Rum National Nature Reserve is internationally important for breeding Manx shearwaters.  It is reckoned that around 23% of the world’s population of this remarkable seabird breed on this Hebridean island.

Sean Carlisle is currently working on Rum as part of a three-year studentship looking at rat ecology and the results of his research will help inform management of the local rat population at the Manx shearwater colony.  Recently he tagged a female rat on the island to test tracking technology and determine if it is possible that rats move between the mountain shearwater colony and the village at sea level.  We sat down with Sean and asked him to explain more about this fascinating project.

Norway rat (Image courtesy AHVLA archive)

 Sean what does the project hope to find out ?

“The GPS tagging project hopes to help understand the movement patterns and home range sizes of Norway rats (perhaps better known as brown rats) on the Isle of Rum.  The idea is that by tracking rats we can decide how far they travel day to day, and if they move between other populations of rats found on the island. The work is part of a Magnus Magnusson SNH/SEPA PhD Studentship looking at the ecology of Norway rats on Rum. The work is supervised by Anglia Ruskin University and the National Wildlife Management Centre, which is part of the Animal Health and Veterinary Laboratories Agency.”

Is it possible that rats might travel from the village to the shearwater burrows and thereby endanger the seabird population?

“One of the key questions for Manx shearwater conservation on Rum is whether the rats are moving to the shearwater colonies seasonally, that’s to say over summer when the birds are on land, or whether the rats even stay near the colonies all year round. It is certainly possible that rats could move up to the hills over summer, but probably unlikely, however no-one really knows, which is why this GPS tagging is very exciting.”

Manx shearwater          ©Laurie Campbell/SNH

Generally do rats range quite far in their hunt for food?

“On average an adult male Norway rat has a home range size of about 500m, twice as much as an adult female. Most of the Norway rat research has however been done on mainland populations; we may find that in an island setting such as Rum, home ranges of Norway rats are different. Norway rats will also occasionally have dispersal events; a rat that usually has a home range size of 500m may travel up to 2km to a new area. We hope to find this out using the GPS tracker also.”

How is the data being picked up?

“The tricky part of the method is that the tagged rats must be recaptured in order to download the movement data. Attached to the rat with the GPS tag is a directional radio (VHF) beacon. This beacon sends out a pulse every 1-2 seconds, which means we can track the rat using an antenna and receiver, then set up traps around its last known location to trap the tagged rat and retrieve the location data. It sounds tricky but our recapture rates of rats on Rum have been pretty high so far.”

The stunning island of Rum    (c) John MacPherson / SNH

What does the data look like and how do you plot the rats’ movements?

“The data is stored on-board the GPS tag and then decoded, once retrieved, via the internet. This creates several types of location files, for the technically minded these are either  KMZ, NMEA, or CSV files. These can then be opened using preferred mapping software (as long as the file type is compatible with the software); the default is Google earth. The outcome is a lovely map of your study site covered in a series of rat location points.”

Has this kind of work been done before and informed any subsequent actions?

“As far as we know no-one has used GPS tags on rats in this context before; GPS tags have however been used to study the ecology of a several different species of mammals and birds including Manx shearwaters, badgers, and wild boar. Generally speaking this type of technology is great for studying anything from foraging behaviour to migration patterns.”

Is there a link to this story and the removal of rats from Canna and Ailsa Craig?

“Understanding the ecology of an invasive non-native species such as the Norway rat is crucial to resolving a lot of the conservation problems we face globally. Norway rats are considered one of the biggest global pests and as such various islands have — in the interests of local conservation — carried out rat eradication programmes.

“Island wildlife in particular appears to be susceptible to the effects of introduced species. In 2004 for example, the Isle of Canna, a neighbouring island to Rum, eradicated Norway rats due to a concern that the Manx shearwater numbers on the island were extremely low at least partly because of Norway rat predation. In Lundy the shearwater population grew after rat eradication in 2005. This apparent success story has been replicated on islands throughout the world, however despite the generally negative effect of rats on seabird productivity, rats and seabirds do sometimes appear to co-exist and an example of this is known on New Island, Falklands.

“A lot is still unknown about the factors that contribute to the effect of rats on seabirds which is why understanding the ecology of the rats is so important.”