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Figure 16.5 Percentage of breeding species in different arid regions that have been classed as nomadic. Total numbers of breeding species in parentheses. Nomadic species reach significantly greater numbers (and proportions) in the warm deserts of the southern hemisphere than in similar deserts in the northern hemisphere (x2 with Yates' correction = 12.17, P = 0.005), and also significantly greater numbers (and proportions) in all southern hemisphere arid regions regardless of winter temperatures (x2 with Yates' correction = 17.76, P < 0.001). The deserts of Australia hold the highest proportion of nomadic species (compared with all other southern hemisphere deserts, x2 with Yates' correction = 89.09, P < 0.001). From table 2.3 in Dean (2004).
hemisphere, nomadic species tend to outnumber regular migrants. This difference between hemispheres is attributed mainly to the greater fluctuation and unpredictability of rainfall in southern regions (associated with El Niño-La Niña events). In Australia, where conditions are most variable, up to 46% of desert bird species can be classed as nomadic, a greater proportion than in deserts in any other region (Dean 2004).
The nomadic birds of the African and Asian deserts are similar, being dominated by sandgrouse (Pteroclidae), larks (Alaudidae) and sparrows, weavers and finches (Passeridae). The Australian deserts are dominated by honeyeaters (Melophagidae), parrots (Psittacidae) and crows (Corvidae), and the New World deserts by finches (Fringillidae). Overall, nomadism is found in about half the bird families that breed in arid and semi-arid environments. It seems more related to diet than to phylogeny, but occurs disproportionately in some families, such as sandgrouse, all of which are nomadic to greater or lesser extent, as are many larks.
In general, nomadism is associated with ecosystems in which the underlying productivity, and the densities of resident bird species, are very low, but in which periodic pulses of high productivity occur, with resources sufficiently abundant to support major influxes of birds from elsewhere. Most nomadic species are specialists, feeding on only a small range of food items, which appear occasionally in extreme abundance. Thus, throughout the world many nomadic birds are seed-eaters, and in deserts they mostly specialise on the masses of grass seeds produced after rain has fallen (equivalent to tree seeds in the northern forests). Others are insectivores which tend to specialise on 'plague' insects, such as noctuid moth larvae, grasshoppers or locusts (equivalent to various defoliating caterpillars in northern forests). Yet others are predators that feed on the other birds that move in, or on the rodents that increase in response to vegetation growth and seeding.
The impact of rainfall on predators can be illustrated by findings at Red-billed Quelea Quelea quelea colonies in the savannah grasslands of Africa. As explained above, these small birds move into areas after rain has fallen in order to feed on the resulting grass seed. They form enormous colonies, containing many millions of pairs, and covering many hectares of scrub, from which the birds fan out over the surrounding land to forage. These colonies attract concentrations of predators. At one large quelea colony in Kruger National Park, Pienaar (1969) counted 200-240 Wahlberg's Eagles Aquila wahlbergi, 800-1160 Steppe Eagles A. nipalensis, 300 Marabou Storks Leptophilus crumenifenus and many other species. At least 60% of quelea nests were torn open by these birds. In the same region at another large quelea colony, Kemp (2001) counted more than 1000 eagles, the majority being Lesser Spotted Eagles A. pomarina, with smaller numbers of Tawny Eagles Aquila rapax, Wahlberg's Eagles and Steppe Eagles, and at least 100 Marabou Storks. Most of these predators were non-breeding migrants from other parts of Africa or Eurasia. Other concentrations of raptors have been found at the smaller colonies of Wattled Starlings Creatophora cinerea, which normally cover a few hectares or less. These birds move in after rain to exploit the resulting mass hatches of locusts or noctuid and other lepidopterous larvae.
Other concentrations of different raptor species occur in association with local rodent outbreaks, again following rainfall. For example, Malherbe (1963) recorded concentrations of several diurnal and nocturnal birds of prey in a semi-arid area in Northwest Province, South Africa, associated with an outbreak of Multi-mammate Mice Praomys natalensis. In such conditions, several local species bred and produced larger than usual broods. During another mouse outbreak in Zimbabwe, one pair of Barn Owls Tyto alba over a 12-month period nested continuously from March to December, laying 32 eggs, almost all of which produced fledglings (Wilson 1970). Normally they would raise only one brood, containing less than six young. Few other bird species live on such an extreme boom-and-bust economy.
All these various nomadic species may appear in enormous numbers when conditions are suitable, but then disappear for up to several years before they come again. They all move in relation to sporadic changes in food supplies, but without this knowledge, their movements would appear random and unpredictable. The importance of food supplies is further shown by the fact that some typi cal nomadic species, such as the Wattled Starling, are regular breeders in parts of their range where food is more consistently available, usually through human action (Broekhuysen et al. 1963). Similarly, in Southern Australia, the White-fronted Chat
Epthianura albifrons is nomadic in the drier parts of its range, but sedentary near the coast (Dean 2004).
In the Australian central desert, rainfall is more irregular in amount, timing and distribution than almost anywhere else on earth (Dingle 2004). Persistent heavy rain can produce widespread flooding, giving rise to large areas of wetland habitat, while subsequent droughts can eliminate these wetlands for years on end (Roshier & Reid 2002). To survive and prosper, birds must therefore move at irregular intervals, to wherever suitable wetlands exist. The many thousands of temporary wetlands can vary in size from a few square metres to thousands of square kilometres. Some are terminal water bodies filled by major drainage systems (e.g. Lake Eyre), others are lakes filled by local drainage, or overflow areas from swollen rivers. These floodlands last at most a few months before they dry out. They are re-created to varying extents, up to several times per 20-year period but at irregular intervals. For example, in eastern Australia, total wetland varies from almost nothing in extreme drought years to around 30 000 km2 in wet years (Figure 16.6).
During their short lives, these wetlands can produce great quantities of aquatic invertebrates and fish. Most Australian waterfowl are highly mobile, and it is not uncommon, as ring recoveries have shown, for individual ducks to move more than 2000 km from one area to another (Roshier & Reid 2002). The most extreme examples include the Grey Teal Anas gracilis, Freckled Duck Stictonetta naevosa and Pink-eared Duck Malacorhynchus membranaceus (Frith 1967). But irregular nomadic movements also occur among Australian Shelduck Tadorna tadornoides, Chestnut Teal Anas castanea, Australasian Shoveler Anas rhynchotis, Hardhead Aythya australis, Hoary-headed Grebe Poliocephalus poliocephalus, Australian Pelican Pelecanus conspicillatus, Black-tailed Native Hen Gallinula ventralis, Red-kneed Dotterel Erythrogonys cinctus, Gull-billed Tern Sterna nilotica and others, each requiring rather specific conditions. In Australia, as in deserts elsewhere, stilts and avocets also benefit from the flooding of normally dry hollows, feeding on the masses of crustaceans that result. Both movement and breeding patterns have arisen to take advantage of such temporary bonanzas (Chapter 11).
Lake Eyrie has been called the sump of Australia: a dry salt plain at times of drought, and a rich shallow lake extending over thousands of square kilometres at times of flood, perhaps once every 20 years. When floods begin, thousands of waterbirds appear from nowhere and immediately begin to breed. One colony of pelicans reached 50 000 pairs, the largest on record. Some birds must travel 1500 km to reach this area from the coast. It remains a mystery how such birds find suitable localities within vast desert areas: whether they search at random over appropriate range or whether they respond to climatic and other clues that indicate the most rewarding directions to fly. Older birds might remember the site, but this still leaves the question of how they know the time is right, as there is nothing regular about the rains that bring the flood. In theory, birds could find suitable areas by travelling with the wind, which blows towards the low pressure areas where rain is falling. This would not always work, however, because flood-waters in inland Australia can sometimes take weeks to reach the lower catchments, long after the weather that created them has passed. Olfaction has been suggested as an alternative mechanism, but is untested in this context (Roshier et al. 2006). Few desert species have been studied in detail however, and it is possible that further research might reveal more regularity in their movements than casual observations suggest.
Typically, many species of the central Australian desert build up in numbers as a result of good breeding in occasional wet years, then move outwards to the more humid peripheral districts in the following dry years (Nix 1976). Many birds probably die before they can return, but some species spend most years in coastal localities and periodically move inland to breed in vast numbers when rain creates suitable conditions. For example, Banded Stilts Cladorhynchus leucocephalus live for years as non-breeders on scattered briny coastal lagoons. But within days of rain falling inland, they concentrate in tens of thousands on newly formed shallow lakes, feeding on the freshly hatched swarms of brine shrimps (Burbidge & Fuller 1982, Robinson & Minton 1989). They breed while conditions last, making repeated nesting attempts, while the young form huge crèches. Water evaporates rapidly, however, and the land soon resorts to its normal parched state. The birds then return to the coast, if necessary leaving the last thousands of eggs and young to die. Years may pass before they can breed again, and not necessarily in the same sites. In recent years, known breeding events have occurred at Lake Eyrie in South Australia in 2000, in the Coorong in South Australia in 2005, and at Lake Corangamite in Victoria in 2006, the latter for the first time in recorded history.
Poor conditions over most of the range, coupled with good conditions in other parts, can sometimes lead to enormous concentrations of birds. For example, in the austral summer of early 2004, an estimated 2.9 million Oriental Pratincoles Glareola maldivarium gathered along 235 km of coastal grassland (Eighty-mile Beach, southwest of Broome) in northwest Australia, feeding on the abundant grasshoppers (Sitters et al. 2004). These birds breed widely over Southeast Asia and migrate mainly to Australia for their non-breeding season. In most years, the birds are thinly scattered over a wide area. This makes them hard to find, and the total flyway population had been previously estimated at only 15 000 individuals. But the circumstances of early 2004 led to a huge localised concentration, and a greatly revised overall population estimate.
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