Abstract: Cellular senescence caused by telomere shortening has been suggested as one potential causal agent of aging. In some tissues, telomeres are maintained by telomerase; however, telomerase promotes tumor formation, suggesting a trade-off between aging and cancer. We predicted that telomerase activity should vary directly with life span. We determined telomerase activity in bone marrow in cross-sectional samples from two short-lived bird species and two long-lived bird species. The two short-lived species had high telomerase activity as hatchlings but showed a sharp downregulation in both the young and old adults, whereas the two long-lived species had relatively high telomerase activity in bone marrow that did not decrease with age. In zebra finches, the age-related change in telomerase activity varied in different tissues. Telomerase activity increased late in life in skeletal muscle, liver, and gonad, but not in blood or bone marrow.