Research showed that the declines in seabird numbers were primarily due to high levels of predation on eggs and small chicks by brown rats.

The island’s rat population has recently increased, probably as a result of milder winters. This has meant that over winter survival rates have increased, thus leading to larger numbers each spring in the seabird colonies.

Research by the National Trust for Scotland, showed rats to be widely distributed round the island in mid-winter (when numbers will be at their lowest) with concentrations along the coast and on the more sheltered lower ground on the eastern half of the island. The map below shows the mid-winter distribution of rats on Canna. It is based on a survey of chew sticks. Chew sticks are tongue depressers soaked in margarine or used cooking oil and placed upright in the ground in a grid pattern over the island. The amount of each stick chewed by the rats gives an indication of rat activity. It can also be used to give a crude estimate of rat abundance, currently thought to be between 5,000 and 10,000 individuals.