There is more to Silvereyes (Zosterops) than
meets the eye!
I should start this post by pointing out
the silver does not refer to the eye, which is brown, but to a bold ring
surrounding the eye, which is not silver but white. Bird names can be baffling
at times!
The Silvereye has a brown eye and bold white eye ring. |
There are seven species of Zosterops
currently recognised in the BirdLife Australia Working List of Australian
Birds. Some have the common name white-eye however the subject of this post,
the Silvereye, is Zosterops lateralis.
Lateralis is a polytypic species, meaning a
bird with strong sub-speciation giving rise to many sub species or races,
lateralis has nine sub species or races. The subject of this post is the
Tasmanian race, Zosterops lateralis lateralis and the South-eastern race, Zosterops
lateralis westernensis, both sub species being found in south eastern Australia
and more specifically in East Gippsland Victoria.
This is the South-eastern Silvereye, Zosterops lateralis westernensis - note the grey flank. The bird is carrying the fruit of a Cherry Ballart (Exocarpos cupressiformis) to a nearby nest. |
The Tasmanian Silvereye Zosterops lateralis lateralis - note the rich brown flanks. |
Before banding of this species in the early
50’s it was thought birds in the Sydney region, lateralis cornwalli, changed
plumage colour over the course of the year in association with breeding.
However banding showed there were many sub species with recognizable plumage
differences, though sometimes subtle, and that the east coast sub species were
strongly migratory with birds moving south in spring to summer breeding grounds
and then in autumn moving back north again. So the plumage colour changes were due
to one sub species leaving and another turning up each spring.
The most southerly east coast sub species
is the Tasmanian sub species lateralis lateralis. This is the only sub species
found in Tasmania, and then only over the summer months when it breeds. Further
north in Victoria we have the South-eastern race, lateralis westernensis, and
further north still the Eastern race, lateralis cornwalli, and even further
north the North-eastern Silvereye, lateralis vegetus.
All of these sub species migrate north and
south each year with the more southerly sub species moving further than the
northern ones. The Tasmanian sub-species twice yearly flies across Bass Strait,
a notoriously fickle stretch of ocean subject to sudden and dramatic weather
changes and extreme sea conditions.
Today most people with some knowledge and
interest in birds will be aware of the epic long distance migratory flights
made between the southern and northern hemispheres by sea birds and shore birds
which cross long stretches of open ocean along their flyways. However the
migratory flights of land birds are less well known, and further, it seems
somewhat inexplicable that tiny land birds such as Silvereyes would migrate twice
a year across a potentially hostile stretch of open sea such as Bass Strait.
To understand why they do this we need to
think in terms of thousands of years and think back to the last ice age when sea
levels were much lower than they are at present and Tasmanian was joined by a
land bridge to the mainland. Under these conditions it can be appreciated that
land birds could develop the habit of moving south to breed in summer and north
again in autumn to avoid cold winters and associated food shortages.
As the sea level rose again, at about 1
metre per century, some land birds continued their annual migrations flying across
increasingly longer stretches of sea until Bass Strait reached its current
configuration about 10,000 years ago. Small land birds that are known or
presumed to undertake annual migration flights across Bass Strait include, Striated
Pardalote, Grey Fantail, Welcome Swallow, Tree Martin,
Satin Flycatcher, Dusky Woodswallow and Silvereye. Presumably these species
have been doing this for the past 10,000 years as it would seem unlikely that a
small land bird would develop an annual migration cycle that involved having to
undertake a crossing of Bass Strait as it is today.
Another factor to keep in mind when
thinking about bird migration is the hard-wired genetic basis for migration.
Birds do this automatically, it is not something they need to learn. They do
not think with intelligence as we understand it, oh autumn is approaching, I
must start heading north. It would appear that Silvereyes, following the summer
breeding in Tasmania when birds have dispersed and taken up individual
territories, gather together with the newly fledged young birds into flocks to
undertake migration north. The migration flights are often at night and follow
well established paths. While the general stimulus for migration may be
hard-wired it may be that young birds do learn to some extent the migration
routes by joining in flocks with older experienced birds.
In addition to following the same migration
routes each year birds also return each spring to the same areas to breed. This
no doubt is why over many thousands of years genetic diversity to the sub
species level is maintained by birds that migrate to and breed in
geographically separate areas. If they did not follow these regular patterns
and intermingled to breed then the gene pool would homogenise and the sub
species would disappear over time. So it is not surprising that a species such
as the Silvereye, which has so many sub species, is a strong migrant with
separate breeding areas.
So each autumn in Victoria we enjoy an
influx of thousands of Silvereyes from Tasmania, the sub species lateralis
lateralis with the rich brown flanks, as they pass noisily through our parks
and gardens stopping here and there on their way north to feed on the fruits of
summer and insects still active ahead of the coming winter. Some Tasmanian
birds reach as far north as southern Queensland with others happy to stay in
Victoria for the winter. And the sub species lateralis westernensis that we saw
breed here in Victoria over summer has headed north for the winter. Of course
there are always a few exceptions to the rule with some sub species failing to
undertake the annual migration.
Tasmanian Silvereye perched in a Tree Violet (Hymenanthera dentata) which is loaded with berries. |
There must have been a hundred or more Silvereyes in and around four Tree Violets all very noisy and active. They flew down a dozen or so at a time from larger wattle trees to feed on the berries. |
An interesting bird the Silvereye. I am particularly
impressed by the stability of the lateralis lateralis genetic code, which has
seen this gregarious and active little land bird continue to undertake the
hazardous crossing of Bass Strait twice a year, without fail, for the last
10,000 years. Of course the rising sea level lead to a 10,000 year isolation
for many species in Tasmania that could not make the sea crossing, including
humans, but that is another story.
A wonderfully informative entry about a great little bird John. I'm with you - the migration across Bass Strait by our little birds has always impressed me.
ReplyDeleteGreat article, thanks.
PW
I'm in North Tasmania, our silvereyes are observed in autumn and into winter, does this suggest a different subspecies...or?
ReplyDeleteI'm in North Tasmania and I observe the white eyes predominantly in Autumn and into winter, does this mean they're a different subspecies...or?
ReplyDeleteLynnie T All Silvereyes in Tasmania are sub-species lateralis lateralis. Not all birds migrate to the mainland with many overwintering in Tasmania. For the birds that stay there would/could/may still be some movement north for winter within Tasmania. Why some birds migrate and others stay I can't say. There is still a lot to learn about birds - their genetics and movements. In East Gippsland there have been many reports of the Tasmania sub-species turning up here in recent weeks.
ReplyDelete