As part of our research project looking at petrel predation by great skuas on St Kilda, this year we have been ringing Leach’s petrels and European storm petrels on Hirta in reasonably large numbers. Petrels have been caught at two study sites (Village Bay and the boulder fields at Carn Mor) using mist nets after dark when the birds visit land. When caught, each petrel is fitted with a minute light-weight alloy ring, engraved with a unique number to identify that individual bird. Measurements of wing-length, weight and breeding status (brood patch scores) are also made for each bird. This helps generates a data-set of measurements for the year and may allow comparisons between birds caught at a breeding site (Carn Mor) and birds prospecting for nest sites, caught in Village Bay. A number of petrels ringed in previous years have been re-captured in 2007 and this helps provide the information for which ringing is most intended: information on the movements and age of individual birds.
Will releasing storm petrel
On the Carn Mor study site we have re-captured several Leach’s petrels that had been ringed previously. Somewhat surprisingly, all these birds turned-out to have been ringed originally at Carn Mor! This provides further evidence of how this species returns to visit the same colony again and again – which is quite remarkable, given that Leach’s petrels spend the entire winter roving vast distances over the open ocean, totally out of sight of land. One Leach’s petrel that we re-captured at Carn Mor had been ringed in 2005. This bird was fully grown when first caught (therefore was at least one year old in 2005), so we know this petrel now must be at least three years old or older. Occasionally, petrels with rings have been found amongst the regurgitated prey remains (pellets) of great skuas. So far this year we have not found any ringed petrels among the skua pellets we have collected, despite recording a high incidence of petrel predation by skuas on Hirta. The only bird ring found in a pellet this year was from a ringed puffin. This unfortunate bird had been ringed on St Kilda as a fledged chick (a ‘puffling’) in 1980.
From a general perspective, the Carn Mor study site is awesome for seabirds and is a real pleasure to observe. Puffins, Manx shearwaters, European storm petrels and Leach’s petrels all breed at the site and are highly active from dusk until dawn. The species’ peak activity is staggered through the night and as we arrive at the site, around sunset, the air is full of hundreds and hundreds of Puffins, swarming together in a great swirl of flight and noise before the light begins to fade. The adults and pufflings are also very vocal from within their burrows at this time of day, and on reaching the site the sudden (and comic!) ‘Brrrrruuuuuuuurrrr’ calls of underground adult puffins can be startling! The puffin activity slowly decreases through dusk, to give way to a variety of very bizarre squawk-purring sounds - the sounds of shearwaters. Calling Manx shearwaters have been likened to ‘cockerels with asthma’ and during the first hour or so of darkness the birds are very noisy, with weird and spooky calls coming from their nest sites in burrows and under boulders. The shearwaters are occasionally seen at this time of night, soaring in off the sea like shadows, along with the first European storm petrels. Relative to the shearwaters, the petrels are extremely tiny and, rather than gliding, they speed around bat-like before invisibly landing. Like the shearwaters, both European storm petrels and Leach’s petrels nest in burrows which they only visit at night. The song of storm petrels begins emanating from the burrows between midnight and 1am and it too has a purring quality, although not nearly as harsh as the Manxies’! The last birds to become active at Carn Mor are the Leach’s petrels. Slightly larger than European storm petrels, the Leach’s petrels come in on an erratic, swoopy, jerky flight and call with an utterly extraordinary ten-syllable ‘chatter’-call, sounding like an arcade machine gone haywire! The wait until 2am is always worth it for the Leach’s petrels. At the darkest point of the night, the air and the earth fill with their calls as a mass-assembly of petrels are seen, heard and felt as they chatter and zoom past incredibly close-by - seabird spectacular!
Will Miles, PhD researcher