MANEN W VAN (2000) Reproductive strategy of Honey Buzzards Pernis apivorus in the northern Netherlands. LIMOSA 73 (3): 81-86.

Reproduction of Honey Buzzards was studied in a 6000 ha woodland area in the province of Drenthe in 1992-98. Breeding and non-breeding pairs were differentiated by means of behavioural clues. In the second part of the nestling stage Honey Buzzards with young in the nest made on average one flight above tree level between nest and foraging sites per two hours (mean length of flights 2342 m). Non-breeders made mainly undirected flights when seen above the canopy. By combining nest searches with checks from tree tops 2-3 km apart troughout the study area, the yearly proportion of breeders in the population was estimated at 47%.

In 12 territories of non-breeding pairs a nest was built in July, clearly too late to start laying. In eight out of ten such territories, the nest contained a clutch the following year, suggesting that many non-breeding pairs build a nest in the year prior to egg-Iaying.

Once pairs started breeding this was continued for 1-2:8 years, on average four years in the same territory. Moulted feathers and sightings of individually recognisable birds indicated that mostly the same partners were involved over the years.

In 41 % of the territorial pairs eggs hatched and 34% succeeded in fledging on average 1.63 young. Overall breeding success was 0.55 young per territorial pair. Yearly breeding performance varied twofold in the number of pairs starting a clutch, threefold in pairs thaI produced fledglings and fourfold in number of fledglings per pair. The number of fledglings per pair seemed related to the diameter of wasp combs found at nests (R2= 0.647, N= 7, P=0,1), but the proportion of pairs laying eggs was not. Thus, Honey Buzzards seem unable lot predicl food availability at the start of the breeding season. Based on Swedish survival data (Tjernberg & Rytlman 1994) and Ihe observed breeding output, and assuming a stabie and closed population, the age at first breeding for Honey Buzzards in Drenthe is estimated at 7.7 years. If breeding in later life is intermittenlor net emigration of young occurs, breeding may start 1-3 years earl ier. In Honey Buzzards Ihe breeding window is narrow (late may -late August) and both partners parlicipate in incubation and feeding nesllings. Successful breeding may require close cooperation between the sexes, only to be achieved af ter several years of finetuning behaviour. European Honey Buzzard Pernis apivorus

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