Genetic and environmental components of phenotypic variation in immune response and body size of a colonial bird, Delichon urbica (the house martin)

Directional selection for parasite resistance is often intense in highly so cial host species. Using a partial cross-fostering experiment we studied en vironmental and genetic variation in immune response and morphology in a hi ghly colonial bird species, the house martin (Delichon urbica). We manipula ted intensity of infestation of house martin nests by the haematophagous pa rasitic house martin bug Oeciacus hirundinis either by spraying nests with a weak pesticide or by inoculating them with 50 bugs. Parasitism significan tly affected tarsus length, T cell response, immunoglobulin and leucocyte c oncentrations. We found evidence of strong environmental effects on nestlin g body mass, body condition, wing length and tarsus length, and evidence of significant additive genetic variance for wing length and haematocrit. We found significant environmental variance, but no significant additive genet ic variance in immune response parameters such as T cell response to the an tigenic phytohemagglutinin, immunoglobulins, and relative and absolute numb ers of leucocytes. Environmental variances were generally greater than addi tive genetic variances, and the low heritabilities of phenotypic traits wer e mainly a consequence of large environmental variances and small additive genetic variances. Hence, highly social bird species such as the house mart in, which are subject to intense selection by parasites, have a limited sco pe for immediate microevolutionary response to selection because of low her itabilities, but also a limited scope for long-term response to selection b ecause evolvability as indicated by small additive genetic coefficients of variation is weak.