Bird watchers have noted that there is an observable difference in the size and colouration of some sedentary, but widely distributed Australian birds. This is understandable, for Australia has a large landmass, and a variable climate. These factors have led to some birds in the above category to develop differences of size and colour. Several rules, if remembered, will help us to understand, appreciate these differences, help with identification and make our birdwatching all that more interesting.

Bergmann’s Rule: This is an observation that as one moves to the north of Australia from the south, birds of a widespread species become smaller. This is seen for example, in the Wedge-tailed Eagle and Masked Owls. (The largest of these species are the Tasmanian races and the smallest are in the north of Australia). Where there is a clear contradiction of "Bergmann’s Rule", it is often evidence that the population involved may represent a different species. It is interesting to note that the Australian Magpie of Tasmania is a contradiction of the rule, being smaller than the Magpie on the mainland. Some have suggested that it is an insular species. Remember this and note it as you travel around.

Geiger’s Rule: This is an observable phenomena seen as one moves from a humid habitat to an arid habitat. Birds of a widespread species found in both habitats become paler as the habitat becomes more arid and dry. This is seen very clearly in such birds as the Black-faced Cuckoo-shrike. Again there are exceptions, such as the Grey Fantail. This again may suggest a different species, as is recognized in the case of the Mangrove Fantail. It is very much identical to a Grey Fantail, but is much lighter still than the Grey Fantail of the arid country.

Check out these "rules" as you travel and see the different birds in our country. They are remarkably true to what one can observe, and show the incredible diversity in the world around us.

This article was published in The Christian Bird Observer’s Magazine, April 2001