Costs and consequences of the structured family

The environment experienced during early life can have profound effects on health and fitness as adults. In species that provide parental care dependent young are often reared together in broods. Close relations consequently provide the social environment in which growth and development takes place. However, parental care is costly and parents often initially 'overproduce' the amount of offspring they can, or are willing to, rear. Consequently offspring demand may often exceed parental supply of resources, such as food, which are necessary for growth and development of young. This creates the conditions for conflicts of interest over the supply of parental investment amongst family members. The amount of resources provided by parents is ultimately dependent upon the availability of resources in the environment. In many birds resource availability during breeding is often unpredictable, and parents may not have full control over the distribution of food among offspring. However, parents can simultaneously track resources in the environment and increase control over within-brood food allocation via maternal effects. Asynchronous hatching, as a consequence of early onset of incubation, is one such maternal effect that leads to the creation of different castes of progeny. First hatched 'core' chicks enjoy substantial fitness advantages over last hatched 'marginal' chicks. Core brood chicks are largely buffered from environmental and developmental uncertainty by marginal chicks, such that the development and fitness of core chicks is largely independent of the number of marginal chicks within a brood. In contrast the fitness prospects of marginal offspring are strongly dependent upon both the core brood size and environmental conditions. The creation of such a 'structured family' through phenotypic handicapping of some offspring ensures parents can more easily tailor brood size to match current conditions and provides insurance against failure of core brood chicks. However, very little is known about the mechanisms utilised by marginal offspring to overcome the handicaps imposed by hatching asynchrony should conditions improve (for example if a core chick dies). In particular, the role of the quality of food available has not been examined. Increased availability of dietary antioxidants for example, such as carotenoids and vitamin E, have been shown to enhance growth, reduce susceptibility to pathogens and increase the ability of chicks to withstand oxidative stress. Evidence suggests that natural selection should favour compensatory resource allocation and growth if poor conditions experienced during early development subsequently improve. However, growth compensation is not free from costs, and it has recently been shown that costs of oxidative stress associated with rapid growth can be reduced by sufficient availability of antioxidants. Consequently the provision of antioxidants may be an important determinant of the ability of marginal chicks to respond to a change in food availability and compensate for a poor start in life. Different selection pressures in relation to sex as adults can also impose differential compensatory responses on males and females to a change in food availability. However, the importance of the quality of food available on the costs and consequences of the structured family has not previously been considered. This project will quantify the effects of compensatory resource allocation in relation to sex and family structure, and examine the cost basis of growth through the manipulation of antioxidant availability and subsequent measurement of oxidative stress in red-winged blackbirds (Agelaius phoeniceus). By focusing on marginal chicks, and their ability to respond to changes in their (social) environment through compensatory resource allocation this work will highlight key selection pressures and will assess plasticity of responses to changes in environmental conditions.