Ecologists are challenged to explain the remarkably diverse ways in which organisms allocate time and energy to reproduction, and to unravel fitness consequences of these complex investment patterns. Because individuals typically have limited time or nutrients available for reproduction, advantages of raising more offspring may be offset by costs of lowered parental survival or progeny quality, as well as diminished future reproductive output; trade-offs such as these form the foundation of life-history theory (Roff 1992; Stearns 1992). Studies of optimal timing of breeding indicate that offspring production and recruitment are generally greater for early breeding individuals in numerous taxa [e.g. plants (Kelly & Levin 2000); fish (Schultz 1993); mammals (see Dobson, Risch & Murie 1999); birds (Perrins 1970; Daan, Dijkstra & Tinbergen 1990)], but forces countering early nesting have been neglected in most avian studies (see Nilsson 1994; Svensson 1997; Brown & Brown 1999; for notable exceptions). Here, we focus on these and related processes by analysing patterns of reproduction and breeding success in large samples of individually marked birds of three species, principally with path analysis.
Path analysis begins with the construction of a diagram(s) portraying possible causal and non-causal relationships among response and explanatory variables in a system based on a priori knowledge, and the method is being used increasingly for testing complex evolutionary and ecological hypotheses (Kingsolver & Schemske 1991; Mitchell 1992; Wootton 1994; Sinervo & DeNardo 1996; Grace & Pugesek 1997). With large sample sizes, path analysis may reveal critical relationships among variables correctly or pinpoint where key experimental manipulations are warranted. Our principal objective was to evaluate sources of variation in offspring survival and recruitment, while testing general ecological hypotheses about natural selection on timing of breeding, brood size and female body size and condition. Previous studies have investigated thoroughly some of these hypotheses but, to our knowledge, this is among the first to simultaneously evaluate effects of multiple, interacting factors on recruitment processes.
HYPOTHESES AND PREDICTIONS
We generated a general diagram about interrelationships among variables that could affect offspring prefledging survival and subsequent recruitment (Fig. 1); other obvious paths not shown may also be retained during analysis, but the main hypotheses and assumptions are explained below. In general, factors positioned to the right are causally affected by those on the left, and all directly or indirectly (i.e. effects are channelled through intermediate variables) influence one or both response variables, fledging success and recruitment (see Methods for variable definitions and further explanation).
Brood size. There is little evidence that brood size affects reproductive success in waterfowl (Rohwer 1992; Johnson, Nichols & Schwartz 1992; Milonoff, Pöysä & Virtanen 1995), but larger broods could contribute more fledglings or recruits simply by virtue of initially having more ducklings; therefore, other things being equal, there is a greater likelihood that more ducklings will survive to be recruited. Females may also devote greater maternal care to larger broods because they perceive them to be more valuable (e.g. Eadie, Kehoe & Nudds 1988), possibly enhancing duckling survival in larger broods.
Duckling body mass. A classic reproductive trade-off hypothesis states that individuals producing more offspring do so at the expense of progeny size or quality. This hypothesis is supported by diverse empirical studies [e.g. arthropods (Fox & Czesak 2000); reptiles (Rowe 1994); mammals (Dobson et al. 1999); birds (Nager, Monaghan & Houston 2000; Williams 2001)], but Bernardo (1996) argued convincingly that the failure to find this trade-off in many studies may be due to confounding effects of factors that shape reproductive allocation regimes (e.g. state-dependent reproductive effort: Schwarzkopf 1992; McNamara & Houston 1996). Thus, in addition to maternal fitness, optimal nutrient allocation to individual eggs and clutches may depend critically on both the expected future value of neonates as well as resources available to invest in offspring (e.g. Eldridge & Krapu 1988; Glazier 2000). This trade-off is expected to be most pronounced in avian species which make large nutrient investments in clutches and have limited post-hatch parental care (Lack 1967; Lack 1968), yet there is disagreement about the existence let alone strength of this trade-off in waterfowl, a group which more than satisfies both of these prerequisites (Blackburn 1991; Rohwer 1991).
Because intraspecific analyses may not account fully for effects of other interacting variables (e.g. Bernardo 1996), we tested specifically for an inverse relationship between mean (per brood) duckling mass and brood size, while controlling effects of other variables. Duckling body mass was placed in the path diagram to the left of brood size because inheritance estimates for egg size (i.e. duckling mass) generally are greater than those for brood size (Lessells, Cooke & Rockwell 1989; Larsson & Forslund 1992; Potti 1999); females are expected to balance energetic investment in clutches by lowering egg number, not by compromising egg size (Smith & Fretwell 1974). Indeed, heavier ducklings may have higher survival than lighter ones because they are better able to endure inclement weather and periods of food shortage, capture prey or evade predators (reviewed by Williams 1994).
Female body mass and wing length. The importance of female size and condition to reproductive investment and success in birds is debated. Some workers report a positive correlation between female size and both egg and clutch sizes (Barbraud et al. 1999), but others have not found these relationships (Winkler & Allen 1996). Nutrient storage and use may be influenced by body size, but the interplay of body size and condition in reproductive effort and success is uncertain (Alisauskas & Ankney 1990; Cooke, Davies & Rockwell 1990; Congdon et al. 1999), and has not been evaluated fully in birds (Barbraud et al. 1999). Larger individuals may be heavier, and size adjustments are performed typically in many animal species to derive an index of ‘condition’. Larger birds, and those with better condition, may have greater breeding success than smaller, light-weight individuals because they may be able to retain larger nutrient reserves after hatching or because larger size confers an advantage when brooding ducklings or in acquiring food (i.e. via social dominance). Therefore, we placed wing length (an index of size) before female mass in the path diagram.
Hatching date and annual effects. The influence of hatching time on offspring survival and recruitment, favouring early hatched young, is a ubiquitous pattern in birds (Perrins 1970; Daan et al. 1988), including some ducks (e.g. Dzus & Clark 1998), but it is unclear if this pattern results from higher fledging success, enhanced first-year survival rates combined with greater dominance (Spear & Nur 1994) or both. Therefore, we tested first whether duckling survival and recruitment declined with hatching date in ducks and then also evaluated the hypothesis that early hatched young have an additional advantage that carries over beyond the fledging stage to recruitment.
If early hatched offspring have higher survival and recruitment than late-hatched ones, what counteracts very early breeding (Nilsson 1994)? Hypotheses invoked to explain delays in early nesting in waterfowl centre primarily on female quality or nutrient reserve acquisition (reviewed by Rohwer 1992). We determined whether there was a cost of very early breeding, by explicitly considering curvilinear relationships between hatching date and survival or recruitment using logistic regression analysis (details below). We also looked for yearly fluctuations in production as part of this analysis.
Female age. Typically, avian reproductive rates and success are age-dependent (Sæther 1990; Stearns 1992), and ducks are no exception (Blums, Hepp & Mednis 1997a). Older birds generally breed earlier than younger ones, they may be larger or have better body condition, invest more heavily in broods, and therefore have higher breeding success, but interactions among these variables are not well quantified. Thus, we gauged effects of age on duckling survival and recruitment by exploring direct and indirect paths to all variables on the right side of the path diagram.
Environmental effects. Adverse conditions during the first few days post-hatching can cause high duckling mortality (e.g. Mendenhall & Milne 1985; Korschgen et al. 1996), so we looked specifically for effects of weather on fledging success. The relative roles of abiotic vs. biotic factors on duckling survival are virtually unknown in most duck species, nor have mediating effects of female age (i.e. behaviour) been estimated. Therefore, we included indirect paths from female age and hatching date to duckling survival via weather conditions.
Duckling production, harvest index and breeding density. Total duckling production in the year of hatching may affect recruitment negatively the following year if ducklings compete for food and space, and survival decreases (Savard, Smith & Smith 1991). Similarly, if a large proportion of fledged ducklings is harvested by hunters, recruitment might increase because of compensatory effects on overwinter survival or reduced competition for breeding space the next spring. Alternatively, excessive harvest of young females might lower recruitment if potential recruits are removed from the population.
If females compete for nesting space or other resource then the number of recruits could be related inversely to breeding density [total number of recorded nests (nesting attempts); see below]; alternatively, if the latter estimate reflects habitat suitability rather than population density (an unknown number of breeding females may have been present but did not nest) then we would predict a positive relationship between recruitment and breeding density (Fig. 1). Female age, hatching date and female or duckling size could mitigate effects of duckling production, harvest or breeding density on recruitment, so we considered indirect paths to female recruitment via these variables.