Barn swallows trade survival against offspring condition and immunocompetence

1. Organisms are continuously faced with the problem of making decisions ab out allocation of limiting resources to maintenance and reproduction. This paradigmatic principle leads to the prediction that each individual will ha ve to trade competing activities affecting fitness, with natural selection favouring the evolution of optimal life-history strategies. 2. In the present study we tested for the existence of a trade-off between parental survival and progeny quality and number in the biparental barn swa llow (Hirundo rustica, Linnaeus) by recording survival from one breeding se ason to the next of adults, whose first brood size had been either increase d or reduced by one nestling. 3. Quality of offspring was expressed as body mass, body size and the abili ty to mount a T-lymphocyte cell-mediated immune response, in vivo, to a mit ogenic stimulus, mimicking the reaction to an antigenic challenge to the im mune system. 4. Both adult male and female barn swallows were less likely to survive whe n their offspring had greater immunocompetence. Adult females were also les s likely to survive when offspring had larger body size but smaller body ma ss. Brood enlargement reduced survival of adult males and had a differentia lly larger negative effect on survival of double-brooded than single-broode d adult females. Double-broodedness did not covary with adult male survival but had a differentially larger effect on survival of adult females with e nlarged compared to reduced broods. Males with relatively long ornamental t ail feathers were more likely to survive than those with short tails. 5. We conclude that parent barn swallows trade their own survival against f eatures of nestlings that affect their probability of recruitment. Because immunity is one of the principal defences available to hosts against parasi tes, this study suggests that host-parasite interactions may have played a part in the evolution of optimal parental strategies.