Trade-offs select for optimal allocation of resources among competing functions. Parents are selected to maximize production of viable offspring by balancing between progeny number and “quality.” Telomeres are nucleoproteins, at the ends of eukaryotic chromosomes, that shorten when cells divide. Because shortening below a certain threshold depresses organismal functioning and rate of shortening depends on environmental conditions, telomeres are good candidates as mediators of trade-offs. We altered brood size of barn swallow Hirundo rustica and found that brood enlargement caused a reduction in relative telomere length (RTL). Reliable signals of offspring quality should evolve that mediate adaptive parental care allocation. Because nestlings with darker coloration receive more care, we analyzed the covariation between RTL and coloration and found that RTL increased with plumage darkness, both within and between broods. Hence, we provide unprecedented evidence that signals relevant to parent-offspring communication reflect telomere length and thus offspring reproductive value.