Responses of known-aged common terns to experimental shortening of the wings

Fourteen male Common Terns (Sterna hirundo) of known age (3-21 yr) were handicapped by shortening their wings early in the incubation period, and were then followed through chick-raising. The study was conducted at Bird Island, Massachusetts, USA, in 1996, a year when chick growth and survival appeared to be limited by food availability. Chick growth and survival were used as indirect measures of allocation of resources to current reproduction. Rates of change of body-mass and feather re-growth (ptilochronology) were used as indirect measures of allocation of resources to self-maintenance. The birds' mates were studied similarly, but were not handicapped. Experimental birds and pairs were compared to controls matched for laying date and clutch-size. Handicapped mates re-grew tail-feathers pulled for ptilochronology significantly more slowly than controls. They and their mates raised significantly more young than controls. Older handicapped males lost more body-mass during chick-mising than their matched controls, whereas younger males lost less. These results conflict with the prediction of life-history theory that long-lived birds faced with increased costs of reproduction should allocate these costs to their offspring earlier than to themselves. However, we point out several problems in using handicapping to test such predictions. The assumed effects of handicapping on the cost of flight and on foraging ability have not been verified or measured. Changes in reproductive effort are not measured directly, and the end points that have been investigated are often ambiguous. The assumption that older individuals consistently have lower residual reproductive value than younger individuals may be incorrect if there is selective survival of high-quality individuals within the study population.