Thursday, 23 June 2011

A storming success (hopefully!)

Just in the last week, the numbers of Storm Petrels present in the colony has noticeably increased. This has also coincided with some fairer and calmer weather, making fieldwork more enjoyable and enabling more people to come out to Mousa and enjoy the spectacle of watching the Storm Petrels returning to the colony as darkness falls. Mousa Boat Trips ( operate two night trips every week between late May and late July, bringing people over to witness this display. Few trips have actually run so far this year, however, due to the relentless strong winds making Mousa Sound impassable.

The Mousa Ferry, which offers daytime and night trips to Mousa during the summer months, at the pier in the West Ham

Due to their small size and subsequent vulnerability to predation, all aerial activity at the colony occurs during darkness. Birds start to arrive about 30 minutes after sunset and, by the time the sun has fully risen, no birds are left to be seen, having either departed to feed far out to sea or tucked themselves up safe in their nesting cavity. At this time of year – midsummer – at 60˚ north, this offers a very narrow window for activity. Birds return to the colony from mid May onwards to re-establish their territory and bond with their mate. Year after year, birds return to exactly the same nest-site to breed with the same mate. Those fortunate enough to visit a Storm Petrel colony at night can observe aerial courtship displays and hear birds furiously singing from their nest-sites.

The 'Simmer Dim' - as dark as it gets in Shetland at midsummer

Saturday’s visitors witnessed a spectacular display of numerous Storm Petrels whizzing around the exterior of Mousa’s Iron Age Broch. This phenomenal structure is at least 2000 years old and it’s 2-metre-thick stone walls offer a maze of cavities and crevices for Storm Petrels to breed in. Flash photography is not allowed, to avoid disturbance to this internationally-important colony, which is recognised and protected under European law; consequently, I can’t show you any images of this spectacle, you’ll just have to come and see it for yourself one day.

Mousa Broch and the boulder beach, home to thousands of breeding Storm Petrels lit up by a full moon. On lighter nights like this, birds tend to return to the colony a little later than on darker, cloudier nights.

Prior to laying, birds regularly visit their nest-site at night and occasionally occupy the site during the day. In late May, daytime inspections of nests revealed attendance at a maximum of 5 nests out of almost 90 that were monitored in 2010. Gradually, occupancy has increased, with a peak (so far) of 31 nest-sites occupied yesterday. The first egg was recorded on 10th June – exactly the same day as the first egg was recorded last year. As of today, 17 pairs have laid an egg. Most of these birds have just laid within the last 5 days. Visibility inside nests is typically limited – one of many challenges of studying a bird that’s only active within the colony during darkness and nests in cavities out of site! Consequently, the egg is often never seen and laying is only confirmed by the presence of a bird on 3 consecutive days. The breeding period is greatly protracted in Storm Petrels; last year the first egg was recorded on 10th June and the last on 2nd August! Previous research in the late 1960s by Derek Scott on Skokholm revealed that it is the experienced birds that lay earlier in the season and the newly-formed pairs and inexperienced birds that lay later on.

Despite the poor season for Shetland's other seabirds, let's hope 2011 brings better fortunes for the Storm Petrels. See the RSPB's latest press release reporting the success of the Mousa colony at

Wednesday, 8 June 2011

Back to basics

Relentless force 6-7 south-westerly winds have not helped the 2011 field season get off to an easy start. Stuck on mainland Shetland for the first 10 days or so after arriving, the forced imprisonment in a caravan at least provided plenty of time to crunch data (groan). The hammering of the rain on the roof and the unsettling rocking of the caravan, however, was not particularly comforting or conducive to focusing on statistics. Terrified that the caravan and I might be blown to Norway at any given moment, I breathed a sigh of relief when the wind eventually abated...though only just for long enough for us to get ourselves to Mousa and settle into some more stable accommodation, albeit not very water-tight!

The Mousa bothy...home sweet home

Needless to say, the bothy needed a bit of a clean and tidy following the accumulation of a winter’s worth of dust and grime and having been providing shelter to wheelbarrows, sheep-shearing gear, fencing etc. It soon felt like home again, though we’ve already spent more time than desirable being enclosed by its four dull, grey, windowless (well, there is just one small window) walls. I would say it could only get better, apart from after only a couple of days off Mousa, we already find ourselves once again stuck on the mainland.

Spring-cleaning of the bothy

In between the showers, we’ve been blown around Mousa for the last two weeks, trying to relocate nests which were monitored last year...not so easy when one boulder looks much like any other. Automated cameras and motion sensors have been re-installed to continue monitoring visitor pressure in different areas of Storm Petrel nesting habitat around the island. On our first full day back (26th May), we checked 47 nests and found 6 of them were occupied by birds. The birds haven’t laid an egg yet, but daytime occupancy suggests these will be early breeders. Last year, I recorded the first egg on 10th June, so, any day now, we might see our first egg. The fortunes of Shetland’s other seabirds this season are not looking too rosy. Gales and large swells have washed out many Guillemot nests at Sumburgh Head, the Arctic Terns (tirricks) at Grutness don’t seem to be doing anything, and even the Bonxies aren’t numbering as many as last year. Fingers crossed, the season holds more promise for the Storm Petrels. The fact that Storm Petrels feed lower in the food chain than other seabirds (mainly feeding on plankton) and can forage at such great distances from the colony, might be expected to make them more resilient to locally-unfavourable environmental conditions. The story will slowly (remember Storm Petrels incubate their eggs for 40 days and rear their chicks for about 60 days!) unfold over the course of the next few months...