Analysis of Barn Owl Pellets from 5 Sites on Arran

 

Alistair Murdoch

 

August 2009, Kilmarnock

 

 

Introduction

 

The mammal list for the Island of Arran is significantly different to that of the adjacent Ayrshire mainland. Species such as fox, stoat and weasel are not found on the island, and amongst small mammal species, mole is absent and the status of bank vole and water vole is unclear.

 

This report focuses on small mammal species, and considers the following:

 

1. A survey of small mammals found in barn owl pellets from sites on Arran.

 

2. An investigation of existing small mammal records for Arran.

 

3. A proposed small mammal list for the Island of Arran.

 

 

The Pellet Survey

 

During July 2009, samples of pellets were obtained from 5 different Barn Owl nest boxes sited in predominately forestry habitats on Arran. Each pellet was analysed by removal and identification of the small mammal skulls present. A total was made for each species at each site (1 individual = 1 prey item):

 

 

Site	One
Habitat	Forestry Restock
No. of Pellets	9
Condition	Not Fresh
Species	Number
Common Shrew	2
Pygmy Shrew	1
Field Vole	12
Wood Mouse	1
Total Prey Items	16

 

 

 

Site	Two
Habitat	Forestry Restock
No. of Pellets	9
Condition	Not Fresh
Species	Number
Common Shrew	5
Pygmy Shrew	1
Field Vole	17
Wood Mouse	3
Total Prey Items	26

 

 

 

Site	Three
Habitat	Plantation/Rough Pasture
No. of Pellets	9
Condition	Fresh
Species	Number
Common Shrew	6
Field Vole	19
Total Prey Items	25

 

 

 

 

 

 

 

Site	Four
Habitat	Forestry Restock/Arable
No. of Pellets	13
Condition	Not Fresh
Species	Number
Common Shrew	5
Pygmy Shrew	2
Water Shrew	2
Field Vole	30
Total Prey Items	39

 

 

 

Site	Five
Habitat	Forestry Restock
No. of Pellets	5
Condition	Not Fresh
Species	Number
Common Shrew	1
Field Vole	13
Total Prey Items	14

 

 

 

The above tables indicate two or more species at each site, although the samples are rather small to convincingly represent small mammal diversity within a 1km radius of the site. Also, it is possible that some of the pellets were from nest boxes used for roosting in winter when the hunting territory may be considerably larger.

 

To create a species list, and give a rough indication of the significance of each species in barn owl diet on an island-wide basis, a grand total for all the pellets from all the sites was calculated and the following table produced:

 

 

 

 

 

Grand Total for 5 Sites (45 Pellets)
Species	Number
Common Shrew	19
Pygmy Shrew	4
Water Shrew	2
Field Vole	91
Wood Mouse	4
Total Prey Items	120

 

Clearly the most significant species in the diet of barn owl on Arran is field vole, accounting for three quarters of prey items in the sample. Common shrew is the second most significant species, with pygmy shrew, water shrew and wood mouse occurring in much smaller numbers. These figures are very similar to a comparable sample from a predominately forestry habitat on the Ayrshire mainland. (Bank vole if present on Arran may not occur in a sample from a forestry site). Thus, five small mammal species are accounted for in this sample, including water shrew which can be difficult to survey by other means.

 

 

Existing Records

 

The table below lists the ten small mammal species known to occur on the Ayrshire mainland and investigates the status of each species on Arran by considering:

 

1. Presence of the species in the pellet survey
2. Distribution indicated in The Handbook of Mammals in Britain
3. Records available on the NBN Gateway

 

The following rationale is used:

 

• A positive for the pellet survey confirms species presence
• Three negatives confirm absence
• At least 1 positive from HBM or NBN suggests further work required

 

 

Ayrshire Mainland Species		Island of Arran Species Records
Common Name	Scientific Name	Pellet Survey	HBM Distribution Map	NBN Gateway
Mole	Talpa europaea	No	Absent	No
Common Shrew	Sorex araneus	Yes	Present	Yes
Pygmy Shrew	Sorex minutus	Yes	Present	Yes
Water Shrew	Neomys fodiens	Yes	Present	Yes
Bank Vole	Clethrionomys glareolus	No	Absent	Yes
Field Vole	Microtus agrestis	Yes	Present	Yes
Water Vole	Arvicola terrestris	No	Present	No
Wood Mouse	Apodemus sylvaticus	Yes	Present	Yes
House Mouse	Mus domesticus	No	Present	No
Brown Rat	Rattus norvegicus	No	Present	Yes

 

 

It is also important to take into account:

 

·        The sample of pellets is fairly small and from forestry dominated habitat.

 

·        The Handbook of British Mammals consulted is the most up to date edition available to me at present.

 

 

·        NBN Gateway records are subject to access protocols.

 

 

A Small Mammal List for the Island of Arran

 

Notwithstanding the above limitations, it is still possible to construct a provisional small mammal list for Arran. Further pellet survey work, opportunity to consult the fourth edition of the Handbook of British Mammals, and a greater understanding of how data is made available in the NBN Gateway will all help to add clarity. Access to other published and unpublished work and local knowledge will also add information. The following points are proposed as a reasonable interpretation of the current situation subject to the above considerations:

 

·        There are no records for mole on Arran and this species is absent from the island.

 

·        Common, pygmy and water shrews are all present on Arran.

 

·        Bank vole is currently unclear. A large pellet sample from a barn owl site adjacent to scrubby, shrubby, bramble and bracken habitat would almost certainly reveal bank vole if it is present on the island.

 

·        Field vole is present on Arran.

 

·        Water vole remains unresolved. Local knowledge will most likely play a key role in confirming the presence of this problematic species on Arran.

 

·        Wood mouse is present on Arran.

 

 

·        House mouse seems likely, but confusion with wood mouse in a domestic setting is common. A large pellet sample from a barn owl site adjacent to occupied human habitation may resolve this species.

 

·        Brown rat is likely and easily resolved by local experience.

 

Therefore the provisional list has nine species: five confirmed by the pellet survey and four requiring further work:

 

 

Provisional List Of Small Mammals for the Island of Arran
Species	Confirmed
Common Shrew	Yes
Pygmy Shrew	Yes
Water Shrew	Yes
Bank Vole	Further pellet analysis required
Field Vole	Yes
Water Vole	Local information and field survey work required
Wood Mouse	Yes
House Mouse	Pellet analysis may resolve and/or pest control ID
Brown Rat	Visual records and/or pest control ID required

 

 

Of the four requiring further work, it is the water vole which presents the greatest difficulty. There are no records on the NBN Gateway and the literature seems at odds: the third edition handbook indicates that the species is present, but The Water Vole (Woodruffe, 2000) mentions a population on Bute but not Arran. A full examination of all the literature and a field survey may yet reveal a population of water voles on Arran. The Ayrshire Rivers Trust is the lead partner for water vole in the Ayrshire Local Biodiversity Action Plan. Whilst initial work has been undertaken, funding for a proper investigation and survey has not yet been obtained.

 

Conclusion

 

This survey was an experimental approach towards remotely obtaining new small mammal records. As such, contact was made with wildlife workers on Arran, and in due course pellets obtained and sent to me on the mainland for analysis and comparison with national records. Five small mammal species are confirmed for Arran, and suggestions made towards resolving another four. I consider this approach a success, and I will seek ways of obtaining a similar arrangement for other areas of Ayrshire with a view to eventually publishing a small mammal report for Ayrshire.

 

Sources of Information

 

Corbett, G. B., and Harris, S. (1991), The Handbook of British Mammals 3^rd edition, The Mammal Society.

 

Taylor, I. (1994), Barn Owls: Predator-Prey Relationships and Conservation, Cambridge University Press.

 

Woodruffe, G., (2000), The Water Vole, The Mammal Society.

 

Yalden, D. W., (2003), The Analysis of Owl Pellets 3^rd edition, The Mammal Society.

 

The Mammal Society: www.abdn.ac.uk/mammal

 

The NBN Gateway: www.nbn.org.uk

 

 

Acknowledgements

 

Many thanks to those on Arran for their encouragement with this survey. Special thanks to Robert Logan for supplying the pellets.