When they are breeding, most bird species tend to remain resident in an area throughout the season, so that the whole breeding range is occupied in a consistent manner from spring arrival to autumn departure. There are, however, exceptions to this general pattern, in that individuals of some multi-brooded bird species nest in more than one locality each year. Typically, they raise one brood in one locality, then migrate up to several hundred kilometres, and raise another brood elsewhere. So-called itinerant breeding was first suspected in the European Quail Coturnix coturnix, in which females arriving in Italy in June-July, apparently to breed, often showed regressing brood patches from an earlier breeding attempt, and were frequently accompanied by young no more than two months old. These young must have hatched from clutches begun in March, a date at which breeding would have been possible only in North Africa. Reports of a general exodus of Quail from Tunisia in spring after breeding support this view, and single birds ringed there in May and early June were recovered in Italy and Albania respectively, 2-3 months later (Moreau 1951, Cramp & Simmons 1980). Late clutches in northern Europe during August-September could result partly from an influx in mid-summer of birds that have previously bred in the Mediterranean region. Apparently the males leave southern Europe once the females have laid, and move on to establish new territories further north, where females join them later. Many females do not reach the most northern areas, so males predominate there (evidence summarised in Aebischer & Potts 1994). European Quail can mature and breed at three months old, so that young produced in the southern parts of the breeding range could breed the same summer in the more northern parts, along with the adults on their second or third attempt. So far, however, they have not been proved to do so.
Itinerant breeding is also shown by the Red-billed Quelea Quelea quelea, which feeds on grass seeds on the African savannahs. As the rain belts spread across tropical Africa, they stimulate the growth and seeding of grasses, providing a sufficient temporary food supply to enable Queleas to breed. After raising their young, the birds move en masse, stopping again in an area where rain has recently fallen, and raise another brood (Ward 1971, Jaeger et al. 1986). Throughout the dry season, these birds subsist on dry seeds picked off the ground but, when the rains break, this seed suddenly germinates, thus removing the food supply, and causing the birds to move on (called the early rains migration). The birds then fly over the approaching rain front to areas where rain fell about two months earlier and new grass seed has already formed (the 'breeding migration'). This new seed, and insects associated with the growing vegetation, enable the birds to breed again. In theory, the birds could follow the rains in successive breeding attempts, each time moving some hundreds of kilometres. Within this framework, the pattern is variable from year to year, depending on regional variations in rainfall and grass seed production. Conditions suitable for rearing young do not last long in any one place, and despite a short breeding cycle of only five weeks, queleas cannot raise two broods in the same place. The adults abandon their young at about three weeks old, with enough body fat to ensure their survival, and continue on the breeding migration. Only by remaining within the slowly shifting zone of seeding grasses are individual queleas able to raise more than one brood per year.
Evidence for this process in Red-billed Quelea is largely circumstantial: (1) many females begin developing the yolks for a second clutch while still feeding the first brood, yet they do not breed again in the vicinity; and (2) in those regions where colonies contain birds likely to be breeding for the second time, adults arrive in badly worn plumage, consistent with a previous breeding attempt. However, at three colonies in Ethiopia birds were sprayed from the air with fluorescent particles, and 2-3 months later some were re-sighted at other colonies 500 and 700 km to the north (Jaeger et al. 1986). In West and southern Africa, there is time in wet years for up to three broods to be raised in different places along a 'breeding migration', and in East Africa up to five, but it is unknown whether these maxima are ever reached.
Some other birds in semi-arid regions of the world, which breed at a particular stage in the dry-wet seasonal cycle, may also raise successive broods in places far apart. Multiple breeding along a migration route, following a rain belt, has been suspected in the Eared Dove Zenaida auriculata in northeastern Brazil (Bucher 1982), and substantial shifts in colony sites during a single season have been suspected in the White-crowned Pigeon Columba leucocephala on Hispaniola (Arendt et al. 1979), in the Tricolored Blackbird Agelaius tricolor in California (Hamilton 1998), and in the Spanish Sparrow Passer hispaniolensis in Kazakhstan and elsewhere (Summers-Smith 1988, Cramp & Perrins 1994).
Individuals of some species seem to move around continually during a breeding season. For example, in a semi-arid area of southeast Australia, membership of three permanent nesting colonies of Zebra Finches Taeniopygia guttata changed continually, due to the frequent arrival and departure of birds from distant colonies (Zann & Runciman 1994). Some 66% of adults stayed for no more than one month and many that stayed longer also left for extended periods. Another Australian species, the Regent Honeyeater Xanthomyza phrygia, has been found to move more than 20 km between successive broods in the same season (maximum recorded distance 260 km), as pairs tracked the occurrence of rich food sources, which appeared sequentially through the season (Geering & French 1998).
Other European birds known sometimes to move long distances between successive breeding attempts include some cardueline finches, such as Common Redpoll Carduelis flammea and Siskin Carduelis spinus that depend on different types of tree seeds (Chapter 18). Other carduelines have been found to move shorter distances (up to a few tens of kilometres) during a breeding season (Newton 2001). Also, over shorter distances, some Sand Martins Riparia riparia have been found to change colonies between successive broods in the same season (Mead 1979b). In North America, the raising of two or more broods, each in a different region, has been suspected in the Phainopepla Phainopepla nitens, Dickcissel Spiza americana and Sedge Wren Cistothorus platensis (Walsberg 1978, Fretwell 1980, Bedell 1996). In addition, females of some shorebirds also move on to breed elsewhere after laying a clutch of eggs, leaving incubation and chick care to their mates. Examples include Eurasian Dotterel Eudromias morinellus (in which some males also move long distances during a breeding season, Whitfield, in Wernham 2002), and probably also the North American Snowy (Kentish) Plover Charadrius alexandrinus.
Itinerant breeding enables species that could raise only one brood in a given locality to become multi-brooded. Pre-requisites are a short breeding cycle with early independence of young. Species with long breeding cycles, with prolonged parental care, would not be expected to become itinerant breeders in seasonal environments, because they would not have time to raise any more than one brood in the time available. The above examples seem to fall into two categories: one involving local movements of up to a few tens of kilometres, and the other longer movements of up to several hundred kilometres between different points on an established migration route. Both types could occur in a much wider range of species than recorded, but could be detected only if newly arrived adults had brood patches, recently fledged young or other signs of recent reproduction, and then proceeded with a new nesting attempt. It is clearly rare or non-existent in the vast majority of well-studied European and North American migratory birds.
It might be thought that many species migrating north through Eurasia or North America in spring could raise more broods by stopping at different latitudes en route than by breeding only for a short season in the north, where only one brood can be fitted into the time available. The prior occupation of more southern habitat by conspecifics may be the main factor preventing this. Moreover, the southern habitat usually remains suitable after the first broods for subsequent ones (Chapter 13). This is not true for known itinerant multi-brooded species, such as queleas Quelea spp., which settle in areas that have only just become suitable, and hence where there is no established prior population, and which are forced to move on if they are to breed again in the same season because local conditions soon deteriorate. A remarkable feature of some of these examples is that individuals move so far between successive breeding attempts that the movement can be fairly described as a migration, inserted within a breeding season. In queleas, the 'early rains migration' is preceded by fat deposition, the amount varying between three populations, according to the distances they have to travel (Ward & Jones 1977).
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