Thursday, 12 March 2015

Kiwis and yellowhammers

The Yellowhammer (Emberiza citronella) is a small bunting found across temperate Europe and Asia. 

Surprisingly, it is also common in New Zealand. 

But how on earth did this species get so deep into the southern hemisphere? 

The blame lies with so-called “acclimatisation societies”. In the days of colonialism, these organisations were set up in many British colonies in the belief that the local fauna was in some way deficient and could be improved by introducing species remembered from the British motherland. It is for this reason that the European Starling is now a widespread pest across North America and the rabbit has had a disastrous impact in Australia.

But Yellowhammers in New Zealand? In the middle of the 19th century, the country’s population was growing rapidly. To a large extent the settlers’ diet depended on introduced cereals. But these crops attracted insect pests such as caterpillars and black field crickets. 

Back in Europe, insectivorous bird helped to keep such pests under control. But Kiwi settlers had cleared away New Zealand’s forests, and many insect-eating native birds had disappeared with them. 

It seemed to make sense to protect the crops by introducing insectivores from Britain. But strangely, the main species chosen by the acclimatisation societies was the Yellowhammer — strange because the main food of this heavy-billed bunting is seeds rather than insects, although they do tend to use invertebrates as an additional food source in the breeding season, when they feed caterpillars and other insects to their nestlings. 

In the 1860s and 1870s, consignments of Yellowhammers were carried on no fewer than 25 ships sailing from London to New Zealand ports. A quarter of these shipments were organised by a family in Brighton, and many of the birds had been trapped around this East Sussex town.

At first Kiwi farmers welcomed the immigrants, but they soon began to realise that the newcomers actually aggravated the problem since, rather than eating the insect pests, they would feed on both the newly sown seeds and the subsequent cereal crops. 

Nevertheless, with government support, the acclimatisation societies continued to promote the introduction of Yellowhammers until 1880, when public pressure forced the rejection of the final shipment. (It was sent on to Australia, where Yellowhammers — unlike several other introduced Old World passerines — failed to thrive.)

From then on, New Zealand treated the Yellowhammer as an unwelcome immigrant and encouraged efforts to wipe it out by shooting, poisoning and egg collection. But, despite a bounty placed on the birds and their eggs, it was too late. With no major competitor among native species, the Yellowhammer rapidly became established. 

Today the Yellowhammer is a common inhabitant of open country across much of the NZ mainland and many of its offshore islands.

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