.

• Loss of feeding resources is likely the biggest contributing factor to the current decline. Meadow creation will therefore be very beneficial to twite populations. Expansion of existing meadows and conversion of some improved grassland near twite nesting areas may prevent or slow the loss of twite from the PDNP.
• 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.

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.

• Allow some stands of mature heather to persist within a varied vegetation structure, to allow nesting habitat while minimising wildfire risk. Rotational burning is unlikely to create this structure and so should be phased out.
• Bracken can provide nesting habitat for twite. Consider preserving bracken on steep slopes within 2km of feeding habitat where vegetation is otherwise unsuitable for nesting.