The prevalence of non-breeders in raptor populations: evidence from rings,radio-tags and transect surveys

Age-specific survival and breeding (ASSAB) models were developed with data from 146 common buzzards (Buteo buteo) radio-tagged in southern Britain dur ing 1990-1998, in a 120-km(2) study area that had on average 25 egg-laying pairs. Survival checks were aided by philopatric behaviour and a maximum an nual tag failure rate of 7%: minimum survival rates, that were estimated by assuming death of buzzards with lost tags, were close to maximum rates tha t were estimated using only the recorded deaths. First-year survival rate e stimates for 35 buzzards fitted in 1990-1991 with 25-30-g backpack radios w ere 69-74% (minimum-maximum), close to the 61-71% for 16 buzzards with 12-g tail-mount radios; the backpacks transmitted for 2-4 yr. Overall survival rates were 66-73% in the first year, 91-97% in the second and 88-91% therea fter. Survival estimates from 258 recent British ring recoveries were lower in the first and second years, at 55% and 75%, but similar (88%) thereafte r. Most deaths were from natural causes (40%) or interaction with artefacts (36%). ASSAB models. from radio-tracking and the observed 1.71 young clutc h(-1), predicted breeding by 16-21% of all the buzzards present in spring, or up to 25% with the minimum likely productivity of 1.4 young clutch(-1) o r 12% net emigration. Ringing data predicted breeding rates of 33-38%. The models were tested with density data from nest surveys and new radio-correc ted-transect and truncation-mark-resighting estimates of buzzard numbers. S urveys in autumn and late winter estimated breeding rates of 21-25%. The hi gh non-breeder density in spring. of three other buzzards for each paired b ird with eggs, has important implications for understanding evolutionary fi tness, predation and population ecology.