Blog post by Nick Wilkinson, Conservation Scientist, RSPB Centre for Conservation Science
The twite is a little known small seed-eating finch, the upland cousin of the more familiar linnet. In the UK, twites breed predominantly in coastal areas of northwest Scotland, with much smaller populations in England, Wales and Northern Ireland.
They nest mainly on moorlands and along sea cliffs and feed in flower-rich habitats, often in adjacent farmland.
The twite is red-listed in the UK owing to a long-term population decline, so we conduct periodic national surveys in order to update our knowledge on their numbers and distribution (as we do for a range of species, e.g. capercaillie, golden eagle and ring ouzel).
These surveys are conducted in partnership between national Government conservation agencies and the RSPB under the Statutory Conservation Agency and RSPB Annual Breeding Bird Scheme (or SCARABBS for short).
Results from the second national survey of twite, carried out in 2013, are published today in the journal Bird Study and indicate a further decline in the UK population.
A team of surveyors made counts of twites across a randomly selected sample of 1km grid squares throughout the species’ range in England, Scotland and Wales, while a complete census was made in Northern Ireland.
Each square was surveyed three times between May and July by surveyors walking transects and making five minute stops to scan and listen for birds in suitable nesting habitat. This was the same method as used for the first national survey in 1999.
Population estimates
Extrapolation of the counts on surveyed squares to the area of the range not surveyed gave a UK population estimate of 7,831 pairs (95% confidence limits: 5835-10143).
This compares with the previous survey estimate of 10 000 pairs (95% confidence limits: 6300-14600) in 1999: a decline of 21%. Although this change in population estimates was not statistically significant, the fall in population size reported by the survey is supported by results from Bird Atlas 2007-11, organised by the British Trust for Ornithology, which recorded an 18% decline in the 10-km square breeding range between 1988-91 and 2008-11.
Scotland held 98% of the UK population with an estimate of 7640 pairs; 18% lower than in 1999. In England, however, the population declined by 72% from an estimated 587 pairs in 1999 to just 164 pairs in 2013. The very small populations breeding in Wales and Northern Ireland were estimated at 16 and 18 pairs, respectively.
This latest survey shows that the islands and seaboard of northwest Scotland continue to be strongholds for breeding twite, where they are strongly associated with areas of extensively managed agriculture.
A previous study by the RSPB found that the distribution of moorland nesting twite on the Western Isles was concentrated close to adjacent farmland, where the mix of extensively grazed pastures and cultivated fallows provide a variety of habitats rich in weeds for adults provisioning nestlings with seed food throughout the breeding season.
Moreover, overall breeding productivity in the Western Isles was higher than in a concurrent study of the declining twite population in the English south Pennines due to lower rates of nest loss and a much greater frequency of repeat nesting.
Collectively, this area supports the large majority of the Scottish and UK twite population, highlighting the importance of the high nature value farming systems that characterise agricultural land use here.
Causes of decline
The causes of twite population declines are not fully understood but are likely related to a reduction in seed food supplies in both breeding and wintering areas due to changes in farming practices, loss of pioneer saltmarsh on their English coastal wintering grounds and, at least in England, loss of suitable moorland nesting habitat due to changes in management and land use.
Conservation efforts
In England, the vast majority of the population was found in the south Pennines. Repeated systematic surveys funded by Natural England (NE) have recorded substantial declines between 1990 and 2014 in what was formerly regarded as a stronghold for twite in England.
Conservation measures to recover the Pennine population have been implemented since 2010, under a project funded by RSPB and NE (through the Action for Birds in England partnership).
These focus on restoring flower-rich meadows and pastures close to twite moorland nesting areas and are being delivered through government funded Higher Level Stewardship agri-environment scheme agreements.
Following monitoring work in 2016 showing a further decline in numbers, we are now undertaking analyses of twite demographic data sets from a number of studies to understand whether low productivity or survival is driving the decline of this population.
Similarly, in Wales, trial conservation management (in this case, rotational grazing of pastures) has been deployed on farmland to increase seed food supplies throughout the breeding season as part of conservation efforts to recover the small population breeding in northern Snowdonia.
Further work is now required to understand the demographic and environmental drivers of population change across breeding populations.
Read the paper for full details of the survey, which was funded by the RSPB, Scottish Natural Heritage, NE and Natural Resources Wales.