Effects of temperature and food availability on the reproductive ecology of an arid-zone bird

Pattinson, Nicholas B

Effects of temperature and food availability on the reproductive ecology of an arid-zone bird

Thesis / Dissertation

2024

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

For arid-zone birds that breed in spring and summer, the heightened energy, water, and time demands of reproduction and development coincide with the hottest period of the year. For these species, rapid anthropogenic climate warming is exacerbating trade-offs between thermoregulation and self-maintenance or reproduction, posing a severe threat to breeding and population dynamics. There has been a surge in research into the effects of high and increasing environmental temperature on avian ecology. However, the potential for resource availability to moderate environmental temperature effects on birds' behaviour, physiology, morphology, and success during breeding remains understudied. I investigated these lesser-known concepts through monitoring breeding, conducting a supplementary feeding experiment (providing breeding pairs with access to either a high supplementation (high supp.) treatment of ~25g of Zophobas morio or a low supplementation (low supp.) treatment of ~5g of Z. morio every day of the breeding attempt), and analysing long-term data from a population of Southern Yellow-billed Hornbills (Tockus leucomelas; hornbills) in the Kalahari Desert. These hornbills are long-lived and have an unusual breeding strategy whereby the female seals herself inside the nest cavity for the majority of the breeding attempt, leaving the free-ranging male as the sole provisioner to the nest. Hornbills do not drink, with their water inputs comprised entirely of dietary and metabolic water. Therefore, ‘food' equates to ‘resources' (i.e., food and water combined) in this species. I found that supplementary feeding affected the thermoregulation of free-ranging male parents, incarcerated female parents in the nest, and chicks: hornbills in the high supp. treatment showed more gradual increases in body temperature (Tb) and increased hyperthermia avoidance in response to rising nest temperature (Tnest), although high air temperature (Tair) and Tnest still resulted in facultative hyperthermic responses. Some negative effects of high environmental temperatures persisted regardless of resource availability: breeding male hornbills showed Tair-dependent nest provisioning patterns and chicks showed impaired development (i.e., reduced structural growth) and increased stress responses (indicated by increased feather corticosterone [CORT]) at high Tnest irrespective of resource availability. Collectively, these patterns suggested there were some mediating effects of resource availability on hornbill breeding ecology, but that high environmental temperatures nonetheless had negative effects. This study took place during a period of unprecedented climate extremes, from 2018 – 2021. Therefore, it includes detailed data from two very different summer breeding seasons: a hot and dry breeding season (2019/20) and a cool and very wet breeding season which included a 1-in-100-year flooding event (2020/21). During an earlier year of the study (2018/19), drought conditions were so extreme that no hornbills attempted to breed at the study site, therefore no data for this breeding season are included in the thesis. In the cool and wet breeding season, I recorded a) higher provisioning effort by male parents, b) improved hyperthermia avoidance and more gradual increases in Tb in male and female parents and chicks, c) higher post-hatch body mass (Mb), faster tail feather growth, and lower tail feather barb density in breeding females, d) less time in the nest (i.e., fledging sooner), faster Mb gain, and longer tarsi at fledge and faster tail feather growth in chicks, and e) higher breeding success, compared to the hot and dry breeding season. These differences were mostly unexplained by environmental temperature effects, suggesting they were driven by higher resource availability or quality in the cool and wet breeding season. The lack of any effects of the supplementary feeding experiment on variables other than thermoregulation indicated that it could be resource quality, rather than availability, that was the most important resource-related factor affecting the hornbill ecology during this study. However, I did not have resource quality data to test that possibility. An alternative explanation is that the supplementary feeding experiment did not provide enough extra food and water to influence factors other than thermoregulation, or the signal was simply swamped by the unprecedented influx of natural resources associated with incredibly high primary productivity in the flood year. In systems or periods where resources are not limiting one might not expect a significant effect of resource supplementation. Either of these could have undermined my ability to detect potential resource availability effects. Crucially, I found no significant Tnest effects on female parent morphology or on breeding success, in strong contrast to previous findings in this study population, and in the long-term breeding data analysed in this thesis. This was likely a result of significantly reduced Tnest caused by new nest boxes with an insulation layer, compared to previous studies on this population using uninsulated nest boxes. These findings highlight the need for multi-season studies, and the possibility that improved design of nest boxes can positively affect nest microclimate, thereby mitigating severe high Tnest effects. Overall, this thesis attempted to investigate iv whether high environmental temperatures are currently limiting because of a concurrent lack of energy and water aggravating costly trade-offs. If so, then provisioning of supplementary food and water may present viable conservation options for severely affected and conservation dependant species. Moreover, successful breeding in high rainfall years (corresponding to high resource availability) despite high environmental temperatures may facilitate population persistence. Broadly, based on the predominant lack of supplementary feeding effects, results suggested resource availability was not a strong driver of variation in hornbill reproductive ecology. Rather, the results indicated that high environmental temperatures were limiting regardless of resource availability (i.e., for provisioning rate and developing chicks), despite improved hyperthermia avoidance and more gradual increases in Tb at high Tnest. The results also indicated potential effects of resource quality, or that the supplementary feeding experiment was not completely effective, in that the amount of supplementary food may still not have been enough, or may not have met the nutrient or water requirements (i.e., food quality requirements) of the female parents or chicks. The amount of food in the high supp. treatment was designed to be ~100% of the daily requirements of the nest based on previous studies of this population, but the males in the cool and wet season showed that they could considerably increase the amount of food provisioned to the nest compared to what had been recorded before, suggesting that the amount of supplementary food was well-below what could possibly be provisioned. I lacked the data to investigate these possibilities. Results, therefore, leave avenues for future research to investigate, for example, quality versus quantity effects or the effects of ad libitum food and water supplementation. This PhD contributes to disentangling the independent effects of high environmental temperatures and resource availability on arid-zone avian ecology. Several key potential effects of resource availability or quality on behaviour, physiology, morphology, and breeding success were identified, which ultimately will hopefully aid in understanding avian ecology and designing future research and conservation interventions in the face of rapid climate warming.

Keywords

arid-zone birds

Reference:

Collections

PhD / Doctoral

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