Birds migration in the tropics and Australasia
Some migration routes
In the tropics there is little variation in the length of day throughout the year, and it is always warm enough for an adequate food supply. Apart from the seasonal movements of northern hemisphere wintering species, most species are in the broadest sense resident. However many species undergo movements of varying distances depending on the rainfall.
Many tropical regions have wet and dry seasons, the monsoons of India being perhaps the best known example. An example of a bird whose distribution is rain associated is the Woodland Kingfisher of west Africa.
There are a few species, notably cuckoos, which are genuine long-distance migrants within the tropics. An example is the Lesser Cuckoo, which breeds in India and winters in Africa.
In the high mountains, such as the Himalayas and the Andes, there are also seasonal altitudinal movements in many species.
Bird migration is primarily, but not entirely, a Northern-Hemisphere phenomenon. In the Southern Hemisphere, seasonal migration tends to be much less marked. There are several reasons for this.
First, the largely uninterrupted expanses of land mass or ocean tend not to funnel migrations into narrow and obvious pathways, making them less obvious to the human observer. Second, at least for terrestrial birds, climatic regions tend to fade into one another over a long distance rather than be entirely separate: this means that rather than make long trips over unsuitable habitat to reach particular destinations, migrant species can usually travel at a relaxed pace, feeding as they go. Short of banding studies it is often not obvious that the birds seen in any particular locality as the seasons change are in fact different members of the same species passing through, gradually working their way north or south.
Relatively few Australasian birds migrate in the way that so many European and North American species do. This is largely a matter of geography: the Australasian climate has seasonal extremes no less compelling than those of Europe; however, they are far less predictable and tend to take place over periods both shorter and longer. A couple of weeks of heavy rain in one part or another of the usually dry centre of Australia, for example, produces dramatic plant and invertebrate growth, attracting birds from all directions. This can happen at any time of year, summer or winter and, in any given area, may not happen again for a decade or more.
Broader climatic extremes are highly unpredictable also: expected seasonal heat or rain arrives or does not arrive, depending on the vagaries of El Niño. It is commonplace to have stretches of five or ten years at a time when winter rains do not eventuate during the El Niño cycle, and equally common to have La Niña periods which turn arid zones into areas of lush grass and shallow lakes. Long distance migration requires a heavy investment in time and body mass—and, given the random nature of El Niño, an investment with an uncertain return.
In broad terms, Australasian birds tend to be sedentary or nomadic, moving on whenever conditions become unfavourable to whichever area happens to be more suitable at the time.
There are many exceptions, however. Some species make the long haul to breed in far distant northern climes every year, notably swifts, and a great many wading birds that breed in the Arctic Circle during the southern winter.
Many others arrive for the southern spring and summer to breed, then fly to tropical northern Australia, New Guinea, or the islands of South East Asia for the Southern winter. Examples include cuckoos, the Satin Flycatcher, the Dollarbird, and the Rainbow Bee-eater.
Others again are altitudinal migrants, moving to higher country during summer, returning to warmer areas in winter such as several robins, or travel north and south with the seasons but within a relatively restricted range. The tiny 10 cm Silvereye is an example: most of the southernmost Tasmanian race crosses the 200 miles of Bass Strait after breeding to disperse into Victoria, South Australia, New South Wales and even southern Queensland, replacing the normal residents who fly still further north, following the band of fertile country along the coast, feeding through the day and travelling mostly at night. The northernmost populations, however, are nomadic rather than migratory, as are the Silvereyes of southern Western Australia, which is bounded by thousands of miles of desert to the north and east, and sea to the south and west.
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