Friday, 18 April 2014

Silvereyes


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.

1 comment:

  1. 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.
    Great article, thanks.
    PW

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