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Scottish crossbill BAP report
The Scottish crossbill is only found in the UK
What are we trying to achieve?
The Government agreed a Biodiversity Action Plan for this species in 1995. The targets for this plan were revised in 2006 and are as follows:
- Achieve an increase in the breeding range of Scottish crossbill from current 83 occupied 10 km squares, to 90 occupied 10 km squares by 2010
- Increase the current breeding population of Scottish crossbill
The Scottish crossbill is regarded as “data deficient” making it difficult to assess its status. It is currently a SPEC1 species.
What is the RSPB doing to help?
For several years, the RSPB was working to establish a field method for identifying Scottish crossbills, because it was not reliably distinguishable from parrot and common crossbills. This hampered work to define their range and population size and to carry out conservation measures. Thanks to the RSPB’s detailed research, it has been established that Scottish crossbills can be identified on calls, so progress has now been made.
During 2006, the RSPB:
- Completed development and published a census method and carried out a pilot survey of the distribution and densities of Scottish crossbills in two forests based on recording the number of birds responding to tape playback of crossbill calls. This tested method can now be applied to a national census
- Advocated sympathetic management of native pinewoods and encouraged its regeneration, through the Caledonian Pine Forest HAP
Summary of progress
The field identification of Scottish crossbills based on a distinctive call type has enabled us to establish the distribution of Scottish crossbills. This now needs to be followed up by a full survey to establish the population size of this endemic species. This survey will take place in January-April 2008. A pilot study has established a census method.
There remained some question as to the taxonomic status of Scottish crossbills in the context of current species concepts.
Biochemical studies revealed no significant differentiation between Scottish, parrot and common crossbills in their mitochondrial DNA and micro-satellites. Therefore, it is possible that speciation had occurred only recently, or that hybridisation is maintaining genetic homogeneity.
In this context, the RSPB’s study of assortative mating as a function of bill and call type was crucial in establishing the extent of inter-breeding. The study established that there was a small percentage (4.3%) of mixed species pairing in relation to bill size, but this was insufficient not to regard the Scottish crossbill as a species. It is likely that this mixed pairing is sufficient to prevent the genetic divergence that one would observe if two populations were totally reproductively isolated.
The RSPB study also confirmed that bill sizes are genetically inherited. This finding was also crucial to establishing that the Scottish crossbill is a species. There remains a loose end with regards to the calls of crossbills. It is important to establish how these calls are inherited and if they remain stable within the life of a crossbill.
A preliminary study of habitat requirements showed that Scottish crossbills have a stronger preference for Scots pine than do common crossbills. In winter, both Scottish and parrot crossbills prefer older Scots pines, which have smaller cones. Many Scots pinewoods may be sub-optimal for pine-eating crossbills in winter because they are managed as plantations and generally have larger cones than in native pinewoods.
Has our work been effective?
Our work on calls has been effective in differentiating between the three species of crossbill, which has enabled us to clarify the true breeding range of the Scottish crossbill.
Caledonian Pine Forest conservation has been carried out under the Caledonian Pine Forest HAP but we will not know how appropriate this is for Scottish crossbill until a full survey is carried out to identify its specific habitat requirements and an effective monitoring scheme has been set up. Once this survey has been completed, we will be in a better position to achieve the revised BAP targets.
What do we plan to do next?
We will:
- Make efforts to encourage landowners and foresters of commercial Scots pine plantations to modify their felling practice to provide older Scots pine (with smaller cones)
- Determine to which conifers Scottish crossbills are best adapted.
- Continue some work on the calls to find out how they are inherited and if they remain fixed for an individual
- Make preparations to conduct a national survey of Scottish crossbills to estimate population size and determine their habitat requirements, planned for 2008
What are the constraints to fully achieving the targets?
At present, the major constraint is lack of knowledge about the habitat requirements of Scottish crossbill. Initial studies suggest that forestry practices may be a key factor. Under current felling practices, Scots pine trees tend to be felled before they reach the older age group, which appears to be preferred by crossbills in winter. In addition, we do not know the extent to which non-native conifers are important for Scottish crossbills.
Acknowledgements
We wish to acknowledge support for work on Scottish crossbills by the Centre for Ecology and Hydrology (CEH), Aberdeen University and the Centre for Ecological and Environmental Modelling (St Andrews University). Scottish Natural Heritage and the Scottish Executive helped fund the development of the census method and pilot survey. We are also grateful to the many individual, private and public landowners who have helped work on crossbills and the conservation of pinewoods.
Last modified: 04 September 2007