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.