22nd February 2018
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Good breeding season in isles

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So far the seabird breeding season in Shetland has been good, with even Arctic terns fledging young. After the disastrous breeding success of recent years, this is welcome news.

Arctic terns, or tirricks, usually return to the islands in the first week of May, having wintered in southern oceans, some reaching as far as the Antarctic pack ice. Nesting in short vegetation close to the sea, and on shingly beaches, they mainly feed on sandeels, the Shetland populations being very sensitive to any changes in food availability.

When sandeels are scarce, breeding success is low. Arctic terns feed by hovering above the sea before the final plunge, or by swiftly snatching prey from the surface. Earlier in the summer, on fine, calm evenings, I watched those hunting insects; hawking like swallows low over the fields.

A more elusive seabird is the storm petrel, the smallest of the UK’s seabirds. One of the best places in Shetland to see them is Mousa where friends and I enjoyed a memorable late evening/early morning visit this week. The breeding storm petrel population on Mousa is estimated at 6,000 pairs – two per cent of the world population. However, during the day their presence is only hinted at by an occasional churring or a waft of musky petrel smell. After dark the island comes alive as these small seabirds, about the size of a swallow, come ashore to change partners, incubating the solitary egg, or to feed the newly-hatched chicks.

Eggs are laid from mid-June through to August and the incubation period is 40-50 days, each partner taking a three day shift. Storm petrels mate for life and return year after year to the same nest site. During the day they feed offshore on small crustaceans, squid and small fish. The first chicks generally hatch in the third week of July, fledging in September. Later chicks fledge in October and even into November. The chicks are brooded for the first week but then remain alone in the darkness of the rock crevice while the parents forage for food, returning after dark to feed their chick on regurgitated food.

The top of the broch is a great vantage point from which to observe the incoming petrels, their white rumps visible in the dim light. One skimmed my friend’s hair while another flopped down at our feet. Descending the narrow stairs, there were churring, hiccoughing calls from all directions from within the stone walls of the broch. As Tom Jamieson shone his red-filter torch (to minimise disturbance to the birds) on the broch walls, they could be seen circling the tower and clinging to the ancient broch stones. Storm petrels also nest in the dry-stane dykes and in the boulder beaches on Mousa.

Elsewhere, common crossbills continue to be recorded throughout Shetland; there was a green sandpiper at Quendale and an osprey at Sandwater.

Joyce J M Garden

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