Andy Douse is Scottish Natural Heritage’s senior ornithologist. He has recently been working on Rum National Nature Reserve counting and observing the Manx shearwater colony. Here he tells us what that work involved.
Despite the fact that it was June, a cold mist blew in off the summit of Hallival, enveloping me temporarily and distancing me from the sanctuary of the hut below.
My routine was well-established : the digital recorder set to the right recording, clip-board and recording sheet ready, and so I pressed the play button. The discordant, demanding call of a male Manx shearwater blared out and I directed it down the open burrow entrance. The recorded call itself lasted for 25 seconds. However, within ten seconds of the start, a high pitched call sounds from deep down the burrow, similar but noticeably different to the recorded call I used.
The response was carefully logged, I packed-up and moved on to the next burrow, only 10m away and started the process over again. Over the month of June I repeated this nearly 2,000 times, different calls directed down 60 burrows. Each burrow got one of four different call types on a rotating cycle. The reason for all this?
Well, I’m trying to work out how best to count breeding Manx shearwaters. Counting shearwaters is harder than you’d think. Yes, you can count the holes in the ground but not all holes in the ground are occupied, and most of the burrows are too long and may have passages so convoluted that seeing what’s at the end is almost impossible.
So we have to ‘encourage’ the occupants to tell us that they are at home. Again that is harder than you might think. They don’t always respond and it’s believed that males only respond to male calls and females only respond to female calls. We need a method that will tell us what how many burrows are actually occupied, one that takes account of the fact that it could be occupied by the male or female of a pair, or possibly both.
Why do we need to know this? The colony on Rum is BIG, indeed very BIG probably, but counting it has always been a challenge and previous counting methods may not have given as accurate figures as we can compile today. We are developing a method to count all the burrows (a challenge in its own right over an area the extent of the Rum Cuillin), but in addition we need to know how many of those burrows are actually occupied, and that’s where I come in.
A month on Rum in June gave me the data I needed. Colleagues in RSPB will do the analysis but it’s clear that we’ll need some combination of male and female calls, but we also need to know when is the best time to this – early incubation or later, and does time of day have an effect on likely response probability? All this is leading up to another census, but hopefully with a result that we can really look upon as being definitive.
The Manx shearwater colony on Rum is undoubtedly of international scientific importance for its size. But beyond that I have to admit a fascination, an admiration and deep respect for a bird that spends about seven months of the year flying down into the waters of the South Atlantic and back again, before settling back again into a burrow, deep on a rocky hillside on Rum, and aiming as they do every year, to raise a single chick.
Find out more about Rum National Nature Reserve @ http://www.nnr-scotland.org.uk/rum/