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 during 1990–1998, in a 120‐km2 study area that had on average 25 egg‐laying pairs. Survival checks were aided by philopatric behaviour and a maximum annual tag failure rate of 7%: minimum survival rates, that were estimated by assuming death of buzzards with lost tags, were close to maximum rates that were estimated using only the recorded deaths. First‐year survival rate estimates for 35 buzzards fitted in 1990–1991 with 25–30‐g backpack radios were 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% thereafter. Survival estimates from 288 recent British ring recoveries were lower in the first and second years, at 55% and 75%, but similar (88%) thereafter. Most deaths were from natural causes (40%) or interaction with artefacts (36%). ASSAB models, from radio‐tracking and the observed 1.71 young clutch−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 or 12% net emigration. Ringing data predicted breeding rates of 33–38%. The models were tested with density data from nest surveys and new radio‐corrected‐transect and truncation‐mark‐resighting estimates of buzzard numbers. Surveys in autumn and late winter estimated breeding rates of 21–25%. The high non‐breeder density in spring, of three other buzzards for each paired bird with eggs, has important implications for understanding evolutionary fitness, predation and population ecology.