Breeding biology of the Golden Oriole Oriolus oriolus in the fenland basin of eastern Britain

Abstract

This paper describes, for the first time, aspects of the breeding biology of the British Golden Oriole population. The Golden Oriole population in the fenland basin of East Anglia became established in 1967 and since the early 1980s the annual breeding population has varied from about 20 to 35 pairs. Males arrived in mid-May, and females about a week later. Nest building starts about five days after female arrival. Mean date for first egg laying was around 1 June. The mean clutch size was 3.53. Observed incubation periods were 18 to 19 days. Mean hatching date was 18 June, and the mean brood size at seven days was 2.60. Nestling periods were from 13 to 15 days, and the mean fledging date was 3 July. Failed clutches may be replaced; no evidence was found for genuine second broods. Estimated mean productivity of all nests including failures was 1.4–2.0 fledgings per nest. The greatest cause of failure was bad weather in June, and nests produced three times as many fledged young in warm dry Junes as in cool wet ones. The breeding habitat is very strongly associated with plantations of early leafing hybrid Black Poplars Populus nigra. A total of 94.2% of all nests found were in these trees, and since 1987 this figure has risen to 100%. The mean height of the nests in poplars was about 7 m below the top of the tree, irrespective of the height of the tree. Most nests were in the north, northeast or east sectors of the tree, and 80% were within two trees of the edge of the woodland. Breeding densities were generally at or below 0.25 pairs/km2, but up to 2.25 pairs/km2. Nestling diet contains many Poplar-feeding Lepidoptera and a surprising number of Bumblebees. Conservation efforts should aim to produce more areas of Poplar woodland with plenty of woodland edges and with 30 ha/km2 of Poplar plantation.