Migration, a common phenomenon among many animal taxa, plays a central role in the spatial dynamics of mobile populations (Dingle 1996; Gill et al. 2001; Dingle & Drake 2007). It is a highly flexible system that has responded historically (i.e. glacial and post-glacial periods) and continues to respond nowadays to environmental changes (Berthold et al. 1992; Fiedler 2003; Pérez-Tris et al. 2004; Visser et al. 2009; Piersma 2011). Migration is widely recognized as an adaptation to spatiotemporal fluctuations of resources and a response to environmental adversity (Gauthreaux 1982; Dingle 1996; Dingle & Drake 2007). Using the most seasonally suitable habitats at each moment in their life cycle, individuals may improve their fitness by increasing future fecundity and/or survival (Gauthreaux 1982; Ketterson & Nolan 1982; Berthold 2001). However, the benefit of increased resource availability and avoidance of harsh climatic conditions may be balanced by costs associated with the migratory process such as an increased risk of predation, exposure to new parasites and of course the energetic cost of movement (Alerstam, Hendenström & Akesson 2003). The balance between costs and benefits may also change in some years because of environmental stochasticity and density-dependent factors (Chapman et al. 2011; Kokko 2011). Also the interindividual variation in the costs and benefits of migration linked to individual characteristics may promote a broad range of migratory strategies within a population (Kaitala, Kaitala & Lundberg 1993).
In many animal populations (including insects, fishes, mammals and birds), the seasonal migration between breeding sites and winter quarters involves only a fraction of the population; this is called partial migration (Dingle 1996). Several, nonexclusive hypotheses have been proposed to explain the individual differences in migratory tendency. Partially migratory populations may consist of genetically different sedentary and migratory individuals (Lack 1944). In this case, migratory behaviour will be fixed at the individual level determining sedentary and migratory morphs (‘obligate partial migration’; Lack 1944; Berthold 2001). The partial migration strategy will be maintained, that is, be ‘evolutionarily stable’, if the pay-offs (lifetime reproductive success) of both morphs in the population are balanced (Gauthreaux 1982). Alternatively, partial migration may respond to a behavioural or a state-dependent mixed evolutionary strategy varying over an individual lifetime (‘facultative partial migration’; Ketterson & Nolan 1983; Kaitala, Kaitala & Lundberg 1993). In this case, the migratory behaviour is expected to be conditional on trade-offs influenced by environment (i.e. density-dependent processes; Kaitala, Kaitala & Lundberg 1993) and individual competitive abilities (linked to body condition, age or sex, the dominance hypothesis) (Ketterson & Nolan 1983; Chapman et al. 2011). Under the conditional strategy, fitness balancing between different migratory strategies is not necessary: migratory and resident strategies can evolve as the ‘best of a bad job’ where individuals maximize their fitness considering their internal and external state (Lundberg 1987). For instance, competitively inferior individuals (e.g. young or individuals in poor body condition) could be forced to migrate to suboptimal areas where their chances of survival may be lower (Lok et al. 2011). The combination of environmental conditions at wintering quarters with the route and distance of migration, which ultimately influences fitness components (i.e. survival and reproductive output), will determine the suitability of a wintering site (Lok et al. 2011). Among dense populations, sexually mature individuals arriving early in the season to breeding areas could benefit from a higher breeding success because of intense intraspecific competition for high-quality breeding territories and/or for mates favouring the reduction in migration distances; that is the arrival-time hypothesis (Kokko 1999). Hence, competition for the best breeding sites could push reproductive individuals to take higher mortality risks by wintering nearer the breeding area than they would have under the sole migration costs (Kokko 2011). In this case, residency is expected to increase with the probability of reproduction, that is when the individuals become sexually mature (Marques, Sowter & Jorge 2010). Although age-differential migration is a widespread phenomenon (Cristol, Baker & Carbone 1999), several studies have also found that individuals increase their fidelity to their previous wintering areas when they become older (Barbraud, Johnson & Bertault 2003; Lok et al. 2011). Familiarity with a known environment where conditions are relatively predictable from year to year may enhance wintering site fidelity (Greenwood 1980). Migratory behaviour is thus under the antagonistic influence of several ecological and endogenous factors; in this context, partially migratory species provide opportunities to understand the mechanisms, environmental or ecological factors causing some individuals to migrate when others remain resident year round, and the consequences of different migratory strategies (Chapman et al. 2011).
The greater flamingo (Phoenicopterus roseus) is such a partially migratory long-lived species with a delayed reproduction (Pradel et al. 1997; Barbraud, Johnson & Bertault 2003; Balkız et al. 2007; Johnson & Cézilly 2007). Some individuals spend the winter near their breeding colonies such as the Camargue (southern France), whereas others migrate to distant wintering areas over the western Mediterranean and North Africa (Barbraud, Johnson & Bertault 2003; Johnson & Cézilly 2007). However, catastrophic mortalities linked to extremely low temperatures happen occasionally near the breeding grounds (Johnson & Cézilly 2007). For example, during winter 1984–1985, consecutive extremely cold days with temperatures down to -11 °C caused the coastal lagoons in the south of France to freeze during 2 weeks causing the death of >3000 flamingos (Johnson, Green & Hirons 1991). These cold spell episodes may explain why many individuals spend the winter as far away as North Africa. On the other hand, wintering migration may be a costly process, especially for young inexperienced flamingos. Because flamingos use temporal and permanent wetlands as stopover sites (Amat et al. 2005), juvenile migration may in particular be limited during drought years. Also, the size of the colony may influence the probability of migration of juveniles through density-dependent processes. The balance between the benefits and costs of migration may vary with local environmental conditions and familiarity with the wintering site but may also depend on age. Indeed, in accordance with the arrival-time hypothesis, sexually mature individuals overwintering near their breeding colony may enhance their reproductive prospects.
Using a long-term (35 years) data set of more than 22 000 greater flamingos born in the Camargue (France) and resighted all over the Mediterranean we studied: (i) the age-related survival consequences of wintering in different areas (i.e. close to the natal area, at medium-distance areas or at long-distance areas); (ii) the impact of the extreme cold winter 1984–1985 on survival of resident individuals; (iii) the influence of the winter NAO index, a proxy for precipitation and droughts over the Mediterranean (Sousa et al. 2011), on juvenile migration and survival; (iv) the influence of the annual number of breeding pairs on first-year migration and (v) the ontogeny of wintering site choice and fidelity (age-specific dispersal among wintering sites). We applied multievent capture–recapture models (Pradel 2005) to estimate simultaneously the probabilities of survival, first migration, fidelity and dispersal between wintering sites.