Even greater flexibility in movement patterns is shown by some species that exploit sporadic habitats or food sources. Such species often appear to be truly nomadic, as they show little or no year-to-year consistency in their movement patterns, but shift from one area to another, residing for a time in whichever parts of their range food is plentiful at the time. The areas successively occupied may lie in various directions from one another. No one area is necessarily used every year, and some areas may be used only at intervals of several years, but for months or years at a time, if conditions are suitable. The birds themselves may in theory move at any time and in any direction between successive breeding sites, and over shorter or longer distances.
The essence of nomadism, then, is its inconsistency, involving 'unpredictable, unseasonal and irregular movements across landscapes and regions' (Dean 2004). Of course, many bird species wander locally over distances of a few kilometres in search of winter food, but so-called nomadic species can travel hundreds of kilometres from one breeding area to another or from one non-breeding area to another. It is partly a matter of scale. However, much of what we know about nomadic movements is based on inference, to a large extent unsupported by ring recoveries. This is a consequence of the mobility of the birds themselves, and of the fact that most of them live in environments that support only low densities of people.
It is in desert regions where nomadism is best developed, being governed by sporadic rainfall, which influences plant growth and invertebrate densities, as well as the availability of surface water for waterfowl. Nomadism seems to occur in response to the variability, rather than the severity, of the desert environment. In the most unpredictable conditions, both the movements and breeding of birds become increasingly aseasonal. Following rain, landbird densities in particular localities can increase more than a hundred-fold within a few days, and water-birds that have not been seen for years can re-appear in huge numbers on new flood waters (for examples see Dean 2004). In such environments, the survival of many bird species depends primarily on their mobility.
In many species, in both deserts and elsewhere, nomadic movements are superimposed on regular north-south migrations so that the population is concentrated at different latitudes at different times of year, but always patchily distributed, wherever food occurs. This is obvious in northern regions in some seed-eating species such as Redpoll Carduelis flammea and Siskin Cardulis spinus, and in some rodent-eaters such as Long-eared Owl Asio otus and Hawk Owl Surnia ulula (Chapters 18 and 19). It is also obvious in some Australian species, such as the seed-eating Emu Dromaius hollandiae (Davies 1976, 1984), Budgerigar Melopsittacus undulatus (Wyndham 1982) and Cockatiel Nymphicus hollandicus (Rowley 1974), or the blossom-eating Black Honeyeater Certhionyx niger (Ford 1978). On the other hand, no regular latitudinal shifts are apparent in other Australian species, such as the seed-eating Flock Bronzewing Phaps histrionica and the rodent-eating Letter-winged Kite Elanus scriptus, which seem to be entirely nomadic (Keast 1959). The latter tends to concentrate at local outbreaks of Long-haired Rats Rattus villossis-simus, which follow the vegetation growth resulting from rain. In all, some 36 species of landbirds move about the interior of Australia, either on a combination of regular migration and nomadism, or on nomadism alone.
Yet other bird species behave nomadically only in the non-breeding season, and return to the same localities to nest each year. Examples from among Eurasian-African migrants include the White Stork Ciconia ciconia, Black Kite Milvus migrans, Steppe Eagle Aquila nipalensis, Lesser Spotted Eagle A. pomarina, Lesser Kestrel Falco naumanni, Red-footed Falcon Falco vespertinus, Amur Falcon F. amu-rensis and Black-winged Pratincole Glareola nordimanni. Typically, these species specialise in their non-breeding areas on sporadic food supplies, such as locusts and grasshoppers, emerging termites, large nesting colonies of Red-billed Quelea Quelea quelea or Wattled Starlings Creatophora cinerea, or small animals disturbed by grass fires. Some of these prey species are available at the same localities for a short time every year, but others only after exceptional rainfall.
According to Dean (2004), some 233 arid-land bird species worldwide could be classed as primarily nomadic. They form varying proportions of desert avifaunas in different regions (Figure 16.5). In deserts in the northern hemisphere, regular seasonal migrants tend to outnumber year-round nomadic species, especially in the colder regions (Dean 2004); but in the warm deserts of the southern
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