Shearwaters and Shags

Monitoring work undertaken on Canna by the Highland Ringing Group since 1969 showed major declines in seabird numbers.

The first decline involved the Manx Shearwater colony, which showed a drastic drop in numbers from 1,500 pairs recorded during the 1980s. These birds nested in burrows along the Tarbert road, just west of the human inhabitated part of the island. The decline was linked with a marked drop in breeding success, due to increased levels of predation. Now perhaps only 1 or 2 pairs of shearwaters survive in isolated areas on some of the more inaccessible cliffs.

The Canna shag colony had been one of the largest in Europe and peaked at 1753 apparently occupied nests in 1984. Formerly there were four large sub-colonies, two in the western half of the island at Garrisdale and the Nunnery, one in the east at Lamasgor and one in the north at Geugasgor. The Nunnery colony was the first to decline dropping from 121 nests in 1997, mostly located under boulders on the raised wave cut platform, to only 21 nests in 2005, all located on inaccessible narrow cliff ledges. Lamasgor saw a similar decline from 231 nests in 1998 to 45 in 2005. Garrisdale then followed dropping rapidly from 221 nests in 2001 to only 18 in 2005. Finally Geugasgor is beginning to follow the pattern dropping from 336 nests in 2003 to only 167 in 2005.

Not all colonies were declining. On Sanday numbers increased from 8 nests in 1995 to 42 in 2005 and at Rhu Langanais where there were no nests in 2000 there are now 18. At these sites birds are nesting on inaccessible ledges on cliffs and in caves. It appeared that a redistribution was taking place with birds deserting traditional boulder areas, where they were easily susceptible to predation from rats and moving to more inaccessible sites. These sites are, unfortunately, rather scarce on Canna hence the overall decline in numbers.