Migration usually refers to the regular annual movements of animals from one location (usually a breeding area) to another location (a non-breeding area). Migrations are much more prominent in the northern hemisphere than the southern hemisphere, primarily because the northern continents extend to higher latitudes than southern continents. Many birds take advantage of the high productivity and long summer days to breed at these high latitudes, but must move away during autumn to avoid harsh winters, returning in spring. Many of these northern birds make intercontinental movements, and some cross the equator shifting to Africa, South America and even Australia, arriving at southern destinations from August to November and departing back to the northern hemisphere from February to May.
Much of man’s initial interests in migrations centred on understanding how the birds navigated, and the morphological and physiological mechanisms that were used to migrate. To make long-distance flights birds need to generate fuel stores, a combination of fat and protein. These stores are combusted during long distance flights to provide energy and are particularly important when crossing inhospitable areas such as oceans and deserts. Although satellite tracking has shown that some birds can fly continuously for days at a time, most species migrate by stopping frequently along the migratory route. During these stops the birds recover from the exertion of an extended period of flight and refuel before making the next part of the journey. This makes migratory birds particularly vulnerable to changes to the destination locations or to key stopovers along the route.
The most conspicuous northern hemisphere birds to visit Australia are a suite of migratory shorebirds including sandpipers, plovers, godwits and curlews. These birds breed in the Palaearctic region and move along the East Asian Flyway to Australia. A series of international migratory bird agreements between Australia and other Asian countries (e.g. China, Japan, Republic of Korea) have been established to help protect key habitats. Within Australia these agreements sit under the Environment Protection and Biodiversity Conservation Act, and signatory countries are obliged to protect the important habitats used by the birds in their respective countries. These agreements, in theory, should protect the habitats used by these birds but in practice this is not the case.
Growing human populations, coastal areas utilised by birds being developed, key wetland areas being reclaimed and area and quality of remaining habitats diminishing, almost all species of migratory shorebirds that visit Australia are in decline. Australian bird populations are also declining. South Australia’s Coorong is a key destination for migratory shorebirds and was listed as a Wetland of International Importance in 1985. However, since 1985 the Coorong has changed and migratory shorebirds are now far less abundant, some experiencing more than 10-fold declines. Changes to the Coorong have been brought about by increasing extraction of water from the Murray Darling Basin, resulting in decreasing flow quantity and timing, and endemic shorebirds such as Pied Oystercatchers and Red-capped Plovers and other waterbirds such as the Fairy Tern using the Coorong have also declined.
Other migratory birds’ movements within Australia are less conspicuous. Short-tailed Shearwaters that breed on offshore islands around the southern coasts of Australia arrive in the tens of thousands at breeding grounds, often on the same day each year, and in many cases occupy the same burrow or one nearby as used in previous years. Other migratory birds are bush birds that move within the Australian continent. The more notable movements involve birds departing southern latitudes, such as Tasmania in autumn and moving northwards. Amongst the species that move are threatened species such as the Orange-bellied Parrot and Swift Parrot. Other smaller birds like the Silvereye, Tree Martin, Fairy Martin, Grey Fantail, Dusky Woodswallow, Flame Robin, Rufous Whistler, White-naped Honeyeater, and Yellow-faced Honeyeater also move during autumn, but not all individuals depart from all locations each year. Although aggregations of some of these species are detected during autumn migration, the movements are more diffuse and many may move as individuals or in small flocks consisting of a handful of individuals. These birds have the potential to forage along the routes that they take and so the need for specific stopovers is not at a premium. The return journeys in spring are even less conspicuous. For these birds with diffuse movements that feed along the way, protecting habitats to allow the movements to continue may be even more challenging than well defined routes with key stopovers. The movements of our bush birds remain poorly documented, yet understanding and documenting the movements will be critical to managing these species into the future. Modern technologies (such as miniature satellite trackers) may eventually allow these movements to be documented.
There are two other types of movements of birds within Australia. Altitudinal movements are prominent in autumn as flycatchers such as robins, whistlers and fantails move to lower altitudes where slightly warmer conditions may provide more favourable conditions for foraging and survival during winter. Even the relatively small elevation gradient provided by the Mt Lofty Ranges is sufficient to stimulate these birds to move down slope as winter approaches, and one of the delights for bird watchers is seeing some of these species in suburban gardens of Adelaide in autumn and winter.
The other movements often attributed to Australian birds, particularly those of the interior, are described as nomadic. These often consist of large numbers of birds such as Budgerigars, Crimson Chats and Pied Honeyeaters appearing in more temperate southern latitudes after a boom period. The boom periods follow a period when significant rainfall has stimulated plant growth in the arid interior. The boom times, however, give way to periods with little rain, forcing the birds to move. These potentially nomadic movements are usually not considered migrations because they lack a regular annual cycle and are not, as yet, predictable.
Guest post by Associate Professor David Paton.
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