1. Organisms are continuously faced with the problem of making decisions about allocation of limiting resources to maintenance and reproduction. This paradigmatic principle leads to the prediction that each individual will have 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 swallow (Hirundo rustica, Linnaeus) by recording survival from one breeding season to the next of adults, whose first brood size had been either increased or reduced by one nestling.
3. Quality of offspring was expressed as body mass, body size and the ability to mount a T-lymphocyte cell-mediated immune response, in vivo, to a mitogenic stimulus, mimicking the reaction to an antigenic challenge to the immune system.
4. Both adult male and female barn swallows were less likely to survive when their offspring had greater immunocompetence. Adult females were also less likely to survive when offspring had larger body size but smaller body mass. Brood enlargement reduced survival of adult males and had a differentially larger negative effect on survival of double-brooded than single-brooded adult females. Double-broodedness did not covary with adult male survival but had a differentially larger effect on survival of adult females with enlarged compared to reduced broods. Males with relatively long ornamental tail feathers were more likely to survive than those with short tails.
5. We conclude that parent barn swallows trade their own survival against features of nestlings that affect their probability of recruitment. Because immunity is one of the principal defences available to hosts against parasites, this study suggests that host–parasite interactions may have played a part in the evolution of optimal parental strategies.