Story: Introduced land birds
Page 8 – Blackbirds and song thrushes
Blackbirds (Turdus merula) are native to Europe, north-west Africa and the Middle East.
English settlers introduced blackbirds to New Zealand because their song was a nostalgic reminder of English life. Between 1867 and 1879 blackbirds were liberated on the three main islands, where they multiplied rapidly.
Within 60 years they had spread to the Chatham, subantarctic and Kermadec islands. On mainland New Zealand, blackbirds are now found in most habitats up to 1,500 metres above sea level.
Within 15 years of introduction, blackbirds were becoming a pest because they damaged fruit in orchards and spread the seed of unwanted plants such as elderberry and blackberry. As well as fruit, they also feed on worms, beetles, caterpillars and other invertebrates. Blackbirds sometimes play a useful role spreading the seed of some native plants.
Blackbird males are black with an orange bill and eye-ring, whereas females are dark brown with a streaky underside. They are about 25 centimetres long and weigh 90 grams. They have a warbling song and a piercing alarm call.
The oldest blackbird recorded in New Zealand was 15 years old.
Male blackbirds are so intent on defending their territory in the lead-up to nesting that they sometimes attack their own reflection. For several years, each spring residents of a Wellington house found their front door glass covered in blood-smeared beak marks and wing impressions.
Blackbirds return to the same breeding territory each year. The female builds a nest in a fork of a shrub or hedge, which it may reuse in subsequent seasons. They lay three or four blue-green freckled eggs, and raise up to three broods a year.
The song thrush (Turdus philomelos) is closely related to the blackbird, but is slightly smaller and lighter – about 23 centimetres long and 70 grams in weight. Both sexes have a yellow-brown back and wings, and lighter underside with regular rows of tapered brown spots. Their song is a series of repeated notes and trills.
Like the blackbird, thrushes were introduced for sentimental reasons and were soon considered a pest for damaging fruit.
Distribution and habitat
Since their introduction in the 1860s and 1870s, song thrushes have colonised all major island groups of New Zealand. They are common in most habitats, except for intact native forest.
Song thrushes specialise in eating snails, including introduced and native land and marine species. Using a rock, they smash the shell repeatedly until it breaks open. They also eat other invertebrates and fruit.
Nesting habits are similar to blackbirds, but thrushes’ eggs are clear green-blue with black dots.