Australian birds-migrants or nomads
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Australian birds-migrants or nomads? Text books often say that few Australian birds regularly migrate from one place to another—they are merely nomads, finding feed where they can get it. Mr Henry Nix, a scientist at the CSIRO Division of Land Use Research, is not so sure. He noticed, for example, that if he travelled to the brigalow in central Queensland late in autumn he saw large concentrations of many bird species that had disappeared from his home base of Canberra some weeks previously. Using a computer model previously developed to pre- dict the timing of plant growth over the whole surface of Australia, Mr Nix simu- lated where a bird would go to obtain enough food to survive. He also checked the breeding times and seasonal movements of birds at six representative locations in Australia and two in New Guinea to see whether these times coincided with pre- dicted peaks in plant growth. The results showed that at each location most species in fact breed in the quarter of the year when plant growth is at a maximum. For example, on the Southern Tablelands of New South Wales near Canberra there is usually a peak of plant growth during spring. Practically all the bird species that eat nectar, fruit, seeds, or insects breed during this period. Once the summer heat sets in, plants grow much less. In winter it is too cold for any plant The rainbow bird and the sacred kingfisher migrate regularly in both eastern and western Australia. growth, and many bird species disappear from the region altogether. Do they go to Queensland, or somewhere else? Mr Nix's analyses of when plant growth begins and ceases at nearly 500 locations in Australia and New Guinea suggest that bird movements would be confined within four distinct regions—eastern Australia, Tasmania, and New Guinea; north-western Australia, West Irian, Timor, and other outlying islands of eastern Indonesia; south- western Australia; and arid and semi-arid central Aus- tralia. Normally, very little mixing would occur between bird populations of these regions. Mr Nix included New Guinea and outlying islands because for birds they comprise one biological entity along with Australia. According to his calcula- tions, many of the migrant birds of south-eastern Aus- tralia over-winter in central and northern Queensland, but many continue through to New Guinea. Bird move- ments in the centre are largely nomadic, but there is some migration northwards in summer and southwards in winter. Perhaps the circular move- ment predicted for birds in south-western Australia is the most intriguing. Some evidence suggests that a num- ber of species—the inland subspecies of the white-tailed black cockatoo for example —really do follow it either wholly or in part. These birds breed in the wheat belt during late spring, and then move southwards and westwards to the more humid coastal areas. In autumn they move to the north and north-west towards Geraldton to over-winter, and then return to the wheat belt to breed. Evidence for the whole of Australia is sketchy, but Mr Nix considers that what there is tends to support his analysis. He points out that his predictions will not apply to every species, and the shapes of the four regions will vary each year depending on the seasonal conditions. Nevertheless, his ideas do create a pattern in what previously seemed like complete chaos. Environmental control of breeding and post-breeding dispersal and migration of birds in the Australian region. H. A. Nix. Proceed- ings of the 16th International Ornithological Congress, 1974 (in press).
Transcript of Australian birds-migrants or nomads
Australian birds-migrants or nomads?
T e x t books often say tha t few Aust ra l ian b i rds regularly migra te f rom one place to ano the r—they are mere ly n o m a d s , finding feed where they can get it.
M r H e n r y N i x , a scientist at t he CSIRO Divis ion of L a n d Use Research , is no t so sure . H e not iced, for example , tha t if h e travelled to the brigalow in central Queens land late in a u t u m n he saw large concentra t ions of m a n y b i rd species tha t h a d disappeared from his h o m e base of Canber ra some weeks previously.
Us ing a compute r model previously developed to p r e dict t he t iming of p lant g rowth over t he whole surface of Austral ia , M r N ix s imulated where a b i rd would go to obta in enough food to survive. H e also checked the breed ing t imes and seasonal movement s of birds at six representa t ive locations in Austral ia and two in N e w Guinea to see whe the r these t imes coincided wi th p r e dicted peaks in plant g rowth .
T h e results showed tha t at each location mos t species in fact b reed in the quar te r of the year w h e n plant g rowth is at a m a x i m u m . Fo r example , on t he Sou the rn Table lands of N e w Sou th Wales near Canber ra the re is usually a peak of p lan t g rowth dur ing spr ing. Practically all t he b i rd species tha t eat nectar , fruit, seeds, or insects b reed dur ing this per iod. Once the s u m m e r heat sets in, plants grow m u c h less. I n winter it is too cold for any p lan t
The rainbow bird and the sacred kingfisher migrate regularly in both eastern and western Australia.
growth , and many bi rd species disappear from the region al together. D o they go to Queens land , or somewhere else?
M r Nix ' s analyses of when plant g rowth begins and ceases at nearly 500 locations in Austral ia and N e w Guinea suggest tha t b i rd movements would be confined wi thin four dist inct regions—eastern Austral ia, Tasman ia , and N e w Guinea ; no r th -wes te rn Austral ia , Wes t I r ian , T i m o r , and other out lying islands of
eastern Indones ia ; sou th western Austral ia; and ar id and semi-ar id central A u s tralia. Normal ly , very little mixing would occur be tween bi rd popula t ions of these regions. M r N ix included N e w Guinea and out lying islands because for birds they comprise one biological ent i ty along wi th Austral ia.
According to his calculat ions , m a n y of the migran t birds of south-eas tern A u s tralia over-winter in central and no r the rn Queens land ,
bu t m a n y cont inue t h r o u g h to N e w Guinea . Bird move ments in the centre are largely nomad ic , b u t the re is some migrat ion no r thwards in s u m m e r and southwards in winter .
Perhaps t he circular move m e n t predic ted for b i rds in sou th -wes te rn Austral ia is t he mos t in t r iguing. Some evidence suggests tha t a n u m ber of species—the inland subspecies of t he white- ta i led black cockatoo for example —really do follow it ei ther wholly or in pa r t . T h e s e birds b reed in the wheat belt dur ing late spr ing , and t h e n move southwards and westwards to the more h u m i d coastal areas. I n a u t u m n they move to t he n o r th and nor th -wes t towards Gera ld ton to over-winter , and t h e n r e tu rn to t he wheat belt to b reed .
Evidence for t he whole of Austral ia is sketchy, bu t M r N i x considers t h a t what the re is tends to suppor t his analysis. H e points out tha t his predict ions will no t apply to every species , and the shapes of t he four regions will vary each year depend ing on the seasonal condi t ions . Never the less , his ideas do create a pa t t e rn in wha t previously seemed like comple te chaos.
Env i ronmenta l control of b reed ing a n d pos t -b reed ing dispersal a n d migra t ion of birds in t h e Aust ra l ian region. H . A. Nix . Proceedings of the 16th International Ornithological Congress, 1974 (in press) .
