Perhaps the most fundamental question in ecological immunology is what maintains variation in immune function? Despite the obvious benefits of guarding against parasites and disease, all animals do not have optimal immunocompetence. Instead, there exists large phenotypic variation in components of immunocompetence within many natural populations. The overall aim of this thesis was to investigate different factors that may affect variation in avian T-lymphocyte cell-mediated immune response in zebra finches (Taeniopygia guttata). Some explanations of inter-individual variability of immunocompetence are based on the assumption that immune defence is partially controlled by additive genetic variation. To test this, a partial cross-fostering experiment was used. Not only was the genetic basis of cell-mediated immune response confirmed, immune response was also found to be condition-dependent, displaying positive genetic covariance
between body condition. This relationship may have important implications for sexual selection models as females may use body condition as an indication of underlying acquired immune quality. Negative genetic covariance was found between immune response and preferential allocation to growth, suggesting that chicks that favoured fast wing growth over rate of weight gain had reduced immune responses. In addition to genetic influence, variation in immune response may be influenced by environmental factors, particularly social conditions. I show that for males artificially given low socio-sexual status, social conditions influence the relative investment in cell-mediated immune response and sexual signalling, with investment biased towards immunity in same sex environments but towards signalling in dual sex environments. This effect was not seen for high status males who instead had similar immune responses and sexual signalling in both social environments. These findings confirm that T
cell-mediated immune responses of zebra finches is intimately related to sexual signalling and mate selection.