Manx shearwaters are a notoriously difficult species to count owing to their nocturnal and burrow-nesting habits. Burrows are usually deep enough to conceal an incubating bird but there may be signs such as disturbed or flattened soil and vegetation, droppings, feathers, or the smell of shearwaters.
Photo: Flattened grass and a feather at the entrance of this shearwater burrow
This year, the number of shearwater burrows were counted within four plots that were established in 2007.
Photo: Shearwater plots marked in red at the top of the slope above the boulder field of Carn Mor
A taped call of a male and female shearwater was played at the entrance of every burrow within the plot, regardless of whether there were any signs of occupancy. The sampling is carried out during the day, when fewer non-breeding immature or prospecting birds are present in the burrows, and the survey is conducted in early June, before any chicks have hatched as they are left unattended by day once they are about a week old.
We know that not all birds respond to the tape so population estimates must be corrected to take account of non-responding birds. One of the established plots was used as a calibration plot and it was visited daily. Each day, all new responding burrows were marked with a flag and the ID number of every responding burrow (whether new or marked on a previous day) was recorded, until no new burrows were found. Exactly 100 burrows were located during a ten day period!
Photo: Arrows mark the locations of burrows with this small section of the calibration plot
Counts of burrows in established plots can be used to monitor population trends over time. This year 153 responses were made in the four plots (equates to 322 Apparently Occupied Burrows when the correction factor is applied). This is significantly higher than previous years and indicates that Manx shearwaters are currently faring well on St Kilda.