For the past few nights Amy and I have been out trying to record the calls of Manx shearwaters from their burrows. This is to build up a set of recordings we can use later in the season for a response rate project we are undertaking with our RSPB Conservation Science department.
Manx shearwaters are nocturnal on land, coming back to their burrows under the cover of darkness in an attempt to avoid marauding gulls. Their bodies are adapted for swimming so their legs are set far back making them relatively cumbersome on land. As a further safety mechanism they breed underground in burrows. All in all this makes them a difficult species to census accurately. This is done, in part, by playing the call down burrows by day and calculating a 'response rate'. It is this element of the census method we are investigating with this project.
Shearwaters have been the unparalleled success story on Ramsey since the RSPB bought the island 20 years ago. Up until 1999 rats roamed at large having been accidentally introduced by shipwrecks in the 1800's. Burrow nesting seabirds have evolved over millions of years to breed on islands free from ground predators. They have no defense against rats, stoats, weasels etc who find their eggs and chicks easy prey.
Following eradication in 1999/2000 the Manx shearwater population on Ramsey has increased from 850 pairs to 3,800 in 2012.
Below is an extract from a 'duetting' male and female in a burrow on the NE slopes on the night of 1st May. It is possible that at least one of the pair in this recording is an immature bird investigating potential nest sites and partners (they do not breed until they are 5 or 6 years old) - certainly after a lengthy spell of interplay the female can be heard to leave the burrow. The noise reaches a crescendo as the bird can be heard to walk right past the microphone on her way out (it's worth listening to the end!). There were plenty of immature birds around that night - they could be seen poking their heads into burrows or tentatively creeping into one before being given an angry rebuff by one of the rightful occupants
The male is the slower. higher pitched call while the female is the lower pitched, harsher call with a slightly quicker tempo.
Click on link below to listen
Manx shearwater (photo: Dave Boyle)