Australia is a vast landmass covering almost 7.7 million km2. Much of it is dry except for the coastal strips and the mountains of the southeast and east, which are however prone to periods of drought.
Movement Patterns
More than 460 species of land-based birds breed on the continent. Their breeding is largely dependent on flowering and insect activity, which in turn is determined by the rather erratic rainfall that can occur anywhere and any time of the year in many parts of Australia. However, extreme cold like that in the Northern Hemisphere rarely occurs.
In response to the unpredictable conditions, many birds become:
- nomadic — constantly moving to wherever conditions are suitable at the time
- irruptive — suddenly leaving an area in large numbers only to appear at another
- sedentary — moving only locally within the bird’s territory or not far from it
- seasonal migration — although occurs, it is typically short-distanced compared to that of Northern-Hemisphere birds. Very few migrants leave Australia entirely to winter closer to or beyond the equator.
Partial Migration Observed
Thirty-five years ago it was thought fewer than 5% of land-bird species in Australia conducted north-south migratory movements, and a vast majority of those originated from Tasmania, a large island state at the southernmost part of Australia. Since then, with improved data, migratory movement has been observed in about 40% of species. What was thought to be nomadic movements in some species turned out to be regular migration between breeding and wintering grounds.
A high proportion of the sedentary species was also found to contain migratory individuals. The term “partial migration” has been assigned to such species. Partially migratory species with separate, morphologically different sedentary and migratory populations are found all over the world. The remarkable aspect of Australian partial migrants is that morphologically indistinguishable sedentary and migratory individuals are frequently found within the same population, rather than from distinct sedentary and migratory populations.
Silvereye: A Typical Partial Migrant
An excellent example is the silvereye Zosterops lateralis in which banded individuals from Hobart in Tasmania have been recovered in Sydney about 1,000 km away in some winters, but observed to have remained in Hobart throughout winter in other years. The question arises as to how a silvereye chooses to migrate or not.
Research has shown that silvereyes carried as much as one-third of their body mass in fat reserve prior to their 240-km return migration to Tasmania over the Bass Strait. However, some carried almost no fat. Presumably these lean birds stop over on small islands en route to feed. But why would they risk taking a hazardous journey unprepared?
When silvereyes captured while overwintering in Hobart and in a mainland site (Armidale, NSW) were tested for their annual movement pattern, it was found that social factors could influence the development of migratory movement in many of these birds. In the field, an individual may be drawn into a migrating flock even though it is physiologically unprepared to migrate.
Behavior Explained
Many terrestrial birds in Australia is gregarious or become gregarious after breeding. In the latter, birds are commonly seen moving in either single- or mixed-species flocks or both. Some of these may develop into migratory flocks. Such flocks have been seen migrating over Bass Strait.
Moving as a member of a group when conditions are variable increases the chance of finding food and, in the case of migration, of making the right decision to migrate or to defer migration. This may explain why many bird populations in Australia are partially migratory.
References
M. Blakers, S.J.J.F. Davies & P.N. Reilly (1984) The Atlas of Australian Birds. Royal Australasian Ornithological Union, Melbourne.
K. Chan (1994) Migratory fattening in an Australian intracontinental migrant. Condor 96: 211-214.
K. Chan (2001) Partial migration in Australian landbirds: a review. Emu 101: 281-292.
K. Chan (2005) Partial migration in the silvereye (Aves: Zosteropidae): pattern, synthesis, and theories. Ethology Ecology & Evolution 17: 349-363.
G.F. Mees (1974) The migration of the Tasmanian race of the Silvereye. Australian Bird Bander 12: 51-54.
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