By Rosemary Penwarden and Derek Onley
Song thrushes have started singing madly again; they're maintaining
their territories and beginning to nest. They usually sing through the
autumn, are far less enthusiastic mid-winter, and then start singing
again in early spring. August is nesting time, usually in the same
place as last year. May's heavy rain got them singing madly for a
while; it brought worms to the surface – easy pickings, and part of a
varied diet that also includes fruit and insects.
Blackbirds, on the other hand, hardly ever sing in the autumn. They
don't usually start their more laid back whistly tune until mid- to
late August, and nest a couple of weeks later than the song thrushes.
Only female blackbirds and song thrushes sit on eggs, so the males of
both species keep singing to defend their territory. They feed the
nesting females to help in their egg production ('courtship feeding'
in ornithological language). Both mum and dad feed the babies – so the
males get too busy to sing once the chicks appear. It takes about two
and a half weeks for the eggs to hatch and another two for the chicks
to fledge. After fledging the males keep feeding the little guys for
another couple of weeks. Females help with this part too, but also
build another nest and lay more eggs.
Song thrushes tend to like the open paddocks more than blackbirds do.
There are hardly any song thrushes in native forest, but heaps of
blackbirds, which are probably one of the commonest birds in New
Zealand now. You will find them not only on your lawn and in sheep and
cow paddocks but also on offshore islands, in forests and even up in
the sub-alpine areas.
One thousand blackbirds and several hundred song thrushes arrived in
New Zealand via the Acclimatisation Society between 1862 and 1878,
introduced for 'sentimental reasons'. By 1900 blackbirds had colonised
the whole of the country including the Chatham and Auckland islands,
and song thrushes had done the same, though they kept out of the
forested areas.
Singing blackbirds and song thrushes both feature in our Orokonui bird
surveys in the spring, but there are more blackbirds. Song thrushes
and blackbirds nest all through the Orokonui Ecosanctuary but song
thrushes feed outside the fence while blackbirds feed both inside and
outside.
The ecosanctuary has given us a wonderful opportunity to look at
nesting behaviour. Outside the predator-proof fence the predation rate
is so high – 70 to 80%, due to cats, rats, stoats and possums – that
the males are singing nearly all the time. This predation rate is par
for the course for all small birds (passerines) in New Zealand. Of the
common native birds remaining in our forests and gardens, all have
learnt to adapt to losing this many young.
Outside the fence blackbirds and song thrushes would have up to five
or six clutches every season. Inside the fence, where the risk of
predation has disappeared, you might expect the Orokonui forest to be
crammed full of blackbirds, but it's not, for two possible reasons.
One: once they reach a certain number of healthy young, they don't
bother nesting again, and two: there is not enough food. Habitat,
which means a dependable food source, is just as important as
predation on bird populations.
The oldest recorded blackbird in New Zealand is 15 years old. Song
thrushes and blackbirds have an amazing variety of tunes. Both can
mimic -- telephones, other birds, whistling the dog. I love the
blackbird's lower pitch and altogether lazier sounding song best, as
if he's singing the blues. When he sings, you know winter is over.
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