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
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.