Bairlein & Henneberg (2000)

aBased on comparison of average habitat state between years, rather than habitat of individuals.

aBased on comparison of average habitat state between years, rather than habitat of individuals.

arrive and nest late (Table 26.2; Chapter 14). Variations in arrival dates sometimes exceed a month, and late-arriving individuals are often in poor condition. All relevant aspects of performance could be influenced by the quality of the individual birds and, without experiments, it is hard to tell to what extent reproductive success follows directly from arrival date, and to what extent from individual quality (though one experimental study confirmed the importance of date as such, Cristol 1995; Chapter 14).

In a few studies, quality of winter habitat and associated food supplies have been found to influence the body condition and spring migration dates of birds. American Redstarts Setophaga ruticilla studied in Jamaica competed for territories in moist forest habitats, forcing some individuals into dryer scrub habitat (Marra et al. 1998). Birds in the better forest habitat showed better body condition than those in scrub. They also showed lower 13C values in their muscle tissue, a pattern mirrored in the available insect prey. The 13C values in the muscle of newly arrived birds in North American breeding areas provided an indication of the habitat those birds had occupied in winter. Individuals that arrived earliest in breeding areas had lower 13C values than those arriving later, indicating that the early birds had come from the best wintering habitat. In a later study, the first arriving birds were found to have better breeding success than the later ones (Norris et al. 2004). This work, involving measurement of carbon isotopes, thus provided another link between the conditions experienced by individuals in winter, their subsequent migration dates and breeding success.

The fact that good winter habitat was limiting for American Redstarts was shown by an experiment in which 28 individuals (largely adult males) were removed from their territories in optimal mangrove habitat (Studds & Marra 2005). They were rapidly replaced by 23 other individuals (females and juvenile males) from poor scrub habitat. Initially, the replacement birds had blood isotope signatures typical of scrub, but two months later they had isotope signatures typical of mangrove. Compared to control birds in scrub, the upgraded birds maintained weight over winter, departed earlier in spring, and returned in greater proportion the next autumn. Insect biomass was greater in mangrove than scrub, suggesting that food availability caused the differences. This experiment gave further indication that prime winter habitat was limited, and that winter conditions influenced the subsequent performance of individuals. Birds able to get a place in good habitat in their first winter may be well positioned to breed productively the next spring, and possibly throughout their lives.

Among Black-tailed Godwits Limosa limosa which winter in Britain and breed in Iceland, a similar pattern emerged, as found from study of colour-ringed individuals identified in both breeding and wintering areas (Gill et al. 2001, Gunnarsson et al. 2005). Those individuals that occupied the best wintering habitat were able to prepare for spring migration earlier, and were therefore first to arrive in breeding areas. They thereby acquired the best nesting habitat, and subsequently had better breeding success, on average, than later arriving individuals that had occupied poorer winter habitat. In this way, as in the American Redstarts, the effects of winter habitat carried through the whole annual cycle.

Of course, things do not always run smoothly. Whatever the date of departure from wintering areas, adverse weather encountered en route may delay arrival in breeding areas, and thus affect reproduction (Johnson & Herter 1990, Richardson

1990). In 1997, many White Storks Ciconia ciconia were late in leaving their African wintering areas, which was attributed to poor food supply (Berthold et al. 2002). In addition, some individuals (including a radio-tagged bird) were delayed for another week en route, as they hit a severe cold spell. These circumstances led to late arrival in European breeding areas, and depressed breeding success over wide areas, providing another link between conditions in wintering and migration areas and subsequent breeding success.

Similarly, events in breeding areas can in turn affect subsequent survival in migration or wintering areas. An example was mentioned above for the Sand Martin Riparia riparia, in which heavy summer rain in breeding areas was associated with poor body condition and reduced subsequent survival of adults (Cowley & Siriwardena 2005). Evidence also emerged for effects of reproduction on the subsequent survival of Barnacle Geese Branta leucopsis (Prop 2004). In females, non-breeders (that made no reproductive commitment) survived better than failed breeders, which in turn tended to survive better than successful breeders, with annual rates of 0.95 (SE 0.03), 0.86 (SE 0.02) and 82 (SE 0.04) respectively. In addition, among females that attempted to breed, variance in survival was explained by the date of nest departure (when they could start feeding well again), whether hatching for successful breeders or desertion for failed breeders. The later this date, the lower the subsequent survival probability. This trend was attributed partly to body condition when intense feeding began again, and partly to the seasonal decline in the nutrient content of plant-food, and the limited time available for pre-migratory fattening. In male geese, which played no part in incubation and were free to feed throughout, survival did not differ significantly between birds of different breeding status.

These and other studies based on ringing or isotope analyses have all provided evidence of apparent carry-over effects in individuals from one time of year to another (Table 26.2). Evidence for effects of reproduction on subsequent survival emerged when experimentally-increased reproductive effort (achieved through increasing brood sizes) resulted in poor body condition, and in reduced subsequent survival in adult Glaucous-winged Gulls Larus glaucescens (Reid 1987), Common Kestrels Falco tinnunculus (Dijkstra et al. 1990), male Pied Flycatchers Ficedula hypoleuca (Askenmo 1979) and female Collared Flycatchers F. albicollis (Cichori et al. 1998). Although poor condition in these birds was experimentally induced, it confirmed that events acting at the time of breeding could influence the subsequent performance of individuals. In all these various species, reduced survival was inferred from the lowered return rates of poor condition individuals the next year, but in none was it certain whether the extra mortality occurred on migration or in winter quarters.

Carry-over effects of these various types can be incorporated in the model in Figure 26.2, given the appropriate information. Increase in the proportion of individuals experiencing a negative carry-over effect would be expected to magnify a subsequent population decline, while increase in the proportion of individuals experiencing a positive carry-over effect could lessen a population decline. To predict changes in population size resulting from habitat loss, it would therefore help to determine: (1) which factors in which seasons produce strong carry-over effects, and (2) the quality, as well as the amount, of habitat that is lost (Norris 2005).

Carry-over effects are evident not only at the individual level, but also at the population level through well-established density-dependent effects. In particular, in any species good breeding success at the level of the population is likely to result in high post-breeding numbers. In species limited in wintering areas, the ensuing mortality is likely to be density dependent, resulting in high mortality following good breeding years, and lower mortality following poor breeding years. Among migrants, mortality over the non-breeding period has been shown to be density dependent in most populations in which the problem has been examined: namely Sedge Warbler Acrocephalus schoenobaenus, Blackcap Sylvia atricapilla, Greater Whitethroat S. communis, Willow Warbler Phylloscopus trochilus, European Pied Flycatcher Ficedula hypoleuca, Common Redstart Phoenicurus phoenicurus, Barn Swallow Hirundo rustica, Redshank Tringa totanus, Mallard Anas platyrhynchos, Northern Shoveller Anas clypeata and Barnacle Goose Branta leucopsis (Jarvinen 1987, Stenning et al. 1988, Mihelsons et al. 1985, Kaminski & Gluesing 1987, Owen & Black 1991, Baillie & Peach 1992, Whitfield 2003). In most of these species, overwinter loss was also the key factor governing year-to-year change in breeding numbers (review Newton 1998b), but it included migration as well as wintering periods. Density-dependent mortality in the winter period alone has been documented in the American Redstart Setophaga ruticilla (Studds & Marra 2005), and it is probably only a matter of time before other examples come to light.

In general, therefore, it seems that different periods in the annual cycles of migratory birds are inextricably linked, and that events occurring during one period can affect performance in a later period. Such effects can occur at the level of the individual (carry-over effects), and at the level of the population (density-dependent effects). It has become increasingly possible to detect carry-over effects through our ability to 'connect' populations in specific breeding and wintering areas, through ringing or isotope analyses, and as information on global climatic conditions has become more generally available.

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