The long-term consequences of conditions during early development for fitness are important for life history evolution, population ecology and the interface between them (Lindström 1999; Cam, Monnat & Hines 2003). Early conditions are likely to influence offspring quality, which is an important factor in main life history trade-offs (Stearns 1992). For example, long-term effects of early conditions shift the optimal solution of the trade-off between quantity and quality of offspring towards fewer offspring of higher quality. In an ecological context, effects of early conditions may amplify a source–sink population structure when breeders in good habitats produce many offspring that additionally have higher fitness prospects. Effects of early conditions on life history traits can also provide feedback on population dynamics. For example, early conditions can affect the prospects of entire cohorts, which may have delayed consequences for population dynamics (Albon, Clutton-Brock & Guinness 1983; Sæther 1997); condition-dependent dispersal is important for meta-population dynamics (Ims & Hjermann 2001).
Conditions during early life can vary as a result of differences in parental quality and environmental quality; the latter are partly under parental control. For example, parental habitat selection affects the spatial environment in which offspring are reared; a mother's choice of date of birth affects the temporal environment in which offspring are reared. Parental and environmental quality are therefore linked intricately and probably often positively intercorrelated (e.g. van de Pol et al. 2006).
Favourable rearing conditions, i.e. being raised with a ‘silver spoon’ (Grafen 1988), are often beneficial throughout life, but are thought to be most pronounced during early life stages (Lindström 1999; Metcalfe & Monaghan 2001). Two hypotheses may explain why the effects of early conditions are most pronounced early in life. First, the environmental stochasticity that individuals experience during life accumulates and this decreases the potential for long-term effects of early conditions. Secondly, selection gradients of fitness components usually become less strong during the course of life (Charlesworth 1980; Caswell 2001). Consequently, parents can expect a higher fitness payoff by improving conditions that enhance the short-term fitness prospects of offspring than by investing that same effort in improving early conditions that pay off later in life.
Many studies report ‘silver spoon’ effects on juvenile survival (e.g. Tinbergen & Boerlijst 1990; Magrath 1991; Green & Cockburn 2001; Perrins & McCleery 2001), natal dispersal (e.g. van der Jeugd 2001) and adult morphology (e.g. Boag 1987; Gustafsson, Qvarnström & Sheldon 1995; de Kogel & Prijs 1996). In addition, several studies suggest that early conditions can also have long-term consequences on fitness components, such as adult survival before recruitment (Harris et al. 1994; Cam et al. 2003) and recruitment probability (Reid et al. 2003). However, less is known about the long-term consequences of early conditions on fitness components during the reproductive stage, such as breeder survival (Perrins & McCleery 2001), quality of the acquired breeding habitat (Verhulst, Perrins & Riddington 1997) and reproductive success (Gustafsson & Sutherland 1988; Haywood & Perrins 1992; Visser & Verboven 1999).
When investigating the ecological and evolutionary consequences of early conditions, cost and benefits should be calculated in terms of fitness. Fitness measures are calculated over (at least) a lifetime [e.g. lifetime reproductive success (LRS), finite population growth rate (λ)] and they are the combined result of many individual fitness components (early as well as late in life). For logistic reasons, most studies investigate only one or a few fitness components. Consequently, little is known about the relative importance of short- and long-term consequences of early conditions on fitness. Integrating all fitness components into one fitness measure is also important because early conditions do not always affect all fitness components positively (e.g. King 2002; Olsson & Shine 2002).
In this paper we use data from a 20-year study to quantify and compare the contribution of short- and long-term effects of early conditions on the fitness of oystercatcher (Haematopus ostralegus L.) offspring. In coastal breeding oystercatcher populations there is usually a clear dichotomy in habitat quality, based on the nesting location relative to the foraging area (Safriel, Ens & Kaiser 1996). Some parents can take their chicks to the food, because breeding and feeding territory are adjacent. Other parents have to bring the food to their chicks, because the breeding and feeding territory are segregated spatially. Parents that can escort their chicks to the food consistently produce two to three times more fledglings per year, because transporting food to the chicks is costly (Ens et al. 1992). Early conditions in oystercatcher are therefore characterized by the habitat type in which an individual is reared. However, as natal habitat quality is probably linked intricately with parental quality (van de Pol et al. 2006) we prefer to use the term natal origin, which encompasses both an environmental and parental (genetic and non-genetic) component of early conditions.
We estimated the fitness consequences associated with the natal origin of offspring by calculating the effects of natal origin on juvenile survival and adult survival, recruitment probability, as well as on their subsequent breeding career (Fig. 1). We combined fitness components to estimate fledgling fitness prospects (LRS and λ). Subsequently, using stage-structured population models, we compared the sensitivity of fitness to long-term and short-term effects of early conditions. Finally, we discuss how long-term effects of natal origin affect life history evolution (settlement decisions) as well as population ecology (source–sink population interactions).