Most birds breed regularly at the same times each year, but others breed at varying times depending on the food resource, with knock-on effects on the timing of moult and movements. Given sufficient food, three main patterns have been recorded among such opportunists. The first involves a substantial extension of the normal breeding season, as found in Galapagos finches and others (Gibbs & Grant 1987). The second involves a main breeding season in spring and an additional one in autumn, separated by moult, gonad regression and re-growth, as noted in the Tricolored Blackbird Agelaius tricolor, Pinyon Jay Gymnorhinus cyanocepha-lus and others (Payne 1969, Ligon 1971). The third involves breeding in different months in different years, whenever food is sufficiently plentiful (often depending on irregular rainfall), as found in the Black-and-White Manakin Manacus manacus, Zebra Finch Taenopygia guttata and others (Snow 1962, Zann et al. 1995).
In the central Australian desert, rainfall is irregular, and at particular localities can fall at different times each year.4 It seems that some desert birds remain ready to breed for much of the year, but nest only when stimulated by fresh rainfall or the resulting surge in food supplies which can occur up to several weeks later (e.g. Serventy 1971, Zann et al. 1995, Leitner et al. 2003). They include not only landbirds, such as the Zebra Finch, which can breed for more than 10 consecutive months if conditions remain suitable (Zann et al. 1995), but also various waterbirds which move rapidly into areas where rain has fallen. They breed in the resulting shallow lakes, and move on again when the lakes dry out (Frith 1959, Burbidge & Fuller 1982; Chapter 16). Examples include the Freckled Duck Stictonetta naevosa, Grey Teal Anas gibberifrons, Pink-eared Duck Malacorhynchus membranaceus and Banded Stilt Cladorhynchus leucocephalus. Such birds have a long 'reproductive window', but nest only at times within this period when other conditions are suitable. In contrast to breeding, moult in these desert birds occurs on a more fixed schedule, but if rain falls unexpectedly, moult may or may not be suspended as breeding begins (for Galapagos Finches see Snow 1966, for Budgerigar Melopsittacus undulatus see Wyndham 1981, for Zebra Finch Taenopygia guttata see Zann et al. 1995).
A similar opportunist strategy is adopted by Common Crossbills Loxia curvi-rostra in boreal regions. These birds depend on conifers, but the seeds of different species become available at different times of year. Seed abundance also varies greatly from year to year, so that crossbills face both temporal and spatial unpredictability in food supply (Chapter 18). They can breed over much of the year,
4In deserts elsewhere in the world, rainfall is more regular in timing, even though it may be slight to non-existent in some years. The local birds therefore tend to breed at the same times in different years, though not necessarily every year. In some arid parts of the southwestern USA, the breeding seasons of some species, such as the Roadrunner Geococcyx californianus, tend to be bimodal, with little nesting in the hottest period from mid-June to late July (Ohmart 1973).
but most nesting occurs some time between September and May, depending on local seed supplies (Newton 1972, Berthold & Gwinner 1978, Benkman 1987).5 Yet despite this variable breeding season, and associated movement patterns, the single annual moult occurs consistently in July-September, sometimes overlapping with breeding (Newton 1972, Hahn 1998). However, despite seasonal overlap at the level of the population, individuals seldom migrate and moult at the same time. Experiments have shown that Common Crossbills and some other opportunist breeders can respond to photoperiod (Hahn 1998), even tropical ones which under natural conditions experience little or no daylength variation (for Red-billed Quelea Quelea quelea see Lofts 1964).
To judge from these various findings, aseasonal opportunistic breeders are not fundamentally different from regular breeders. Both groups depend partly on an endogenous rhythm, and both can respond to photoperiod in captivity, but opportunist breeders respond more strongly to prevailing ecological cues (notably food supplies) to fine-tune the timing of their nesting and movements (Wingfield 1980, Hahn et al. 1997). Some opportunist breeders can delay or arrest moult or movement in order to breed if food happens to be plentiful at the time these other activities would normally occur. Many northern birds, too, show great flexibility in the timing and extent of their movements, which vary from year to year in relation to food availability (so-called irruptive migrants, Chapters 18 and 19).
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