Field biologists often work with animals for which there are no prior history. A marker of an animal's age would offer insight into how age and experience affect reproductive success and other life history parameters. We previously reported that length of telomere restriction fragments shorten predictably with age in the captive zebra finch (Taeniopygia guttata). This paper reports that telomeres can also be used to gain knowledge on the age structure of wild, long-lived common terns (Sterna hirundo). Although ages cannot be determined precisely from telomere lengths alone, birds can be classified into broad age-classes. This technique can provide useful information about the age of individuals in cases where their previous histories are unknown.