26th September 2018
Established 1872. Online since 1996.

Crossbills and golden plovers take a bow

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NOTABLE migrants still around this week were crossbills at Lerwick, Busta House and Cunningsburgh.

Common crossbills are thick-set, powerful finches and, as the name suggests, the large bill has the mandibles crossed at the tips. The males are a colourful orange-red while females are greenish-grey with yellow rumps. They are noisy birds, often calling attention to themselves by loud metallic “chip, chip” calls.

Crossbills are specialist feeders on conifer seeds, especially those of spruce, larch and pine. Agile and acrobatic, they behave like small parrots, using their feet to hold down cones and then prise apart the scales with their bills to extract the seeds. They also eat seeds, berries, buds, shoots and invertebrates. Crossbills arriving in Shetland can only find suitable coniferous trees at a few sites such as Kergord so need to vary their diet to find enough food. They breed across Europe, North Asia and North America where there are forests of conifers. Birds arriving in Shetland are from populations in Scandinavia with most occurring in the summer. They breed very early in the year and irrupt from their breeding grounds if there is in insufficient food available. Common crossbills also breed in Scotland and there is also a separate species of Scottish crossbill which has been recognised since 1980.

Waders are still on breeding territories this month. One of the most attractive is the golden plover with its spangled gold and black upperparts and black and white underparts. On the list of breeding birds in Shetland since 1733, golden plovers breed on blanket bog, moorland and on serpentine heaths in Unst and Fetlar. The Shetland breeding population comprises about six per cent of the British breeding population, while the entire Scottish population is about 80 per cent.

Golden plovers start visiting territories in February and March although eggs are not laid until May. The nest is a scrape in the heather or grass and the chicks leave soon after hatching. The adults are wary during the breeding season uttering liquid, mournful calls which echo over the moorland. A wailing song accompanies the butterfly display flight. Most food is taken from the surface or by shallow probing to depths of 1-2 cm, and consists mainly of earthworms, beetles and other invertebrates and also some plant material such as berries and seeds.

In the clear waters off West Burra last week, we enjoyed the sight of several jellyfish pulsing past.. These were moon jellyfish, the adults of which can grow as large as 40cm across. Moon jellyfish can be easily recognised by the four pink-purple rings near the centre of the bell which are reproductive structures. The pale blue translucent bell consists of two bell-shaped layers of cells, separated from each other by a thick layer of jelly. As the jelly is largely made up of water, stranded jellyfish quickly die and then disintegrate as the water evaporates. Suspended from the underside of the bell is a short tube, at the tip of which is the mouth. In the moon jellyfish the edges of the mouth form four frilly projections called oral arms. From the margins of the bell hang numerous, short hair-like tentacles which contain stinging cells. When a small animal touches the tentacles minute harpoons are shot out which contain a paralysing poison. In moon jellyfish the harpoons are not powerful enough to pierce human skin so they are harmless to us.

Moon jellyfish are distributed widely. In the North East Atlantic they occur from Arctic Norway to tropical areas and are also found throughout the Mediterranean and the Black Sea regions. They can be seen in coastal waters in swarms from April to September, feeding on a range of plankton, including fish eggs and small fish. These jellyfish can use the sun as a compass and form breeding aggregations in late summer.

Joyce J M Garden