OpenUCT :: Browsing by Author "Cunningham, Susan J"

Browsing by Author "Cunningham, Susan J"

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Open Access
The anatomy of the bill tip of kiwi and associated somatosensory regions of the brain: comparisons with shorebirds
(Public Library of Science, 2013) Cunningham, Susan J; Corfield, Jeremy R; Iwaniuk, Andrew N; Castro, Isabel; Alley, Maurice R; Birkhead, Tim R; Parsons, Stuart
Three families of probe-foraging birds, Scolopacidae (sandpipers and snipes), Apterygidae (kiwi), and Threskiornithidae (ibises, including spoonbills) have independently evolved long, narrow bills containing clusters of vibration-sensitive mechanoreceptors (Herbst corpuscles) within pits in the bill-tip. These ‘bill-tip organs’ allow birds to detect buried or submerged prey via substrate-borne vibrations and/or interstitial pressure gradients. Shorebirds, kiwi and ibises are only distantly related, with the phylogenetic divide between kiwi and the other two taxa being particularly deep. We compared the bill-tip structure and associated somatosensory regions in the brains of kiwi and shorebirds to understand the degree of convergence of these systems between the two taxa. For comparison, we also included data from other taxa including waterfowl (Anatidae) and parrots (Psittaculidae and Cacatuidae), non-apterygid ratites, and other probe-foraging and non probe-foraging birds including non-scolopacid shorebirds (Charadriidae, Haematopodidae, Recurvirostridae and Sternidae). We show that the bill-tip organ structure was broadly similar between the Apterygidae and Scolopacidae, however some inter-specific variation was found in the number, shape and orientation of sensory pits between the two groups. Kiwi, scolopacid shorebirds, waterfowl and parrots all shared hypertrophy or near-hypertrophy of the principal sensory trigeminal nucleus. Hypertrophy of the nucleus basorostralis, however, occurred only in waterfowl, kiwi, three of the scolopacid species examined and a species of oystercatcher (Charadriiformes: Haematopodidae). Hypertrophy of the principal sensory trigeminal nucleus in kiwi, Scolopacidae, and other tactile specialists appears to have co-evolved alongside bill-tip specializations, whereas hypertrophy of nucleus basorostralis may be influenced to a greater extent by other sensory inputs. We suggest that similarities between kiwi and scolopacid bill-tip organs and associated somatosensory brain regions are likely a result of similar ecological selective pressures, with inter-specific variations reflecting finer-scale niche differentiation.
Open Access
Can sociality buffer the impacts of climate change on a cooperatively-breeding bird, the southern pied babbler Turdoides bicolor?
(University of Cape Town, 2020) Bourne, Amanda R; Cunningham, Susan J; Ridley, Amanda R; Spottiswoode, Claire
Increasingly harsh and unpredictable climate regimes are affecting animal populations everywhere and understanding how species respond to current environmental variability is important for predicting vulnerability to climate change over longer timescales. Species living in characteristically harsh and unpredictable arid and semi-arid ecosystems are useful models for studying impacts of climate variability and change because these ecosystems are experiencing rapid increases in both average and maximum temperatures, as well as increased interannual rainfall variation, as a result of anthropogenic climate change. That animals living in highly variable environments are disproportionately more likely to engage in cooperative breeding implies that this strategy may buffer individuals against the negative effects of adverse climate conditions. An aspect of species' vulnerability to climate change that remains relatively unexplored is whether responses to environmental stressors might therefore be mitigated by sociality, particularly in those species in which group members are highly cooperative. In this thesis, I use behaviour, morphology, and physiology data that I collected over three consecutive austral summer field seasons (2016-2019) and A. Prof. Amanda Ridley's 15-year life history dataset (2003-2019, to which I contributed the last three years of data) for a cooperatively-breeding bird, the southern pied babbler Turdoides bicolor. I investigate the impacts of temperature, rainfall, and group size on interannual survival, behaviour, physiology, growth, and reproduction in southern pied babblers, taking a multidisciplinary approach combining behavioural ecology, life history, and ecophysiology. In order to avoid disturbance to the study population, I validated and implemented a non-invasive method for collecting physiological measurements (daily energy expenditure and water turnover). I also tested for the influence of interactions between weather and group size variables because the presence of significant interactions would provide evidence in support of a moderating effect of sociality. I found that exposure to high temperatures significantly constrained successful breeding and the interannual survival of both breeding adults and juvenile birds, and explored the mechanisms behind these observed relationships: adjustments in parental care behaviour, body mass loss, reduced nestling growth rates, and the physiological costs of care at high temperatures. Higher rainfall and larger groups sizes were generally associated with higher reproductive success and survival, but I found no evidence for an interaction between weather variables and group size: individuals across all group sizes experienced similar effects of conditions. I therefore conclude that 1) pied babblers will increasingly face challenges for population recovery and persistence in the near future as survival and reproduction are increasingly compromised by ever higher temperatures, and 2) a life history strategy that relies on the presence of helpers for successful breeding is unlikely to buffer individual group members against climatic variability and climate change in this cooperatively breeding species.
Open Access
Does the availability of shade limit use of water troughs by desert birds?
(2015) Abdu, Salamatu; Cunningham, Susan J; Ryan, Peter G; McKechnie, Andrew E
Climate change poses a major threat to living organisms, with maximum temperatures expected to continue to rise over the next few decades. Hot desert environments are particularly at risk because they experience high environmental temperatures, scarce vegetation, low productivity and unpredictable water sources. Endotherms such as birds face the challenge of maintaining a stable body temperature while avoiding dehydration. This study was carried out in the southern Kalahari, in South Africa's Northern Cape, where about 50% of bird species (36 species) depend on free-standing drinking water. Livestock farms within this area provide artificial water points, which benefit birds as well as livestock. This study determined the role of shade and cover in the use of these artificial water points by birds. An experiment was conducted at six waterholes using the Before-After, Control-Impact (BACI) design. After an initial baseline was established, three waterholes were shaded while the other three were left unshaded. Camera traps were used to record the pattern and intensity of water use by bird species at different times of the day and at varying air temperatures. A total of 36 bird species drank at the water holes, but data analysis was confined to the ten most abundant species. Of the ten, six species responded to the presence of shade/cover, with four species reacting positively (Cape Glossy Starling Lamprotornis nitens , Red - headed Finch Amadina erythrocephala , Black - throated Canary Serinus atrogularis , and Laughing Dove Spilopelia senegalensis ), four showing no significant change in drinking patterns, and two showing a decrease in visitor numbers when the site was shaded (Cape Turtle - Dove Streptopelia capicola, Namaqua Dove Oena capensis). This suggests that providing shade at waterholes is not a universal solution to the problem of increasing heat stress experienced by birds coming to drink. Certain species such as the Laughing Doves and Cape Turtle-Doves avoided waterholes during the warmest time of the day while the Namaqua Doves were frequent visitors at this time. However, the Laughing Dove took advantage of the shade provided at midday (warmest temperatures) as their numbers increased. The Red-headed Finch and Black-throated Canary also increased at water holes with temperature irrespective of the time of day. These patterns imply that the provision of shade modifies the behavior of some bird species in response to predation risk or heat stress. These species utilized shade at different times of day and with varying intensities as temperatures rose.
Open Access
Handling the heat: keeping water available for birds of the Tankwa Karoo Desert
(2023) Morar, Sean; Cunningham, Susan J; Lee, Alan; van Wilgen, Nicola; Foden, Wendy
Climate change poses one of the biggest risks to biodiversity globally. Africa is warming at twice the global average rate and African bird species are therefore at risk, particularly in desert environments where they are already operating near physiological limits. Desert air temperatures in the subtropical latitudes will rise by 3°-5°C by the end of the century. When air temperatures are higher than body temperature, birds must use evaporative cooling to avoid hyperthermia. However, evaporative cooling requires water and a tradeoff exists between hyperthermia and dehydration. For drinking species which need open water sources in order to rehydrate, this tradeoff is exacerbated by high operative temperatures on the ground near the water's edge during periods of extreme heat. This study assessed whether providing artificial shade at waterholes can buffer impacts of high temperature by increasing accessibility of water to birds, potentially facilitating persistence of desert bird communities under climate change. I used a before-after control-impact experimental design to test the impact of shade provision on visitation rates, species richness and the timing of visits by desert birds to waterholes in the Tankwa Karoo National Park of South Africa. Providing shade reduced afternoon waterside operative temperatures below lethal limits for many small passerine species and successfully buffered birds' visitation rates to the water's edge during hot periods, although species richness did not change. Overall bird visitation rates to waterholes increased in the afternoon relative to control sites in response to the addition of shade but decreased in the morning, likely due to a perceived/actual predation risk while operative temperatures were low and shade structures provided no thermal benefit. Responses to shade were species-specific, with the overall positive response driven in part by two common species: Grey-backed Sparrow-larks Eremopterix verticalis and Tractrac Chats Emarginata tractrac. Two species (Cape Sparrows Passer melanurus and White-throated Canaries Crithagra albogularis) responded negatively to the provision of shade. There was an observed shift in bird visitation rates from the morning to the afternoon at shaded structures, suggesting temperature as a primary driver in the use of shade. As temperatures increase under climate change, birds may be forced to drink more in the afternoon due to rising water demands for thermoregulation. Shade structures such as those trialed in this study may have the potential to facilitate this shift and keep water accessible to desert birds in the face of temperature increases.
Open Access
How air temperature affects flight initiation distance in arid-zone birds
(2016) Pistorius, Penny; Cunningham, Susan J; Martin, Rowan O
Arid zones, such as the Kalahari in southern Africa, are experiencing an increase in the number of hot days, as well as in the intensity of heat waves. Research is being conducted to try and understand how increasing temperatures will impact bird communities and population persistence in these areas. Understanding the mechanisms through which birds are vulnerable to climate change is key to answering these questions. This study investigated how changes in air temperature affect arid-zone birds' response to predators. The flight initiation distances (FID), i.e. the distance from a predator when the prey initiates escape, of eight bird species in the Kalahari were measured over a range of air temperatures during October and November 2015. The results from this study show a negative relationship between air temperature and FID. This relationship was weak across the full range of temperatures observed during the study period (11.9 ⁰C - 40.4 ⁰C), with a large amount of variability in FID not explained by air temperature. However, high air temperatures (> 35 ⁰C) were important in explaining some of the variation in FID. Starting distance, time of day, bird species, plant species, bird location (on the ground / in the canopy / at the top of the tree / in a dead tree), bird exposure (whether the bird was in the sun or in the shade) and bird activity (whether mobile / stationary) were consistently found to explain some of the variation in FID in this dataset. The relationships between the behavioural variables (plant species, bird location, bird exposure and bird activity) and air temperature were investigated and revealed a potential mechanism through which air temperature may also indirectly be affecting FID. The reduction in FID at high air temperatures, and thus the potentially increased risk of predation, may have negative consequences for these species in the Kalahari in light of rising air temperatures associated with climate change
Open Access
How does temperature affect Fork-tailed Drongo, Dicrurus adsimilis, foraging effort, nestling provisioning and growth rates?
(2017) Olinger, Ryan; Cunningham, Susan J; Flower, Thomas
Global average temperatures are predicted to increase by as much as 5ᵒC by 2100. Therefore, it is imperative to identify how species are affected by temperature increase to ensure that proper conservation and management of wildlife can be implemented. Species which already live in warmer climates, such as deserts, may provide the first clues to the impacts of increasing temperatures. Birds represent an ideal indicator because research has shown that rising temperatures affect their behaviour, including foraging and provisioning of young, thereby reducing reproductive success. However, it remains unclear whether this results from physiological constraints on behaviour, from declining food availability and or access to prey items at high temperatures. In this study, I aimed to identify how temperature affects the foraging and provisioning behaviour of wild Fork-tailed Drongos, Dicrurus adsimilis, when supplemental food is provided. Additionally, I aimed to identify how nestling growth rates change with temperature. The results of two supplemental feeding experiments indicated that as temperatures increased, adults significantly reduced their willingness to forage, reduced provisioning to offspring and exhibited increased rates of heat dissipation behaviour. Furthermore, when provisioning nestlings, adult drongos consumed a larger proportion of supplementary food at higher air temperatures, and provisioned less. This suggests that they suffer greater provisioning costs at such times. Surprisingly, despite finding several negative effects of temperatures on adults' willingness to forage and provisioning, there was only a weak and non-significant effect of temperature on nestling growth rates. This suggests that adults may be able to compensate for reduced foraging at high temperatures. My study shows that the increased thermoregulatory costs of activity at high temperatures can constrain foraging and provisioning, even when food remains abundant. However, whether this impacts the fitness of current offspring or parents future reproductive success remains unclear.
Open Access
Identifying biologically meaningful hot-weather events using threshold temperatures that affect life-history
(Public Library of Science, 2013) Cunningham, Susan J; Kruger, Andries C; Nxumalo, Mthobisi P; Hockey, Philip A R
Increases in the frequency, duration and intensity of heat waves are frequently evoked in climate change predictions. However, there is no universal definition of a heat wave. Recent, intense hot weather events have caused mass mortalities of birds, bats and even humans, making the definition and prediction of heat wave events that have the potential to impact populations of different species an urgent priority. One possible technique for defining biologically meaningful heat waves is to use threshold temperatures (T thresh ) above which known fitness costs are incurred by species of interest. We set out to test the utility of this technique using T thresh values that, when exceeded, affect aspects of the fitness of two focal southern African bird species: the southern pied babbler Turdiodes bicolor (T thresh = 35.5°C) and the common fiscal Lanius collaris (T thresh = 33°C). We used these T thresh values to analyse trends in the frequency, duration and intensity of heat waves of magnitude relevant to the focal species, as well as the annual number of hot days (maximum air temperature > T thresh ), in north-western South Africa between 1961 and 2010. Using this technique, we were able to show that, while all heat wave indices increased during the study period, most rapid increases for both species were in the annual number of hot days and in the maximum intensity (and therefore intensity variance) of biologically meaningful heat waves. Importantly, we also showed that warming trends were not uniform across the study area and that geographical patterns in warming allowed both areas of high risk and potential climate refugia to be identified. We discuss the implications of the trends we found for our focal species, and the utility of the T thresh technique as a conservation tool.
Open Access
Life on the edge: do body size and drinking dependency influence how birds deal with the heat in South Africa's most extreme desert?
(University of Cape Town, 2020) Orolowitz, Matthew; Cunningham, Susan J
Climate change-induced increases in air temperature pose a substantial risk to birds inhabiting arid environments. Terrestrial endotherms, such as birds, can respond to high temperatures by moving to cooler microsites, increasing heat dissipation behaviour and/or suppressing activity. Previous studies have suggested that larger bird species may have a greater suppression of activity (e.g. foraging) than smaller species at high air temperatures. However, this body mass effect may be confounded by drinking behaviour, since different species have diverse drinking ecologies. Using four species of lark that inhabit the Tankwa Karoo National Park, I investigated whether foraging activity and other heat-influenced behaviours were influenced by body mass or drinking behaviour when comparisons were constrained within a single family. These lark species were: Red-capped Lark (24 g), Spikedheeled Lark (25 g), Karoo Lark (29 g) and Large-billed Lark (45 g). There was a two-fold difference in body mass between the lightest (Red-capped Lark) and the heaviest (Large-billed Lark). Moreover, two of these lark species drink surface water (drinking larks; Red-capped and Large-billed Lark) and two do not (non-drinking larks; Spike-heeled and Karoo Lark). I also collected data on other passerines present in the Tankwa Karoo for comparison to the larks. Black bulb thermometers were used to measure the thermal landscape and a combination of instantaneous scan samples and focal observations to record bird behaviour. Black bulb temperatures were as much as 8.16 °C cooler in shaded than in sunny locations. Similarly, black bulb temperatures were as much as 8.02 °C cooler off the ground than on the ground. The results from scan sample data showed limited support that foraging was negatively correlated (although non-significant) with mass between lark species as temperatures increased; however, data from focal observations suggested larks that obtain all water from food had a greater reduction in foraging as temperatures increase than larks that drink free surface water. Within scan samples, heat dissipation and shade-seeking behaviour appeared to be more strongly influenced by whether the species drinks free surface water or not than by differences in body mass. Furthermore, drinking larks dissipate heat at lower temperatures and seek shade at higher temperatures than non-drinking larks. Foraging intensity was higher in the sunny microsites as compared to shaded microsites for all species, suggesting that drinking larks might gain an energetic benefit due to increased heat tolerance. Therefore, non-drinking species may be vulnerable to foraging-thermoregulation trade-offs under climate change. However, ongoing drying trends in the Tankwa Karoo and reduced availability of surface water may make drinking species more vulnerable to climate change in the future than non-drinking species.
Open Access
Mechanosensory structures in the beaks of probe-foraging birds in relation to their foraging ecology
(2022) du Toit, Carla J; Cunningham, Susan J; Chinsamy-Turan, Anusuya
Some taxa of probe-foraging birds (ibises, kiwi and scolopacid shorebirds) possess the sensory of capability of “remote-touch”, allowing them to detect mechanical vibrations in their foraging substrates using a specialised bill-tip organ in their beaks. This enables them to remotely detect the location of prey submerged in opaque substrates in the absence of all other sensory cues. The bill-tip organ that facilitates remote-touch is made up of mechanoreceptors housed in dense clusters of foramina in the distal portions of the beak bones (each unit of foramen and associated receptors is referred to as a “sensory pit”). Previous research showed that in ibises (Family: Threskiornithidae), species which live in more aquatic habitats tend to have more extensively pitted beak bones (i.e., the relative size of the bill-tip organ increases with increasing aquatic habitat usage of the species) than species living in drier habitats. The first three data chapters of this thesis investigate this trend, using three species of southern African ibises. These three species represent a spectrum of habitat usage, ranging from mainly terrestrial (Hadeda Ibises) to mainly aquatic (Glossy Ibises), with African Sacred Ibises a generalist species. My main hypothesis is that the interspecific differences in bill-tip organ morphology are related to differences in the moisture content of the birds' foraging substrates, as this affects how well these substrates transmit vibrations that the birds are sensing using remote-touch. The morphology of the bill-tip organs of the three species (Chapter 2) and their foraging behaviour in the wild (Chapter 3) indicate that species which forage in less saturated substrates have higher densities of mechanoreceptors in their bill-tip organs, suggesting that they are more sensitive to vibratory cues. This follows logically from the fact that drier substrates transmit vibrations more poorly than wetter ones, thus I hypothesize that species which forage frequently in dry substrates may have faced evolutionary pressure selecting for more sensitive bill-tip organs. My data on foraging behaviour of all three species of ibis in the wild suggests that bill-tip organ pitting extent on the beak bones is linked to depth of probing, which is in turn related to the penetrability of their probing substrates. As substrate penetrability is strongly affected by moisture content, the extent of pitting on the bill-tip organ is a good osteological correlate for the water content of the foraging substrate in the absence of soft tissue histology in ibises. Experiments using captive Hadeda Ibises (Chapter 4) provide further support for the hypothesis that species foraging in drier substrates require more sensitive bill-tip organs as their success rate using remote-touch was positively affected by substrate moisture content. Additionally, as this species' recent range expansion across southern Africa has been closely tied to increased soil irrigation in urban and agricultural habitats, I suggest that this in part due to Hadeda Ibises being better able to detect prey in more saturated substrates. The final data chapter of this thesis concerns the evolution of the remote-touch bill-tip organ in modern birds: the three families which possess remote-touch capability are widely phylogenetically separated, indicating that it evolved convergently. Kiwi (order: Apterygiformes) present an interesting case, as they are part of the palaeognath clade of Neornithes and are the only members of this clade which use remote-touch probeforaging. However, various other palaeognathous birds (ostriches & emu) possess a bill-tip organ, though its function in these taxa is unknown. I show that all species of modern palaeognathous birds (including the extinct moa and elephant birds) have the same beak morphology (bony pits containing numerous mechanoreceptors). This is at odds with the fact that none use the organ or possess the neuroanatomical correlates that would allow them to do so, indicating that the organ is vestigial in most palaeognaths. I thus hypothesized that the trait is plesiomorphic in palaeognathous birds, inherited from a common ancestor that used remote-touch probe-foraging. As the bill-tip organ is characterized by pitting in the beak bones, I was able to study the fossilized beaks of the oldest known palaeognaths, the lithornithids (which evolved during the Cretaceous period). By comparing them to an extensive sample of extant birds' beak bones, I showed that these ancient palaeognaths had bill-tip organs which were probably capable of remote-touch. Aside from supporting the hypothesis that the remote-touch bill-tip organ in palaeognaths is plesiomorphic, this indicates that remote-touch is one of the oldest documented foraging specialisations in modern birds.
Open Access
Temperatures in Excess of Critical Thresholds Threaten Nestling Growth and Survival in A Rapidly-Warming Arid Savanna: A Study of Common Fiscals
(Public Library of Science, 2013) Cunningham, Susan J; Martin, Rowan O; Hojem, Carryn L; Hockey, Philip A R
Frequency, duration, and intensity of hot-weather events are all predicted to increase with climate warming. Despite this, mechanisms by which temperature increases affect individual fitness and drive population-level changes are poorly understood. We investigated the link between daily maximum air temperature (t max ) and breeding success of Kalahari common fiscals ( Lanius collaris ) in terms of the daily effect on nestling body-mass gain, and the cumulative effect on size and age of fledglings. High t max reduced mass gain of younger, but not older nestlings and average nestling-period t max did not affect fledgling size. Instead, the frequency with which t max exceeded critical thresholds (t crit s) significantly reduced fledging body mass (t crit = 33°C) and tarsus length (t crit = 37°C), as well as delaying fledging (t crit = 35°C). Nest failure risk was 4.2% per day therefore delays reduced fledging probability. Smaller size at fledging often correlates with reduced lifetime fitness and might also underlie documented adult body-size reductions in desert birds in relation to climate warming. Temperature thresholds above which organisms incur fitness costs are probably common, as physiological responses to temperature are non-linear. Understanding the shape of the relationship between temperature and fitness has implications for our ability to predict species’ responses to climate change.

Browsing by Author "Cunningham, Susan J"

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