Climatic variations have an important role on the population dynamics of vertebrates through their influence on their demographic parameters such as survival and breeding success of individuals (Coulson et al. 2001; Stenseth et al. 2002). Large-scale seasonal indexes, such as the NAO (North Atlantic Oscillation) index or the SOI (Southern Oscillation Index), account for major variations in weather and climate around the world and have been shown to affect ecological patterns (e.g. geographical ranges, abundance, diversity) and processes (e.g. birth and death rates) in both marine and terrestrial ecosystems (e.g. Stenseth et al. 2002, 2003, 2004). The NAO was one of the first weather index known to be associated with multiple ecological processes in the Northern Hemisphere and affects environmental conditions in the North Atlantic and the European continent (Hurrell 1995). On the other hand, the Southern Oscillation is the atmospheric component of a single large-scale coupled interaction called the El Niño–Southern Oscillation (ENSO). The ENSO is the most prominent known source of inter-annual variability in weather and climate, showing stronger effects in the South Pacific regions, but also affecting weather variability all around the world (Stenseth et al. 2003). This is because of teleconnections, that is, the physical relationships that result from the dynamics of atmospheric and oceanic waves (e.g. Murphy et al. 2007). One of the stronger ENSO teleconnections is with global tropical cyclone activity in the North Atlantic, with more and fewer hurricanes than average in La Niña and El Niño years, respectively (Holland 2009). These climatic variations may also affect environmental conditions for the whole food chain by changing food quality or availability at all trophic levels (Planque & Taylor 1998; O'Brien et al. 2000). For example, there is a linkage between the SOI in the Pacific and coastal upwellings off West Africa in the Atlantic through an atmospheric teleconnection. While El Niño suppresses upwelling along the coast, La Niña increases it (Roy & Reason 2001; Rouault, Pohl & Penven 2010). All these environmental changes may specially influence population dynamics of marine organisms, such as seabirds, for which oceanographic conditions may have stronger effects than on terrestrial species (e.g. Frederiksen et al. 2008; Barbraud et al. 2011). Seabirds spend most of the time at sea, and they travel long distances in one season being subjected to potentially different conditions during the breeding and the non-breeding periods (both during migration and wintering areas).
Here, we exploit an analytical tool recently developed in mark–recapture modelling, namely the multi-event approach (Pradel 2005), to analyse whether climatic variation influences several demographic parameters of the seabird Cory's shearwater Calonectris diomedea at two colonies in the western Mediterranean. Such methodological approach allowed us not only to assess the potential association between climatic indexes and adult survival (e.g. Sandvik et al. 2005; Oro et al. 2010) or transience (Tavecchia et al. 2007) but also to analyse simultaneously their effects on other parameters usually difficult to estimate, such as the probabilities of reproductive skipping (i.e. to take a sabbatical year) and those of nest dispersal (Barbraud & Weimerskirch 2010; Sanz-Aguilar et al. 2011). The fact that Cory's shearwater is a seabird that carries out trans-equatorial Atlantic migration from their breeding colonies to their wintering sites (e.g. González-Solís et al. 2007; Dias et al. 2010) made such species a good candidate to test several hypotheses about the influence of global climatic indexes on most of its demographic parameters.
Seabirds, as long-lived species, show extreme values in life-history traits, with very high adult survival, low fecundity and delayed sexual maturity (Weimerskirch 2002) and individual fitness depending primarily on adult survival (Roff 1992; Crone 2001). So we tested the life-history theory prediction that climate should influence more the trait less associated with fitness, that is, breeding success, whereas the most sensitive parameter, that is, adult survival, should be more buffered against that component of environmental stochasticity (Saether & Bakke 2000; Grosbois & Thompson 2005; Sandvik et al. 2005). This is the so-called environmental canalization of fitness components, higher for adult survival in long-lived organisms (Gaillard & Yoccoz 2003). Although its frequency and environmental drivers still remain unclear for most seabird species, intermittent breeding (or skipping) and transience should have an intermediate sensitivity to climatic variability (e.g. Weimerskirch, Jouventin & Stahl 1986; Bradley, Wooller & Skira 2000; Cubaynes et al. 2011). The same sensitivity should apply for dispersal, for which a much better knowledge on the environmental drivers (e.g. predation, disturbance and massive reproductive failure) is accumulated, especially in gulls and terns (e.g. Cam et al. 2004; Braby et al. 2012). So we tested the hypothesis that under harsh climatic conditions during wintering, birds would arrive in poor physical condition to the breeding sites, which would increase the probabilities of becoming a transient, either because they die after breeding (i.e. a cost of reproduction, see Sanz-Aguilar et al. 2008) or because they permanently disperse to another colony (Tavecchia et al. 2007, 2008). After such bad wintering conditions, individuals may also opt for a more conservative strategy, that is, to skip reproduction (Cubaynes et al. 2011), or if they widow or divorce, they can also disperse to alternative sites within the colony (i.e. nest dispersal, Kim et al. 2007). In summary, we expected that climatic indexes should have a more pronounced effect on breeding success than on transience, reproductive skipping and nest dispersal probabilities, whereas their influence on adult survival should be much lower.
What should be the influence of each of the climatic indexes? Since Cory's shearwaters winter in the Atlantic Ocean and mainly in its Southern areas (see González-Solís et al. 2007), NAO but specially SOI should have a direct influence on adult survival, and secondarily on transience, reproductive skipping and nest dispersal. Survival can be specially affected by hurricanes, and the Atlantic tropical hurricane season, mostly governed by the SOI, coincides with shearwaters migration to their wintering sites. On the other hand, reproduction occurs in the Mediterranean, and thus, breeding success should be more affected by the NAO than by the SOI.