Browsing by Author "Sumasgutner, Petra"

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    A mechanistic approach to understanding the colour polymorphism in black sparrowhawks (Accipiter melanoleucus)
    (2021) Nebel, Carina; Amar, Arjun; Sumasgutner, Petra
    Species that exhibit large variation in phenotypic traits, are commonly considered to have a stronger evolutionary potential. However, how they are capable to maintain polymorphisms remains a fundamental problem in evolutionary biology. Colour polymorphic species provide ideal study systems to explore the processes that lead to variation maintenance. About 3.5% of all bird species are colour polymorphic, but it is an especially common phenomenon in Accipitridae (22%), which indicates that it has an adaptive function in this bird group and makes them ideal model systems to study evolutionary processes. The black sparrowhawk (Accipiter melanoleucus) occurs in two discrete colour variants: dark and light. The two morphs differ in the expression of white and black feathers on the breast, belly and underwing coverts. The morph has been associated with ambient light-dependent foraging success and activity behaviour: Dark morphs forage more and have higher foraging success under low light conditions whereas light morphs forage independently of light levels but are better foragers under bright light conditions. This is hypothesized to be due to a crypsis advantage for the morphs under these conditions. During the winter breeding period, the predominating low light conditions on the Cape Peninsula (Western Cape, South Africa) could create an advantage for dark morphs, which should result in their higher survival and higher breeding success. However, this is not the case and only when the two morphs come together to breed, there is a fitness difference: mixed-morph pairs (that consist of a dark and a light morph) have higher breeding success than like-morph pairs (that consist of the same morph) and offspring of mixed-morphs have higher survival rates. This higher success of mixed-morph pairs is hypothesized to be due to emergent pair-level properties with the two morphs being able to expand the hunting niche as a pair. This ‘complementarity hypothesis' is based on previous research conducted on the study system. The aim of this PhD was to explore the mechanistic background of colour polymorphism maintenance in the black sparrowhawk. (i) I performed an experiment in which I test whether there is a morph- and ambient light-dependent crypsis advantage in the black sparrowhawk. I measure the reaction time of feral pigeons towards a simulated hawk attack but did not find indication of such an effect as pigeons reacted the same towards the two morphs. In line with the complementarity hypothesis, (ii) I found that mixed-morph parents provide food more consistently to the nest than like-morph parents. This results in a more predictable food supply for nestlings and buffers against long periods of malnourishment. However, (iii) I was unable to determine the mechanistic link between food supply and higher survival: nestlings of mixedand like-morph pairs had the same levels of innate immune function. Thus, an improved innate immune function in nestlings of mixed-morph pairs is unlikely associated with their higher survival rates. (iv) I performed individual-based model simulations which incorporate multiple key fitness parameters and found that complementarity – in combination with morphdependent seasonality-associated fitness effects – explains the stable colour morph equilibrium in this population. I conclude that emergent pair-level properties which arise due to the complementary nature of the two morphs play an important role in maintaining polymorphism in this species. Complementarity might not only be restricted to colour polymorphic species but could be present in other polymorphic traits that allow parents to behaviourally complement each other when raising their young.
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    Fluctuating human activity and associated anthropogenic food availability affect behaviour and parental care of Red-winged Starlings
    (2018) Catto, Sarah; Cunningham, Susan; Amar, Arjun; Sumasgutner, Petra
    Increased food availability associated with urbanisation is widely recognised as one of the key factors influencing avian demography. Temporal fluctuations in food availability, tied to variation in human presence, are of particular interest as they occur frequently in urban environments, but their impacts on the survival and reproduction of birds have not been particularly well-studied. In this study, I explored whether breeding Red-winged Starlings at a university campus in Cape Town, South Africa alter their behaviour and parental care of nestlings in response to fluctuating numbers of people and associated food over a relatively short timescale. I used data from nest watches and behavioural observations collected during both incubation and nestling periods to test whether differences in food availability due to day status (week days with thousands of students present versus weekends with substantially fewer students) affected a number of behaviours related to parental care. I found that, with less available food on weekends, parent birds appeared to trade off feeding their offspring for maintaining their own energetic requirements, meaning that nestlings received less food on weekends. I also found that parents preferentially fed their nestlings natural food on week days, despite an increased availability of anthropogenic food. This suggests that, with increased food availability, birds in this system may use anthropogenic food to supplement their own diets, allowing them to prioritise the feeding of natural food to their chicks. These results provide evidence that fluctuating food resources impact the behaviour and parental care of starlings, but it is still unclear what effect they have on the individual health of adult birds and their developing young. To further enhance our understanding of some of the ecological implications of urbanisation, future research should prioritise understanding the potential health impacts such a variable urban diet may have on the birds exploiting it.
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    How common ravens (Corvus corax) exploit anthropogenic food sources through time and space in a semi-transformed, alpine environment
    (2021) Jain, Varalika; Sumasgutner, Petra; Cunningham, Susan; Bugnyar, Thomas; Loretto, Matthias
    From large-scale agriculture and farming to concentrated fishing discards, garbage dumps, game carcasses and bird feeders, human action has been increasingly affecting natural systems and animal species through the deliberate and unintentional provisioning of food resources. Anthropogenic food sources (AFSs) are often more spatially concentrated, easily accessible, abundant and stable than natural food sources. The common raven, Corvus corax, is a behaviourally flexible and ecologically adaptable species that has managed to thrive in human transformed landscapes by exploiting these anthropogenic sources of food. The aim of this research was to investigate how raven individuals vary in their use of different AFSs through space and time. I used data from a long-term GPS tracking initiative in the Upper Austrian Alps to investigate (1) the space-use of non-breeding raven individuals across this landscape to answer the questions: (2) what types of AFSs are most extensively used by ravens in this landscape, and what factors predict individual variation in AFS use (i.e., apparent reliance on and access to resources), specifically (3a) the number of AFSs visited and (3b) the probability of being at AFSs at any given point in time. Movement patterns can reveal information on the foraging decisions made by individuals, including how they use different AFSs. Non-breeding raven individuals exhibited great variation in how they moved around and used the landscape. The number of AFSs visited, but not the probability of being at an AFS (at any given point in time), varied among individuals with different ranging behaviour and of different age class (i.e., juvenile and adults) and origin (i.e., captive-bred-released and wild-caught), suggesting that experience affects AFS-use. Non-breeders differed in their use of AFSs by season, visiting the highest number of AFSs but having the lowest probability being present at an AFS in winter, potentially indicative of high foraging competition under stressful environmental conditions. They were also found to extensively exploit resources in spring, both visiting high numbers of AFSs and having a high probability of being present at an AFS, perhaps due to decreased competition (e.g., from breeders) and increases in food availability. The category (i.e., wildpark, refuse site, hut) of AFSs also influenced the probability of an individual being present at the site, likely because of differences in resource quality, quantity and replenishing rate. A very few foraging sites were highly popular, while over half attracted less than 5 individuals throughout the study. By exploiting AFSs, raven population numbers have increased across their range, raising conservation concerns (i.e., predation on threatened species and human-wildlife conflict). With a better understanding of the patterns of AFS-use and the factors influencing these patterns, I suggest that strategies to manage ravens in this semi-transformed, alpine environment should focus on controlling the supply of food at AFSs at a regional scale.
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    How does urbanisation affect the breeding performance of African Crowned Eagles (Stephanoaetus coronatus)?
    (2019) Muller, Rebecca; Amar, Arjun; Sumasgutner, Petra; McPherson, Shane; Downs, Colleen
    Birds face many challenges from the process of urbanisation. Those species that are able to occupy urban areas offer opportunities to understand processes of acclimatisation to urban life and may help in the development of urban spaces for the benefit of wildlife. In many bird species, individuals that occupy territories in more urban areas show lower productivity and lower body condition of nestlings, which is thought to be mediated by food availability. Most of the studies exploring this issue were done on passerines and carried out in the global north, with very few studies exploring this topic on non-passerines, and even fewer in Africa. Studies addressing urban productivity in apex predators with slow life histories that are often of conservation concern are largely missing. Here, we explore the breeding performance of the African Crowned Eagles (Stephanoaetus coronatus) across an urbanisation gradient in KwaZuluNatal, South Africa. Specifically, we explored the hypothesis that living in an urban environment allows this species, which is typically a biennial breeder, to breed annually more often (i.e. increased breeding rate), and whether this might increase the productivity of this species. We also explore whether there may be any hidden costs of such a breeding strategy by examining the condition of chicks for pairs which had successfully bred in the previous year. We found that Crowned Eagles breeding in more urban areas attempted to breed more often (i.e., higher breeding rate), but that these birds also suffered from lower breeding success. These two contrasting responses counteracted each other and meant that overall productivity (number of young produced per occupied territory) was not influenced by urbanisation. Breeding annually did not appear to have a negative cost on the chick condition, as offspring in the year following a successful breeding attempt did not have lowered body condition. This species appears to be well adjusted to breeding in an urban environment. Crowned Eagles are currently considered vulnerable in South Africa, and ensuring that an urban population of this species is able to persist can help secure the conservation status of this charismatic species.
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    Landscape homogenization due to agricultural intensification disrupts the relationship between reproductive success and main prey abundance in an avian predator
    (2019-08-06) Sumasgutner, Petra; Terraube, Julien; Coulon, Aurélie; Villers, Alexandre; Chakarov, Nayden; Kruckenhauser, Luise; Korpimäki, Erkki
    Abstract Background Selecting high-quality habitat and the optimal time to reproduce can increase individual fitness and is a strong evolutionary factor shaping animal populations. However, few studies have investigated the interplay between land cover heterogeneity, limitation in food resources, individual quality and spatial variation in fitness parameters. Here, we explore how individuals of different quality respond to possible mismatches between a cue for prey availability (land cover heterogeneity) and the actual fluctuating prey abundance. Results We analyse timing of breeding and reproductive success in a migratory population of Eurasian kestrels (Falco tinnunculus) breeding in nest-boxes, over a full three-year abundance cycle of main prey (voles), and consider several components of individual quality, including body condition, blood parasite infection, and genetic diversity (n = 448 adults) that act on different time scales. Older individuals, and kestrel parents in higher body condition started egg-laying earlier than younger birds and those in lower body condition. Additionally, egg-laying was initiated earlier during the increase and decrease phases (2011 and 2012) than during the low phase of the vole cycle (2013). Nestling survival (ratio of eggs that fledged successfully) was higher in early nests and in heterogeneous landscapes (i.e., mosaic of different habitat types), which was evident during the increase and decrease phases of the vole cycle, but not during the low vole year. Conclusions We found a strong positive effect of landscape heterogeneity on nestling survival, but only when voles were relatively abundant, whereas a difference in the timing of breeding related to territory landscape heterogeneity was not evident. Therefore, landscape heterogeneity appeared as the main driver of high reproductive performance under favourable food conditions. Our results show that landscape homogenization linked to agricultural intensification disrupts the expected positive effect of vole abundance on reproductive success of kestrels.
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    Street-wise : does prey abundance buffer black sparrowhawks (Accipiter melanoleucus) from the negative health impacts of urbanisation?
    (2016) Suri, Jessleena; Amar, Arjun; Sumasgutner, Petra; Hellard, Eléonore
    As trends in urbanisation continue globally, there is a growing need to understand the impacts of urban development on wildlife. Whilst urban impacts on patterns of diversity and abundance of species have been well-studied, there remains a distinct lack of understanding around the impacts on ecological interactions. Different species have different levels of tolerance to urban disturbance, some species even appear to thrive in urban areas and make use of human-subsidised resources; but the physiological costs and trade-offs faced by urban-dwelling species are poorly understood. Given that their range in South Africa has only recently expanded into the human-dominated landscapes of the Western Cape, the Black Sparrowhawk (Accipiter melanoleucus) provides an excellent opportunity to explore some of these questions. In this study we explored how urbanisation may affect the health of this raptor on the Cape Peninsula, South Africa. If the health of this species is negatively influenced by levels of urbanisation, this might be driven by differences in diet and prey availability across the urban spectrum. Thus, we explored this potential mechanism by examining diet composition and assessing prey abundance within different territories and habitat types. The health of nestling Black Sparrowhawks was evaluated through their immune response (Heterophil/Lymphocyte ratio in white blood cell counts), body condition (based on morphometric measurements) and blood parasite infection (presence and abundance of Haemoproteus and Leucocytozoon). Trends in diet composition along a gradient of urban cover were determined through the analysis of prey remains collected in the immediate nest surroundings and differences in prey abundance were determined through point counts in different habitat types.