BIJLSMA RG (1995) Golden Orioles Oriolus oriolus and poplars Populus spec. below sea level. LIMOSA 68 (1): 21-28.

Large parts of Lake IJsselmeer have been reclaimed since the 1930s, the polder reclaimed most recently being Zuidelijk Flevoland (43 000 ha in 1968). Forests were planted in 1947-55 in the Noordoostpolder, in 1958-64 in Oostelijk Flevoland and in 1973-82 in Zuidelijk Flevoland. The forests are mainly deciduous, with poplar covering 28.5% of the total acreage (Tab. 2). Some characteristics of commonly used cultivars of poplars are listed in Tab. 1. Breeding birds, among which Golden Oriole, of forests in each of the three polders mentioned were studied in 1989, 1990 and 1994. The area studied encompassed 6168 ha, 5148 ha of which forested. The distribution of Golden Oriole territories over available tree species was unequal (Tab. 2). Coniferous trees were mostly avoided, whereas poplars were used more often than expected by chance. Not all cultivars of the poplar were, however, equally favoured by Golden Orioles. Cultivars like Robusta, Zeeland, Heidemij, Flevo, Donk and Rap were preferred, whereas Golden Orioles were absent or scarce in others (Tab. 3). While foraging, both sexes spent most time in the canopy (Tab. 4). Identified prey species included caterpillars of Lackey Malacosoma neustria and Poplar Hawkmoth Laothoe populi, and imagos of Poplar Clearwings Paranthrene tabaniformis and Poplar Hawkmoths. Few tree species were visited while foraging, mainly poplar (mostly Robusta, Heidemij, Zeeland and Oxford) and oak, probably coinciding with caterpillar abundance. Relative caterpillar densities in poplar plantations were highest in the cultivars Oxford and Robusta, as shown by the percentage of leaves damaged (Tab. 5). Distribution and abundance of Golden Orioles is thought to depend on structural features of poplar cultivars (preference for' open' cultivars with thin branches and numerous forks in the canopy, and older than 6-8 years) and arthropod abundance. Onset of laying took place between 22 May and 11 June, the latter being a repeat-laying (Fig. 2). Of 15 nests found, 11 were in poplar (6 X Robusta, 3X Zeeland, 2 X Spijk), and one each in willow, ash, oak and common hazel. Mean nest height was 19 m (SD=7.2 m, N=15), and mean nesting tree height 24 m (SD=9,4 m). Excluding nests in trees other than poplar, these values were 21 m (SD=3.8 m, range 16-30 m, N=ll) and 29 m (SD=4,4 m, range 25-38 m). Lowest nest height was 5 m (in oak and common hazel). Clutch size was 2X C/2, 4 X C/3 and 3 X C/4, brood size (not always in the same nests) 4 X B/2, 2X B/3 and 3X B/4. Six out often nests were successful; of these nests three had a smaller brood than clutch size because of hatching failure (twice one egg) and death of one young (later removed by parent). Four nests failed completely, of which three through predation. Eurasian Golden Oriole Oriolus oriolus

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