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Offspring survival rates in altricial birds during the post-fledging period are an essential factor in determining the fitness of parents and have a significant impact on general population dynamics. However, our current knowledge of post-fledging mortality and its causes remains fragmentary in most bird species, and even less information is available on the mortality of individuals of the same species in different environments. In order to address this gap in our knowledge, we studied fledgling mortality and its causes in Tengmalm’s owls (Aegolius funereus) during six breeding seasons in Central and North Europe using radio-telemetry. A total of 80 nestlings from 18 nests in Czechia (2010–2012, 2015) and 60 nestlings from 24 nests in Finland (2019, 2021) were radio-tracked during the post-fledging dependence period. The overall survival rate was much higher in Czechia (83%) than in Finland (53%), with predation identified as the primary cause of mortality in both areas. Avian predation was far higher in Finland, but mammalian predation was equivalent at both study sites. Pine martens (Martes martes) and goshawks (Accipiter gentilis) were the most common predators in Czechia and Finland, respectively. Starvation and disease, or mostly a combination of both, formed the second most common cause of death in both areas but were much more frequent in Finland than in Czechia. Offspring survival in both study sites was considerably higher in years of food abundance than in those of food scarcity. We suggest that the interactive effects of infections and poor body condition due to scarcity of main prey species induced higher mortality rates in offspring, particularly in the more challenging environment of North Europe. In contrast, fledgling owls were found to be able to fight off infections more successfully during rich food seasons. Finally, we encourage researchers to pay greater attention to the mutual influences of parasites and their definitive hosts and stress the importance of using radio or satellite tracking for mortality studies to identify causes of death more accurately.

Abstract We equipped 17 captive red deer males (Cervus elaphus) with GPS collars to measure inter-individual distances throughout the 5-months of the antler growth period. We expected some individuals to associate regularly with others while others would not. We predicted that males aggregating with others within a socially stable environment (Associates) would benefit from a form of “social buffering” and would likely have lowered cortisol (C) and testosterone (T) concentrations. Males only irregularly joining social groupings would experience elevated levels of aggression; according to the “Challenge hypothesis”, their T and C concentrations should increase. Interacting with a higher proportion of Associates did indeed reduce C concentrations. Conversely, avoiding Associates and challenging other males stimulated the T secretion. Admittedly, males avoiding regular proximity to others tended to develop the largest antlers. They probably benefited from frequent successful agonistic threats to conspecifics, resulting in elevated T concentrations. Regular association with tolerant, conspecifics and “social buffering” did not seem sufficient for producing larger antlers despite reducing C concentrations. Alternative social strategies were adopted within the same group of individuals and showed how the trade-off between these strategies could have an essential impact on C and T concentrations.

The knowledge about the mortality rate of offspring is crucial for estimating bird population dynamics and conserving species with declining populations. Parents of predatory birds provide food for their offspring during the post-fledging dependency period, which is frequently described as essential due to inexpert flying skills. Using radio telemetry, we studied fledglings’ probability of dying by starvation and predation in Tengmalms’ owls (Aegolius funereus). Nestlings (21 and 39) from 10 and 14 broods in 2019 and 2021, respectively, were equipped with leg-mounted tags and monitored throughout the post-fledging dependency period in west-central Finland. In total, 28 out of 60 fledglings did not survive the post-fledging dependency period (12 died due to starvation, and 16 were predated). The fledglings’ probability of dying by starvation and predation was 3.7 and 2.4 times higher, respectively, in the decreasing (2019) than during the increasing (2021) abundance of main foods (voles), showing that prey availability is essential for survival during the post-fledging dependency period. The probability of starvation increased with augmenting agricultural lands in the home range and increasing precipitation after fledging, which indicated that parent owls avoided hunting in open areas and during rainy nights. The predation rate during the post-fledging dependency period increased with augmenting cover of old-growth forests in the home range. This result suggested that coverage of old-growth forests is nowadays so small in the study area that they act as ecological traps for newly-fledged owlets. The reason is that the main avian enemies of Tengmalm’s owls’ (goshawks and Ural owls) also prefer old-growth forests for breeding and hunting.

The objective of the study was to investigate the effects of housing (deep litter + concrete floor vs. deep litter + ground soil with the possibility to dig burrows), and genotype (Mecklenburg or Hyplus) on aggressive behaviour, social contacts, does’ and kits’ injuries, and progeny mortality. Twelve groups of six rabbit does (n = 72) were assigned to four treatments (two housing systems and two genotypes). Aggressive behaviour of does, number of injuries on does and kits, and postnatal kit mortality were recorded. The effects of housing and genotype were tested using multivariate GLMM Models. We found that the housing treatment in interaction with the genotype had a significant effect on aggressive behaviours in group housed does (F3,12 = 14.34, p = 0.0003), where the lowest incidence of aggression was in Mecklenburg does housed on ground soil. Reduced aggression was reflected in a lower number of injuries in does (F3,68 = 10.51, p < 0.0001), number of injuries in kits, and kit mortality (F3,1 = 4.59, p < 0.0001, F3,54 = 43.94, p < 0.0001). The results indicate that the proper combination of genotype and housing should be carefully considered for breeding to reduce aggression and injury in group housed does.