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