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.