Mysteries of Eggshell Maculation: An Investigation of the Relationship between Eggshell Maculation and Female Body Condition

Abstract

Avian eggshell phenotype varies enormously between and within species. Eggshell colouration results from two pigment types: biliverdin, responsible for blue–green colour, and protoporphyrin, responsible for red–brown colour and maculation patterns. Both are derived from the haem cycle, which binds oxygen to red blood cells. Both pigments play roles in oxidative stress in laying females. Eggshell pigmentation has consistently captivated scientists. Additionally, understanding eggshell pigmentation variation is crucial for understanding birds' evolution, ecology, and behaviour. There have been several hypotheses offered to explain the variation in eggshell pigmentation patterns. The association between eggshell pigmentation and female body condition is at the foundation of several of the hypotheses proposed to explain the functional significance of eggshell pigmentation. The hypothesis posits that the pigmentation variation reflects the female body condition; this hypothesis is one of the most frequently proposed for explaining eggshell pigmentation variation in the last two decades. However, it has received conflicting findings. The conflicting evidence concerning the relationship between eggshell pigmentation and female body condition suggests that several factors may contribute to the conflicting findings. Therefore, the first part of my thesis (described in Chapter 2), aimed to summarise the available evidence in a quantitative way, by conducting a meta-analysis of the studies that examined the association between female body condition and eggshell pigmentation. In doing so, I found support for the assumption of a positive relationship between female body condition and eggshell pigmentation. However, some cases showed an opposite trend of association, particularly when employing digital analysis to evaluate the egg brown background and maculation. However, these findings were not statistically significant. In Chapter 3 I assessed the correspondence and consistency between three distinct methods used to measure eggshell maculation patterns: (1) Visual Scoring; (2) SpotEgg software; and (3) Nature pattern Match (NPM) on the same eggshell samples obtained from blue tits (Cyanistes caeruleus) and lesser black-backed gulls (Larus fuscus). The comparative analysis indicated a high level of correspondence between the Scoring and SpotEgg methods, suggesting their compatibility in both species. In contrast, the NPM method diverges significantly from the outcomes of the former methods, revealing that it represents a distinct approach to quantifying eggshell maculation patterns. Therefore, in Chapter 4 I explored whether the variables used in Visual Scoring and SpotEgg relate to relationships between maculation variations and female body condition. I employed the SpotEgg method to assess eggshell maculation patterns, including total maculated area of the eggshell, maculation spread, and spot size. This assessment involved manipulating the body condition of female blue tits by removing the first four eggs. The aim was to test the hypothesis that eggshell characteristics reflect the female body condition, and this investigation was conducted in two distinct locations, urban and rural. Spot size displayed differences among the experimental treatments, locations, and laying orders. The spot size decreased in females of lower body condition, especially those in the treated group (where the first four eggs were removed) and those in later-laid eggs. Furthermore, maculation spread exhibited variation between the two habitats, independent of the experimental treatment. Specifically, eggs in rural habitats displayed a concentration of maculation toward their blunt ends, whereas those in urban habitats showed a more even spread. However, the total maculated area did not differ among experimental treatments or locations. Overall, this thesis has significantly contributed to the elucidation of the intricate relationship between female body condition and eggshell pigmentation quantitatively through a meta-analysis and a field experiment, shedding light on the potential implications for avian reproductive costs and, thus, offspring fitness. Despite the valuable insights gained, it is imperative to acknowledge the limitations that warrant further investigation as discussed in Chapter 5.