Genetic Scorecard Indicator - Manx shearwater

Although some (mainly small) colonies have been extirpated historically and more recently, new small colonies have been established, and large ones have grown. In the absence of direct genetic evidence, high-quality demographic data indicate population growth, with no sign of diversity loss.

Population definitions:

The Manx shearwater is a highly mobile migratory seabird species breeding across 28 colonies in the UK, ranging from a single pair to around half a million. Despite strong philopatry (Harris, 1966; Harris, 1972; Brooke, 1990) and indications of moderate structure (Rodríguez et al., 2020), the dispersal capacity is vast, and UK colonies are here treated as a single population.

Ne500: The proportion of populations that have an effective population size of more than 500. 

• Proportion of populations with Ne > 500 in Scotland = 1/1 

(point estimate 590,418 breeding individuals, range 462,630–823,404 breeding individuals)

• Proportion of populations with Ne > 500 in UK = 1/1 

(point estimate 1,639,516 breeding individuals, range 1,214,662–2,037,624 breeding individuals)

PM: Proportion of populations that existed in 2000 that still exist in 2025.

• Proportion of populations maintained in Scotland = 1/1
• Proportion of populations maintained in UK = 1/1

Functional variation

No phenotypic or ecotypic differentiation is reported in the UK population, hence there is no indication of loss of functional variation.

Divergent lineages

While some genetic differentiation between colonies has been detected (Rodríguez et al., 2020), no appropriate larger-scale genetic evaluation has been carried out. Given colony distribution, high dispersal capacity, limited between-colony dispersal (Harris, 1972; Brooke, 1978), and lack of phenotypic differentiation, distinct divergent genetic lineages are unlikely. 

Hybridisation/Introgression

There are no known cases of hybridisation.

Low turnover - constraints on adaptive opportunities

Annual juvenile survival to breeding is low (33%; Brooke, 1990), with near-annual breeding starting at 5–6 years (range 3–9; Perrins et al., 1973; Brooke, 1990). One egg is laid, averaging 0.697 fledglings (Horswill & Robinson, 2015). Breeding adults are long-lived (92.5% annual survival), with a generation time of 19.8 years (Bird et al., 2020), constraining adaptive potential.

Scotland

• Risk: Moderate
• Mitigation: Partially effective

Great Britain/UK

• Risk: Moderate
• Mitigation: Partially effective

Overall Genetic Health status explanation

Although no direct genetic assessments exist, the very large population suggests current good genetic health; however, adaptive constraint through long generation time and restriction to relatively undisturbed islands make the species sensitive to rapid change. Brown rats can devastate colonies, and commercial fishing of prey species creates competition, with both prey availability and rat density interacting with temperature to heighten sensitivity to global warming.

In situ genetic threat level

• In situ Risk for Scotland: Moderate
• In situ Risk for UK: Moderate

Very large and increasing population size for a vagile species breeding on islands with various levels of protection; however, generation time is long, breeding is confined to a small number of colonies, and several potential threats can have swift negative impacts and require close monitoring of population changes. 

Confidence in in situ threat level

• Confidence score for Scotland: High
• Confidence score for UK: High

High-quality demographic data; more extensive direct genetic data desirable.  

 Majority of Manx shearwater colonies within Important Bird Areas (IBAs) overlapping with Special Protection Areas (SPAs). The species is strictly associated with the coast, and particularly islands, identified as a key focus by the Scottish Biodiversity Strategy (SBS). Rat radication campaigns on several breeding islands (Ratcliffe et al., 2009; Stanbury et al., 2017; Burnell et al., 2024). The fishing of sandeels has been banned in Scottish waters and the English waters of the North Sea since 2021.

The vast majority of Manx shearwater colonies, and thus breeding adults, occur within Important Bird Areas (IBAs) overlapping with Special Protection Areas (SPAs). The species is strictly associated with the coast, and particularly islands, identified as a key focus by the Scottish Biodiversity Strategy (SBS), and is on track to meet the SBS 2045 goal “Scotland’s internationally important species will have increased in numbers and have healthy resilient populations.” Probably the main threat is predation by native and non-native mammals, particularly brown rats. Eradication campaigns on several breeding islands have led to rapid recoveries on some, continued abandonment on others, and further campaigns and priority areas have been identified (Ratcliffe et al., 2009; Stanbury et al., 2017; Burnell et al., 2024), but must be paired with biosecurity, as rats re-established on Canna after eradication (Furness et al., 2024). Another potentially large threat is food scarcity due to fishing. The fishing of sandeels has been banned in Scottish waters and the English waters of the North Sea since 2021 (with International Arbitration Panel ruling against challenges by the EU in 2025), although harvesting of other prey species continues. 

Population

Demographic

High-effort detailed monitoring carried out 2015–2021 with >99% of the population covered by counts of occupied nest sites and <1% of the total population assessed without detailed monitoring (Bolton & Thompson 2023).  

N pops assessed/monitored in Scotland = 1/1 

11 colonies in 6 geographic clusters.

N pops assessed/monitored in UK = 1/1 

Genetic

N pops assessed/monitored in Scotland = 0/1
N pops assessed/monitored in UK = 0/1

 Population monitoring would benefit from broader sampling of most or all colony clusters combined with a deeper genetic sampling (multi-locus or single nucleotide polymorphism-based).