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Surveying shearwaters

28 June 2012

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.  Large entrance to a shearwater burrow, with a feather indicating activity

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.

Shearwater calibration plot on grass slope above Carn MorPhoto: 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.Tape player marking the entrance of a shearwater burrow

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!Lots of shearwater burrows in the calibration plot

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.

Gina



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