1 Introduction
Global environmental change is predicted to lead to warmer average temperatures, and more extreme weather events (Stott 2016). If these extreme weather events affect demographic processes, such as the reproduction, survival or dispersal of individuals, environmental changes may contribute directly to wildlife population dynamics (Shriver 2016; Saracco & Rubenstein 2020). In birds, juvenile life stages are often critical to the growth rate of populations (Newton 1989; Robinson et al. 2004; Clark & Martin 2007; Finkelstein et al. 2010), and to understand the consequences of environmental change on populations, we require a better understanding to what extent extreme events affect juvenile survival.
The survival of juvenile birds from fledging to their first reproduction is generally lower than the survival of adults (Maness & Anderson 2013; Naef-Daenzer & Grüebler 2016; Newton et al. 2016). Because juveniles are more susceptible to extreme weather events (Robinson et al. 2007), juvenile survival can vary enormously among years partly due to environmental conditions (Gaillard & Yoccoz 2003; Harris et al. 2007; Souchay et al. 2013).
The period from fledging to first reproduction in young birds includes several distinct life-history stages such as the post-fledging period in the natal home-range, dispersal and migration, wintering, and habitat selection and settlement at the first breeding site. All these different stages involve distinct challenges and therefore may impose differential costs on juvenile survival (Robinson et al. 2004; Ward et al. 2004; Grande et al. 2009; Grüebler et al. 2014a; Buechley et al. 2021). Environmental conditions are known to affect survival during certain life-history stages more than others (Reid et al. 2008; Dybala et al. 2013; Maness & Anderson 2013), and the effects of anthropogenic environmental changes on juvenile survival may therefore differ between the life-history stages of the pre-recruiting phase. To understand which life-history stages are particularly important for population dynamics, and to predict the consequences of environmental changes on populations, the contribution of environmental variation in each life-history stage to pre-reproductive survival must be understood (Robinson et al. 2004; Low & Pärt 2009; Cox et al. 2014; Grüebler et al. 2014a).
The life-history transition of fledging generally results in high mortality as fledged birds need to survive independently in unfamiliar environments (Low & Pärt 2009; Cox et al. 2014; Naef-Daenzer & Grüebler 2016). Following that period, young birds departing from their natal site face two further significant challenges in consecutive life-history stages. First, they move through and explore new, unfamiliar, and potentially inhospitable environments during natal dispersal (Robinson et al. 2004; Bowler & Benton 2005; Low & Pärt 2009; Clobert et al. 2012; Roque et al. 2021; Stillman et al. 2021). Second, they face the reduced availability and accessibility of food, and simultaneously increased thermoregulatory costs during winter (Altwegg et al. 2006; Thorup et al. 2013; Rubáčová et al. 2021). However, whether mortality is mainly associated with the post-fledging and dispersal phases, or with the environmentally challenging period during winter is poorly studied and may vary among species (Dybala et al. 2013; Grüebler et al. 2014a).
Here we examined the first-year survival of little owls (Athene noctua ) at biweekly temporal resolution to determine season-specific survival probabilities from fledging to the first reproductive attempt. The little owl is a small generalist mesopredator, inhabiting various open landscapes of Europe and Asia (Glue & Scott 1980). Many populations of little owls in central Europe have decreased in recent decades, and harsh winters with extended periods of snow cover have resulted in occasional population collapses (Van Nieuwenhuyse et al. 2023). Demographic analyses have indicated that juvenile survival and immigration are key demographic factors explaining differences in population growth rates (Schaub et al. 2006; Le Gouar et al. 2011). Therefore, it is important to understand the critical bottlenecks in the first year of the little owls’ life (Thorup et al. 2013; Tschumi et al. 2019) and to identify the environmental factors affecting survival in different juvenile life-history stages (Thorup et al. 2010; Le Gouar et al. 2011; Perrig et al. 2014). We have previously shown that survival of juvenile little owls was very low just after fledging, varied with fledgling body condition associated with nestling food supply, and increased over the first two months post-fledging (Perrig et al. 2017). However, it is unclear whether the subsequent dispersal and wintering stages impose an equal or different toll on the survival of juvenile little owls, and what effect extreme winter weather events have on the number of little owls surviving the first year.
In this study, we investigated two main hypotheses considering the period between independence and the first breeding season. First, we hypothesized that survival during autumn, when juveniles dispersed from parental territories, would be lower than in winter and the following spring because of the risk of exploring unfamiliar environments. Second, we predicted that survival during winter would be reduced depending on the severity of winter conditions because of limited access to food resources (Altwegg et al. 2006; Le Gouar et al. 2011; Rubáčová et al. 2021). We estimated survival probabilities from the post-fledging period to the first breeding season and could thus identify the most important seasonal bottleneck within the first year of juvenile little owls. This information will be critical to understand the potential future effects of a changing climate on population dynamics of little owls.