Malaria is a scourge of humankind, but many birds seem to shrug it off. Although they are chronically infected with malaria parasites, their behavior seems unaffected, and they mostly reproduce and raise young just as well as noninfected birds. That was a puzzle not just for ornithologists but also for evolutionary biologists, who have long theorized that parasites inevitably take a toll on fitness. The birds' healthy appearance turns out to be deceiving, however. Drawing on data from a 3-decade study of great reed warblers in southern Sweden, researchers report this week in Science that long-term infection with malaria significantly shortened the birds' lives. The analysis also revealed a possible explanation: The blood cells of infected birds also had shorter telomeres, stretches of DNA that cap the ends of chromosomes and protect them during cell division. In many species, shorter telomeres are associated with aging and shorter life span. The shorter lives had a steep cost when it came to reproduction: lost breeding opportunities. On average, uninfected birds raised more than eight offspring to fledglings, infected birds just four.