4 Discussion
We show that winter survival in juvenile little owls was highly
dependent on the number of days with snow cover, but that annual
survival in years with mild winters was characterized by remarkably
equal survival probabilities throughout seasons once individuals had
survived the first weeks after fledging. Overall, only a fifth of
juvenile little owls survived to the age of one year, and harsh winters
with extended periods of snow cover reduced annual survival probability
by > 30%. However, the most important mortality bottleneck
was the month after fledging, which accounted for 40-60% of the total
first year mortality. Supplementary food provided to nestlings increased
annual survival by 23.5% due to its beneficial effect on post-fledging
survival. A changing climate and food supply during the nestling stage
may therefore have similarly large effects on the survival of juvenile
little owls during their first year of life.
Our results are consistent with previous findings that harsh winters
affect survival of little owls (Le Gouar et al. 2011; Thorup et al.
2013; Michel 2016), but our study was able to disentangle mortality
during the post-fledging period in summer, during dispersal in autumn,
and in winter. We show that winters with constant snow cover over two
months result in less than half of juveniles surviving a winter (Table
1). Snow cover and frosty conditions are widely known to prevent owls
and other resident bird species from accessing food, thus leading to
reduced body condition and increased foraging efforts, which ultimately
result in increased mortality rates (Sonerud 1986; Kostrzewa &
Kostrzewa 1991; Altwegg et al. 2006; Riegert & Fuchs 2011; Naef-Daenzer
& Grüebler 2016). Young owls may also become more vulnerable to
predators when they struggle to find suitable shelters and suffer from
excessive thermoregulation costs (Bock et al. 2013; Grüebler et al.
2014b; Naef-Daenzer & Grüebler 2016). As juvenile survival represents a
key factor in little owl population dynamics (Schaub et al. 2006; Le
Gouar et al. 2011), our results suggest strong population effects of
extended periods of snow cover in this species, which is consistent with
past evidence of population collapses after harsh winters from the last
century (summarized in Van Nieuwenhuyse et al. 2023).
We found that survival increased for food supplemented and for heavier
birds, but these effects occurred only in the post-fledging period and
were not detectable in subsequent seasons. A previous study revealed
that early-life effects associated with nestling food supplementation
affected departure decisions from the parental home-range, but not
movement decisions during natal dispersal after departure (Fattebert et
al. 2019). Similarly, early-life conditions affected survival primarily
during the post-fledging period, but not afterwards, and we speculate
that the selection imposed by early post-fledging mortality reduces the
influence of early-life conditions at later stages in life (Sergio et
al. 2014; Sergio et al. 2019). Nonetheless, given the magnitude of the
difference in survival between food-supplemented and un-supplemented
(natural) little owl fledglings during the post-fledging period, 23.5%
more food-supplemented fledglings survived the first year (Fig. 2).
Thus, favourable natal conditions in our study species may affect
population level processes mainly by non-random selection of juveniles
in the nestling and post-fledging period, with an effect of similar
magnitude as that imposed by harsh winters.
Increased predation risk of inexperienced birds is widely known as a
main factor for lower juvenile than adult survival probability during
the first year of life in general (Clutton-Brock et al. 1985; Sunde
2005; Maness & Anderson 2013; Naef-Daenzer & Grüebler 2016), and in
little owls in particular (Naef-Daenzer et al. 2017; Šálek et al. 2019).
Juvenile predation rate is often further increased under poor food
conditions (Rohner & Hunter 1996; Coles & Petty 1997) and when
juveniles explore unknown areas (Bélichon et al. 1996; Lima 1998; Yoder
et al. 2004). Elevated costs during natal dispersal have led to the
theory that the dispersal stage is a bottleneck with respect to survival
and evolutionary ecology (Bowler & Benton 2005; Benard & McCauley
2008; Bartoń et al. 2012; Bonte et al. 2012; Hardouin et al. 2012; Väli
et al. 2021). Our results show that in little owls the survival during
the autumn dispersal season was not noticeably reduced. However, because
we focused on a fixed temporal definition of the autumnal dispersal
season, we were not able to investigate survival during the actual
dispersal movement of individuals. Main natal dispersal movements are
generally of short duration in little owls (median = 10 days; Fattebert
et al. 2019). Mortality might be considerably increased during the few
days of active dispersal without affecting our overall survival
probability of the season. More detailed investigations of survival in
relation to individual movements may reveal more nuanced patterns in
survival probability and will illuminate the costs of dispersal in
little owls (Yoder et al. 2004). Nestling food supply and its effects on
post-fledging survival may therefore be a more important factor for
juvenile survival and the growth rate of populations than the dispersal
or wintering stages (Low & Pärt 2009; Cox et al. 2014; Grüebler et al.
2014a; Naef-Daenzer & Grüebler 2016; Martin et al. 2018).
We found only a relatively small and uncertain effect of increased snow
cover on detection probability, which was generally very high in our
study. Thus, while some temporary emigration is possible and accounted
for by our model, we can be confident that our estimate of lower
survival in harsh winters is not affected by little owls temporarily
leaving the study area during periods with extensive snow cover. We
cannot exclude the possibility that little owls performed permanent
escape movements that have been recorded in other owl species during
severe winter conditions (Sonerud 1986; Mysterud 2016; Gura 2023),
because permanent emigration out of the monitored area and mortality are
confounded in our data set. However, permanent emigration would be most
likely during the autumn dispersal phase (Hauenstein et al. 2019; Van
Nieuwenhuyse et al. 2023), but we did not find a reduction in apparent
survival during this season with the highest mobility of juveniles. We
therefore suggest that permanent emigration likely only accounts for a
small proportion of the estimated mortality.
In summary, the survival of little owls during the first year can be
characterized by two bottlenecks, differing in the underlying
mechanisms. Juveniles first encounter a survival bottleneck in summer
immediately after fledging from the nest, and another bottleneck in
winter if environmental conditions reduce the accessibility of food.
Contrary to the general hypothesis of elevated costs during natal
dispersal, our results indicate that the autumn dispersal season in
little owls is not more hazardous than other seasons during the first
year of life. Increasingly warmer winters with less snow cover, which
are expected under climate change, will therefore likely have beneficial
effects on the juvenile survival probability and thus, on population
dynamics of little owls.