Months of the year
Figure 13.12 Seasonal changes in the distribution of four migratory raptors in West Africa. Grey shading - frequent sightings; blue shading - infrequent sightings. In this region, migratory raptors stay in the southern woodlands during the local dry season, while food is plentiful and hunting conditions are good. When it rains heavily and the grass grows rapidly to 1.3 m high, the migrants move north to the short grass areas, where the rains are later and lighter and produce less growth of vegetation. The birds thus manage to remain in a fairly favourable environment all year and, while in the north, they breed, taking advantage of a short seasonal surplus of food which is not fully utilised by the sparse resident population. Species differ in the extent of their migrations, and in the periods spent at different latitudes, depending on their particular needs, but the general northward passage occurs at the start of the rains in 'spring' and the southward passage at the end of the rains in 'autumn' (From Thiollay 1978).
way. As expected, many species in the Andes, where winters can be harsh, move to lower elevations for the non-breeding season.
Recent estimates of the numbers of intra-continental migrants in South America range from 220 to 237 species, as compared with 338 species of Nearctic-Neotropical migrants (Rappole 1995), but the South American figure could well be an underestimate, considering the richness of the avifauna, and the relatively poor state of knowledge. One-third of all known migratory species belong to the Tyrannidae (New World Flycatchers), reflecting the overall preponderance of this family across the continent (Chesser 1994, Jahn et al. 2004).
Overall, about 40% of Australian landbird species are known to migrate in at least part of their range. Partial migration occurs in about 44% of 155 non-passerine species and 32% of 317 passerines examined (Chan 2001). Similar findings emerged from an analysis of bird count data in which 37% (145/393) of species were detected as making movements (Griffioen & Clarke 2002).
East of the Great Dividing Range, rainfall is generally adequate, and seasonal temperature changes largely influence seasonal productivity. Movement is generally northward toward the subtropics and tropics for the winter. Some species in the southern parts of their range are completely migratory, but many species are partial migrants that occupy much the same geographical range year-round, with seasonal south-north shifts in the centre of gravity of their populations. These shifts are mostly over distances of less than 1000 km. Eastern examples include the Scarlet Honeyeater Myzomela sanguinolenta and Yellow-faced Honeyeater Lichenostomus chrysops (Clarke et al. 1999, Munro 2003). Nevertheless, some species perform longer movements. They include the many waterbirds that migrate from Australia to New Guinea in the non-breeding season, including egrets, ibises, pelicans and ducks. For three species of egrets that come from southeastern Australia, this entails a journey of more than 3000 km (Geering & French 1998). Further south, five species leave Tasmania completely for the winter, spending their non-breeding season in Australia or beyond, namely the Swift Parrot Lathamus discolor, Orange-breasted Parrot Neophema chrysogaster, Pallid Cuckoo Cuculus pallidus, Shining Bronze Cuckoo Chrysococcyx minutillus and Satin Flycatcher Myiagra cyanoleuca (Chan 2001, Dingle 2004). Other eastern species are altitudinal migrants on the Great Dividing Range. They include such conspicuous species as Yellow-tailed Black Cockatoo Calyptorhynchus funereus, Golden Whistler Pachycephala pectoralis, Regent Bowerbird Sericulus chrysocephalus and others. Whereas these and others move to lower ground for the winter, others, such as the Eastern Spinebill Acanthorhynchus tenuirostris, move uphill to exploit Banksia collina pollen at higher elevations in winter.
Except for the monsoonal north and small areas in the southwest and south-centre that are Mediterranean in climate, rainfall over most of Australia west of the Dividing Range is sparse and erratic, making it the driest continent overall. Because bird breeding in these circumstances is frequently tied to erratic rainfall, migration can be complex and variable, but many species show an underlying north-south pattern (Nix 1976; Chapter 16). Comparing different parts of Australia, the proportions of birds that are migratory decline with increase in the amount and evenness of the annual rainfall. Where the annual total exceeds 125 cm, and is well distributed through the year, 70-85% of honeyeater species (Meliphagidae) are year-round residents, but in the central desert, where annual rainfall is 20-28 cm and erratic, fewer than 50% are residents, and many seem to perform nomadic movements in response to rainfall patterns (Keast 1968b; Chapter 16).
New Zealand now holds relatively few native landbird species, and only three of these spend the winter elsewhere. They include the Long-tailed Cuckoo Eudynamis taitensis which winters in a wide arc of islands from New Guinea across the Marshall and Caroline Islands to the Marquesas in the east; the Shining Bronze Cuckoo Chrysococcyx lucidus which migrates to the Bismarcks and Solomons; and the Double-banded Plover Charadrius bicinctus which migrates to southeast Australia for the winter. The last two species also breed within Australia.
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