Corticosterone (CORT), the main glucocorticoid in birds, plays a fundamental role in maintaining homeostasis and energy-balance, and is therefore tightly linked to an individual’s energetic state and the prevalent environmental conditions. CORT also has pleiotropic effects, ranging from reproductive function, the regulation of behaviour, morphology and immune function. Thus, inter-individual variation in CORT can potentially underpin a range of life-history traits, and through its pleiotropic effects act as a physiological mediator of reproductive decisions, causing birds to direct resources towards reproduction or self-maintenance dependent upon energetic condition and/or environmental quality. In turn, the role of CORT as a mediator of life-history traits has lead to the suggestion that inter-individual variation in CORT may be associated with individual differences in fitness. Despite this, the causes and consequences of large inter-individual variation in baseline CORT, specifically during reproduction, remain relatively unknown. The main aim of this thesis was to address these knowledge gaps by monitoring a nest-box population of blue tits, Cyanistes caeruleus, breeding on the east banks of Loch Lomond, UK over three years (2008-2010), and measuring baseline CORT concentrations in both adult and nestling birds at a standard stage of breeding in each year. Although environmental quality is often linked to variation in baseline CORT in breeding birds, this has rarely been investigated at the individual level. Chapter 2 focuses on the relationship between foraging conditions measured at the territory-scale and baseline CORT in adult and nestlings in 2008-2010. Synchrony with the peak in caterpillar abundance was the only factor to influence nestling CORT, and only in 2008. However, I found that synchrony between breeding and the peak in caterpillar abundance, weather variables and the density of oak trees influenced baseline CORT in adult birds. Importantly, the relationships between adult baseline CORT and these foraging conditions were only evident in some years; when conditions were most demanding. In addition, the effects of the foraging conditions measured upon adult baseline CORT appear to be synergetic and/or additive in nature. As inclement environmental conditions are often associated with elevated baseline CORT and reduced fitness in birds, it has been suggested that elevated baseline CORT should also be associated with reduced fitness (the ‘CORT-Fitness’ Hypothesis). However, this may not be the case, as modulation of CORT in the face of environmental challenges can adaptively influence physiology and behaviour to improve breeding performance and/or survival. In Chapter 3, I tested these assumptions and my results indicate that the foraging conditions linked to maternal baseline CORT differ to those associated with a proxy of fitness i.e. reproductive success. Specifically, maternal baseline CORT appears to be linked with factors that affect energetic demand, i.e. movement between trees, rather than reproductive success, i.e. total number of prey provided to offspring. In addition, in 2009 only, maternal baseline CORT was positively correlated with fledging number. In Chapter 4, I investigate whether there is a link between maternal baseline CORT and brood sex ratio adjustment over three years. I discovered that maternal baseline CORT was not correlated with brood sex ratio in any year. Maternal body condition, however, was linked to brood sex ratio adjustment in one year. Furthermore, experimental manipulation of maternal CORT during egg laying did not result in brood sex ratio adjustment or affect maternal condition, hatching success or chick development. Chapter 5 investigates the role of maternal baseline CORT in reproductive trade-offs. I reduced the costs of egg laying through supplemental feeding and compared maternal baseline CORT, brood care and maternal return rates between manipulated and control mothers. Reducing costs negated the physiological stress associated with provisioning effort in manipulated mothers and improved their return rates the following year compared with controls. Therefore, maternal CORT may mediate reproductive trade-offs in this species. As baseline CORT is often linked with energetic status and environmental conditions, and there is some evidence that CORT affects feather growth, I hypothesised that it may be linked to the expression of UV colouration in the crown feathers of female birds (Chapter 6). The results show that baseline CORT was indeed negatively correlated with UV colouration, and that UV colouration was positively correlated with reproductive success consistently over the three years, thus suggesting this trait signals maternal quality. Finally, Chapter 7 summarizes the main findings and considers how my results add to our knowledge base and discusses pertinent avenues of future research. This thesis presents compelling evidence that inter-individual variation in baseline CORT is significant, as the results show that it is associated with foraging conditions, reproductive success and may also influence reproductive trade-offs and UV plumage colouration. However, the results do not support a role for baseline CORT in brood sex ratio adjustments in blue tits. The results also reveal the complexity of the relationships between inter-individual variation in baseline CORT, environmental conditions and reproductive success. Specifically, both foraging conditions and proxies of fitness, i.e. reproductive success were linked to baseline CORT differently between years, most likely due to the contrasting conditions experienced in those years. Therefore, although inter-individual variation in CORT is linked to life-history traits in breeding birds, relating this variation to individual fitness is challenging. Furthermore, there remains a lack of knowledge concerning the repeatability of baseline CORT concentrations in blue tits. Ultimately, my thesis suggests that in order to achieve a full understanding of how inter-individual variation in baseline CORT is linked to fitness, single year or short-term studies are inadequate; instead, researchers must relate individual differences to long-term measures of fitness.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:630963 |
| Date | January 2011 |
| Creators | Henderson, Lindsay J. |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/3055/ |
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