1. Introduction
Survival and reproduction are two key demographic processes for all organisms, yet both are energetically costly and are in competition for the same limited resources (Williams 1966; Stearns 1992). High investment in reproduction one year diverts energy away from other processes, potentially reducing future survival probabilities and, particularly in long-lived species, impacting on an individual’s lifetime reproductive output (Williams 1966). Long-lived species therefore more often act as ‘prudent parents’, prioritising their own survival over reproduction (Sæther et al. 1993; Cam et al.1998). This strategy is widespread among long-lived seabirds (Bleuet al. 2016), especially among pelagic foragers (e.g., albatrosses; Jouventin & Dobson 2002 and king penguinsAptenodytes patagonicus ; Le Bohec et al. 2007), which regularly undertake long and energetically costly foraging trips. However, these nonbreeders are often not accounted for in population studies, despite their ability to obscure low population growth rates and reduce the reliability of population estimates (Lee et al.2017).
The drivers of reproductive skipping are predominantly grouped into adaptive and non-adaptive explanations. Adaptive explanations suggest animals actively avoid reproduction when conditions are poor (e.g., low food availability; Gauthier-Clerc et al. 2001) to enhance survival, ensure future breeding opportunities and improve lifetime reproductive output. Alternatively, reproductive skipping may be driven by non-adaptive individual constraints. For example, unavoidable events like pair-bond breakdown or forced nest site relocation may occur (Bradley et al. 2000; Jeschke et al. 2007; Salas et al. 2020). Additionally, the intrinsic quality of animals will vary regardless of these mechanisms, with some ‘higher quality’ individuals consistently achieving higher survival and reproductive rates (Cam et al. 1998; Jenouvrier et al. 2015). In reality these mechanisms co-occur. For example, ‘lower quality’ individuals are more likely than ‘high quality’ individuals to adaptively skip reproduction under poor conditions (Robert et al.2012; Souchay et al. 2018). The extent and drivers of reproductive skipping therefore vary not only between species and populations, but also at the individual level. Characterising inter-population variation in reproductive skipping is therefore vital to understand better localised population dynamics and to identify areas in need of conservation action. However, in many species reproductive skipping is yet to be evaluated; the endangered African penguin (Spheniscus demersus ) is a key example of this, and the focus of our study.
The African penguin is endemic to South Africa and Namibia, feeding on sardine (Sardinops sagax ) and anchovy (Engraulis encrasicolus ) stocks in the Benguela upwelling system (Wilson 1985). Since 1989, the African penguin population has declined by almost 65%, with the fastest declines occurring at colonies in South Africa’s Western Cape (Sherley et al. 2020). While historically driven by egg collection and guano scraping (Frost et al. 1976; Crawfordet al. 2018), current declines are predominantly attributed to reduced prey availability (Crawford et al. 2011; Sherley et al. 2020). Shifting geographic distributions of sardine and anchovy, the main prey of African penguins, apparently exacerbated by competition with fisheries, have been linked to both reduced survival (Sherleyet al. 2014, Robinson et al. 2015) and lower breeding success (Crawford et al. 2006; Sherley et al. 2013, 2021). However, population growth is strongly influenced not only by survival and breeding success, but also the proportion of the population that breeds each year (Cam et al. 1998; Le Bohec et al. 2007). Characterising the presence and drivers of reproductive skipping is therefore fundamental to improve our understanding of African penguin population dynamics to guide future protection of this endangered species.
Here we use mark-recapture data from two African penguin colonies in the Western Cape, spanning an 8-year period (2013–2020). Changes in survival and breeding propensity were examined over time and between colonies, with a focus on understanding the presence and trends in reproductive skipping behaviour. The effects of food availability on survival and breeding propensity were also examined, in line with previous work linking food availability to changes in survival and breeding success in this species (e.g., Crawford et al. 2011).