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Individual Responses of Adult Little Owls (Athene noctua) to Environmental Conditions


Michel, Vanja Tamian. Individual Responses of Adult Little Owls (Athene noctua) to Environmental Conditions. 2016, University of Zurich, Faculty of Science.

Abstract

Life-history theory predicts that individuals need to adjust their behaviour to specific environmental conditions in order to maximise their fitness. Thus, to understand how the environment affects demography and population dynamics, it is crucial to study the behavioural responses of individuals to different aspects of the environment as well as the consequences of these responses for reproduction and survival. Two aspects of the environment affect animals across all life-history stages. First, the availability of food resources determines the amount of energy available for different life-history functions. If resources are limited during the breeding season, parents need to decide how to allocate limited energy among reproduction and self-maintenance. As a result, individuals may adjust breeding site selection, range use, home-range size, or parental effort to the availability of food. Second, predators influence population dynamics and the spatial distribution of animals directly by lethal predation and indirectly by eliciting anti-predator behaviours.
In this thesis, I used the little owl (Athene noctua) as a study system to investigate three issues faced by adult animals: the allocation of energy between reproduction and self-maintenance as a function of the habitat conditions (Chapters 1-3), the decision between philopatry and breeding dispersal (Chapter 4), and the behavioural response to predation risk (Chapter 5). Using very high frequency (VHF) radio-telemetry, I tracked individuals throughout the year, allowing me to calculate season-specific home-range sizes and survival rates with a high temporal resolution. A nestling food supplementation experiment enabled me to investigate parental responses to the nutritional state of the nestlings and to evaluate food limitation of reproduction in habitats of different quality.
In Chapter 1, I showed that, irrespective of the parental home-range size, clutch size was positively correlated with the proportion of food-rich habitat close to the nest, suggesting that the distance between high quality food resources and suitable nest sites plays an important role for the productivity of little owls. Paternal home-range size was positively correlated with the survival of the father and the nestlings. In homogenous landscapes, parents seem to expand their home-range to include additional food-rich habitat, thereby gaining a net benefit in terms of reproductive success and parental survival. In Chapter 2, I showed that nestlings in poor habitats only received two thirds of the biomass delivered to nests in high-quality habitats, resulting in lower survival of nestlings in poor habitats. Food supplementation increased the survival of adult males and females, not only during the breeding season but also during the following periods of the year, suggesting that little owl parents are limited by the availability of food resources (Chapter 3). Thus, the first three chapters of this thesis provide insights into the mechanisms regulating the trade-off between current reproduction and self-maintenance under varying habitat conditions. I conclude from the results that habitats of low quality affect individual life histories and local demographic rates both in the short term (reduced current reproductive output), and in the long term (reduced survival prospects and future reproductive success of adults).
In Chapter 4, I investigated the patterns, proximate causes and consequences of breeding dispersal. Little owls showed an overall dispersal rate of 20% from one year to the next. Reproductive failure strongly increased the propensity of individuals to disperse. In addition, individuals that had already shown breeding dispersal were more likely than others to change their territory again. On average the reproductive success of dispersers was similar to philopatric individuals, but dispersers reared 0.6 fledglings more after dispersal compared to the preceding year. Our data confirm the strong site fidelity of adult little owls, with 98% of the little owls staying within 2 km of their previous nest site. However, depending on habitat-linked brood loss, the rates of breeding dispersal may be considerable. Therefore, I suggest that breeding dispersal strongly contributes to the small scale dynamics and turn-over within little owl populations.
Finally, in Chapter 5, I used the playback of little owl and tawny owl (Strix aluco) calls to model the site occupancy of the two species across the whole study area and to test whether the distribution of tawny owls affects the distribution of little owls. Site occupancy of little owls strongly decreased within 150 m from the forest in the presence, but not in the absence of tawny owls. In addition, little owls strongly avoided areas closer than 150 m to forest edges during their night-time range use. These results suggest that little owls perceive the predation risk caused by tawny owls and adapt their spatial behaviour accordingly.
I draw two main conclusions from this PhD thesis. First, habitat-specific food supply during the breeding season is of crucial importance for both juveniles and adults. On one hand, it strongly affects productivity by regulating the clutch size as well as the survival of juveniles. On the other hand, it is an important determinant of the survival prospects of adults, thereby affecting lifetime reproductive success. On the individual level, nest site selection determines the access to resources. Thus, settlement decisions and habitat selection represent key ecological mechanisms affecting individual fitness. Second, both habitat characteristics and the presence of predators strongly influence the spatial distribution and dynamics of little owls. Site-occupancy patterns suggest that little owls settle in areas, which minimize predation and secure optimal food resources. Since breeding dispersal mainly occurred in response to reproductive failure, habitat characteristics linked to reproductive failure further affect local population structure and dynamics. In conclusion, the results provide insights into the ecological mechanisms that shape the spatial distribution and the turn-over of individuals within a population.

Abstract

Life-history theory predicts that individuals need to adjust their behaviour to specific environmental conditions in order to maximise their fitness. Thus, to understand how the environment affects demography and population dynamics, it is crucial to study the behavioural responses of individuals to different aspects of the environment as well as the consequences of these responses for reproduction and survival. Two aspects of the environment affect animals across all life-history stages. First, the availability of food resources determines the amount of energy available for different life-history functions. If resources are limited during the breeding season, parents need to decide how to allocate limited energy among reproduction and self-maintenance. As a result, individuals may adjust breeding site selection, range use, home-range size, or parental effort to the availability of food. Second, predators influence population dynamics and the spatial distribution of animals directly by lethal predation and indirectly by eliciting anti-predator behaviours.
In this thesis, I used the little owl (Athene noctua) as a study system to investigate three issues faced by adult animals: the allocation of energy between reproduction and self-maintenance as a function of the habitat conditions (Chapters 1-3), the decision between philopatry and breeding dispersal (Chapter 4), and the behavioural response to predation risk (Chapter 5). Using very high frequency (VHF) radio-telemetry, I tracked individuals throughout the year, allowing me to calculate season-specific home-range sizes and survival rates with a high temporal resolution. A nestling food supplementation experiment enabled me to investigate parental responses to the nutritional state of the nestlings and to evaluate food limitation of reproduction in habitats of different quality.
In Chapter 1, I showed that, irrespective of the parental home-range size, clutch size was positively correlated with the proportion of food-rich habitat close to the nest, suggesting that the distance between high quality food resources and suitable nest sites plays an important role for the productivity of little owls. Paternal home-range size was positively correlated with the survival of the father and the nestlings. In homogenous landscapes, parents seem to expand their home-range to include additional food-rich habitat, thereby gaining a net benefit in terms of reproductive success and parental survival. In Chapter 2, I showed that nestlings in poor habitats only received two thirds of the biomass delivered to nests in high-quality habitats, resulting in lower survival of nestlings in poor habitats. Food supplementation increased the survival of adult males and females, not only during the breeding season but also during the following periods of the year, suggesting that little owl parents are limited by the availability of food resources (Chapter 3). Thus, the first three chapters of this thesis provide insights into the mechanisms regulating the trade-off between current reproduction and self-maintenance under varying habitat conditions. I conclude from the results that habitats of low quality affect individual life histories and local demographic rates both in the short term (reduced current reproductive output), and in the long term (reduced survival prospects and future reproductive success of adults).
In Chapter 4, I investigated the patterns, proximate causes and consequences of breeding dispersal. Little owls showed an overall dispersal rate of 20% from one year to the next. Reproductive failure strongly increased the propensity of individuals to disperse. In addition, individuals that had already shown breeding dispersal were more likely than others to change their territory again. On average the reproductive success of dispersers was similar to philopatric individuals, but dispersers reared 0.6 fledglings more after dispersal compared to the preceding year. Our data confirm the strong site fidelity of adult little owls, with 98% of the little owls staying within 2 km of their previous nest site. However, depending on habitat-linked brood loss, the rates of breeding dispersal may be considerable. Therefore, I suggest that breeding dispersal strongly contributes to the small scale dynamics and turn-over within little owl populations.
Finally, in Chapter 5, I used the playback of little owl and tawny owl (Strix aluco) calls to model the site occupancy of the two species across the whole study area and to test whether the distribution of tawny owls affects the distribution of little owls. Site occupancy of little owls strongly decreased within 150 m from the forest in the presence, but not in the absence of tawny owls. In addition, little owls strongly avoided areas closer than 150 m to forest edges during their night-time range use. These results suggest that little owls perceive the predation risk caused by tawny owls and adapt their spatial behaviour accordingly.
I draw two main conclusions from this PhD thesis. First, habitat-specific food supply during the breeding season is of crucial importance for both juveniles and adults. On one hand, it strongly affects productivity by regulating the clutch size as well as the survival of juveniles. On the other hand, it is an important determinant of the survival prospects of adults, thereby affecting lifetime reproductive success. On the individual level, nest site selection determines the access to resources. Thus, settlement decisions and habitat selection represent key ecological mechanisms affecting individual fitness. Second, both habitat characteristics and the presence of predators strongly influence the spatial distribution and dynamics of little owls. Site-occupancy patterns suggest that little owls settle in areas, which minimize predation and secure optimal food resources. Since breeding dispersal mainly occurred in response to reproductive failure, habitat characteristics linked to reproductive failure further affect local population structure and dynamics. In conclusion, the results provide insights into the ecological mechanisms that shape the spatial distribution and the turn-over of individuals within a population.

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Additional indexing

Item Type:Dissertation
Referees:Keller Lukas F, Grüebler Martin Urs, Naef-Daenzer Beat, Manser Marta B
Communities & Collections:07 Faculty of Science > Institute of Evolutionary Biology and Environmental Studies
Dewey Decimal Classification:570 Life sciences; biology
590 Animals (Zoology)
Language:English
Date:2016
Deposited On:07 Nov 2016 10:47
Last Modified:07 Nov 2016 10:48

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