Nutritional Aspects of Breeding in Birds

This study set out to investigate the relationships between dietary nutrition, female quality and egg production. Representing a considerable investment of her resources, a female bird deposits all the chemical nutrients required for the growth of an avian embryo within a sealed unit over a short period of time. Variation in both the total amount of resources allocated to a clutch of eggs and the distribution of those resources within a clutch can have a profound influence on both her offspring's and her own fitness. Cockatiels (Nymphicus hollandicus) were used as a model for a parrot species, to examine the interaction between the body condition of females before breeding and the quality of diet they obtain during egg formation on subsequent clutch production. Feeding a supplement of high quality nutrients had a positive influence on breeding performance. Clutch mass was 32% larger in birds that received the supplement all through the breeding cycle, versus the birds who only received a maintenance diet over the same time period. Clutch size rather than egg size was increased. High quality nutrition offered during the period of egg production did not produce significantly larger clutches. Instead there was a significant increase in clutch mass when a feeding supplement was offered during the pre-laying period, suggesting an important contribution from endogenous reserves to egg production. Feeding a supplement of high quality nutrients did not appear to affect egg mass, an attribute often used as a measure of egg quality. Chicks hatching from eggs laid by birds fed a supplement of high quality nutrients tended to gain weight and grow skeletally faster than those hatching from eggs laid by birds with no exposure to the high quality diet. To examine interspecific variation in egg composition, eggs of 18 bird species, representing 10 avian orders, a range of dietary habits and many of the eight classes of developmental maturity of chicks at hatching, were collected and analysed. The gross composition (lipid, protein and water contents), amino acid and essential fatty acid profiles of their eggs are presented. Differences in egg composition are discussed in terms of developmental maturity of chick at hatching, and maternal diet. Variation in egg composition and therefore quality, also occurs within a species and this was examined in detail by manipulating the egg production of lesser black-backed gulls Lams fuscus. Birds were experimentally induced to lay extended clutches, presumably representing their physiological and nutritional extremes of egg production, and a number of egg composition variables was measured. It is known from previous work that changes in egg composition have a substantial effect on offspring survival. The objective of this study was to examine the chemical composition of experimentally induced extended clutches in more detail, in order to elucidate which aspects of the eggs are involved in this trade-off. The gross composition (lipid, protein and water contents), amino acid and essential fatty acid profiles of their eggs are presented. An enzyme-linked immunosorbent assay was developed to measure variation in yolk immunoglobulins, the avian equivalent of maternally-derived passive immunity, across extended laying sequences. The weight of amino and fatty acids declined in absolute terms within an extended sequence, but relative to egg mass remained at the same concentration. Earlier laid eggs contained significantly greater quantities of vitamin E and carotenoids, a phenomenon also observed in normal three-egg clutches. Both compounds are powerful antioxidants that protect both against peroxidative damage during development and the oxidative stress associated with hatching. Caroteniods are also believed to enhance the immune system and last-laid eggs have been demonstrated to contain significantly less immunoglobulin G (IgG) than earlier laid eggs. Smaller eggs contained most major nutrients in the same proportions as larger eggs, suggesting a blueprint for egg composition exists within the female, with limited scope for variation. That the last egg laid in extended clutches was not smaller than third eggs in normal clutches indicates the probability of a minimum size threshold below which an egg is unlikely to hatch and survive. Thus differential mortality of chicks hatching from eggs laid later in a sequence may result from them having suffered more oxidative stress during development or having an increased susceptibility to pathogens. Both the crystallographic and chemical structure of lysozyme have been documented for many bird species other than domesticated birds, yet there has been comparatively little work done on levels of lysozyme activity in wild bird eggs and their ecological importance. Investigating lysozyme activity in eggs of experimentally extended clutches laid by L. fusais, within a clutch, later-laid eggs exhibited demonstrably less lysozyme activity and therefore had a lower lysozyme concentration. Since egg mass also declined significantly with lading position, these later-laid eggs were also more likely to contain less lysozyme in absolute terms. On its own, the decreasing lysozyme concentration observed in experimentally extended gull egg clutches, may not be biologically significant but combined with declines in other components of the egg already described it may contribute cumulatively to the decreased hatchability and fledging success observed in previous research. These investigations of chemical composition of eggs, both within and between species, demonstrate the complexity of the avian female's reproductive system. Female body condition is an integral part of that system and it would appear to influence egg production long before oviposition.