Plucking analysis is a standard method for assessing the breeding season diet of goshawks Accipiter gentilis. Results may be biased, however, due to differences in the detectability of remains of certain prey species and to selective prey transportation by hawks. This study investigated prey choice of three goshawk breeding pairs in the city of Hamburg, Germany, during 1997–99. Predation data obtained by continuous radio-monitoring of the adult males were used to quantify biases of simultaneous plucking analyses, which were conducted in the nesting territories of the corresponding breeding pairs. A comparison of estimated average food requirements with actual biomass intake observed in two of the monitored goshawks (with sufficient sample sizes) indicated that it was possible to record virtually all kills that occurred during tracking sessions. Therefore, radio-tracking data could be used to evaluate the accuracy of plucking analysis. Comparisons showed that the proportions of small birds and of feral pigeons Columba livia f. domestica were underestimated by the scanning method by 10% and 13%, respectively. The latter finding was unexpected as pigeons have large, conspicuously coloured feathers that are easy to detect. However, as goshawks plucked large prey more thoroughly at the capture site than smaller prey, a comparatively small number of their feathers were present in the nesting areas. This source of bias in plucking analysis has passed unnoticed in previous studies. It may have important implications for the interpretation of results of future projects that aim to estimate predation rates in goshawks and other bird-eating raptor species.

The impact of food competition from the red fox Vulpes vulpes on the breeding density of the goshawk Accipiter gentilis was studied in a 650 km2 area in southern Norway by investigating the density of goshawks and their grouse prey before, during, and after a period when the red fox population was low because of an epizootic of sarcoptic mange Sarcoptes scabiei. The number of goshawk pairs per 100 km2 was approximately nine/year in the 1950s, three/year in 1972–75 and in 1980–85, and four/year in 1988–91. In each of these periods, nesting territories of goshawks were regularly spaced. There was a positive correlation between the breeding density of goshawk and the population size of grouse. Both grouse numbers and goshawk breeding density increased at the same time that fox populations declined in the 1980s, while the re-establishment of the fox population in the 1990s was associated with decreases in grouse and goshawk numbers. During the partial absence of red foxes, there was a continuously high grouse population. It is concluded that the red fox may influence the goshawk breeding density negatively by limiting the numbers of grouse.