Parents are selected to maximize their fitness by allocating care among the
ir progeny in relation to the differential reproductive value of offspring.
Nestlings have been hypothesized to signal need for parental care reliably
through their begging behavior, but offspring condition as reflected by th
eir reproductive value may likewise affect begging and hence provisioning.
We assessed the relative importance of need and condition in determining be
gging behavior and feeding rate of nestling barn swallows (Hirundo rustica)
through short-term starvation, a challenge to their immune system with a f
oreign antigen negatively affecting condition, and brood size manipulation.
Food deprivation but not condition or brood size manipulation increased ne
stling begging rate. Parents fed offspring depending on both need and condi
tion but only when feeding broods that were reduced or of normal size. In e
nlarged broods, offspring received less food per capita than in reduced bro
ods, and parents did not discriminate among nestlings relative to their nee
d or condition. Thus, nestlings signal their need by increased solicitation
. Parents allocate food to offspring dependent on both need and condition,
with these effects depending on parental workload as determined by experime
ntal brood size.