|Subject:||Re: Bird Migration|
|Date:||Tue, 26 Aug 2003 13:21:47 +1000|
Further to Hugo's examples of migratory and non-migratory forms of the one species.
Before venturing into honeyeaters I was involved in research on egrets. Cattle Egrets are fascinating. They one of the most successful self-colonising birds in the world spreading from core populations in Africa and SE Asia to every continent bar Antarctica (and getting close there - they have been found on Macquarie Island!). The movement patterns (migration if you will) of Cattle Egrets lends itself to this success. For those who have access to a decent library, and are interested, have a look at Maddock M. & Geering, D.J. 1994. Range expansion and migration of the Cattle Egret in south-eastern Australia and New Zealand: Implications for the spread of a species. Ibis 65:191-203.
In short, individual Cattle Egrets undertake a range of migrations. Take a typical breeding colony near Grafton in northern NSW, for example. First year Cattle Egrets from this colony would winter as far away as south-east South Australia, Victoria and Tasmania. Some birds even made it to New Zealand. These birds would return to the natal colony to either breed or, often, to see how it's done. The next year many of these birds would be in the same paddock as the previous winter. We had numerous examples of this in Melbourne and Gippsland where we had plenty of observers looking for our tagged birds. One bird returned to the same sporting field in Sale, Vic. five consecutive winters, returning to it's natal colony each summer. At the other extreme there were Cattle Egrets from the same colony that would "migrate" across the river and winter in paddocks with several kilometres of the br eeding colony. Again the same pattern emerged. Within the two extremes we would have birds wintering 100km, 200km , 500km etc from the natal colony. Wintering flocks would often contain birds from the whole range of breeding colonies in northern NSW and southern Queensland. Most would intimately exhibit this loyalty to the wintering site and most would return to the natal colony to breed.
However, there were exceptions. A certain proportion of the population would change wintering sites, some settling into a routine after the second winter. Some birds wouldn't return to the natal colony but breed at another colony, up to 500km away from where they were born.
This whole strategy is the key to the species success. Imagine the first Cattle Egrets to migrate to New Zealand. How did they know it was out there? Did they know it was out there? The same could apply to Cattle Egrets coming to Australia from SE Asia or, even more amazingly, from Africa to the Americas!. There is a very good chance that these adventurous birds could perish either by not finding land or finding no suitable habitat. If the entire population did this then there's a good chance of total failure. However, by having a mix of migratory strategies within the population you have birds that are pioneers while there are others that "stay at home" thus ensuring that if the pioneers don't return then a nucleus of a breeding colony remains. If all birds returned to the natal colony then expansion is not likely to succeed. There must be birds prepared to breed away from the natal colony, joining other egrets - in the case of the first Cattle Egrets in Australia other species of egret, to push the frontiers of the empire.
I don't know about you but I find this mighty fascinating stuff!
Regent Honeyeater Recovery Coordinator
NSW National Parks & Wildlife Service
P.O. Box 2111
Dubbo NSW 2830
Ph: 02 6883 5335 or Freecall 1800 621 056
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