During 2003-05, 23 nests of European Honey-buzzards near Bialowieza in eastern Poland were visited 105 times to record growth rates of nestlings and prey remains. In 3 out of 8 nests in 2004, but none in 2003 and 2005, biting flies were recorded (unidentified, possibly Culicoides spp, or Simulium spp.) during a 10-day period in mid-July, when conditions were slightly cooler than in the rest of July. The presence of biting flies was not linked with age of nestlings (from 14-28 days old) nor with tree species. Nestlings had swollen eyelids during the presence of biting flies (exclusively on nests which had been in use by Honey-buzzards the year before), but not when these were absent (other types of nests). A 32-days old chick with swollen eyelids was found under the nest on 19 July 2004; this chick had been well fed (suggesting ample food) and had no siblings (sibling aggression, not prevalent in Honey-buzzards anyway, ruled out). It may have jumped the nest due to harassment by biting flies. Out of 40 fledglings in 2003-05, possible harassment by biting flies would have resulted in the death of a single chick. This compares favourably with the mortality caused by Goshawk Accipiter gentilis predation (4 chicks) and the destruction of a nest during high winds (2 chicks dead). It is therefore unlikely that biting flies exert a prominent impact on nest choice and nest site choice in Honey-buzzards. This is also bom out by the fact that biting flies were never recorded on 100s of Honey-buzzards nests on dry sandy soils in The Netherlands, nor on 1000s of nests of tree-nesting Common Buzzards Buteo buteo, Northern Goshawks and Eurasian Sparrowhawks Accipiter nisus in the same region (data W. van Manen & R.G. Bijlsma). However, Buzzard chicks in wetlands in The Netherlands were harassed by Culicoides spp. when taken to the ground (apparently not when on the nest), unlike the chicks of the ground-nesting Eurasian Marsh Harrier Circus aeruginosus (data R. Kleefstra).