VAN NOORDEN B (2015) Breeding biology of the Icterine Warbler Hippolais icterina in the Peel region (Southeast Netherlands). LIMOSA 88 (4): 153-163.

During 2002-2014, 698 nests of the Icterine Warbler were investigated in De Peel, a study area in the province of Limburg (Southeast Netherlands). Besides data derived from numerous nest visits, video surveillance of two different nests yielded data on feeding behaviour. Breeding density fluctuated between 3.5-10.2 territories and 4.0-14.5 nests per km hedge (Tab. 2). Mean nest height was 1.07 m (N=642) and ranged from 0.3 to 3.6 m. Favourite shrub species used for nest building were Blackberry Rubus sp. (71%) and Elder Sambucus nigra (9%). Earliest egg laying date was 12 May and the mean was 8 June (N=430) (Tab. 1, Fig. 1). For first clutches only, mean egg laying date was 1 June. Clutch sizes ranged from 2-6 eggs (1.7% each), usually four (32%) or five (59%) eggs; the mean was 4.53 eggs (4.75 excluding replacement clutches). Incubation periods varied between 12 and 14 days (N=23). Replacement broods after failed first breeding attempts were common, but second broods after successful first broods were observed only twice. Material for replacement nests was regularly reused from the old nest. Mean feeding frequency of young was one feed every 2.2-2.8 minutes, increasing with chick age (Fig. 4). Extrapolated to the entire nesting period this corresponded to 1482- 1560 feedings per chick. Average duration of feedings was 5.7 seconds (1-22 s). At chick ages of 0-2 days, all faecal sacs were ingested by the parent birds; at 4 and 7 days this had decreased to 66% and 14%, respectively. At older ages all faecal sacs were taken away. During the course of the nestling period, the number of feedings per faecal sac initially increased, peaking at an age of 7 days, and dropped slightly afterwards (Fig. 5). Diptera (mainly flies) formed 95% of the identified food items delivered to 8 days old chicks of one nest (Tab. 3). Duration of the nestling period was 12 (38%), 13 (50%) or 14 days (12%) (N=16). Overall breeding success (number of fledglings as % of number of eggs laid) was 58.9% (classical, N=548 nests) and 38.5% (Mayfield, N=535) (Tab. 1). Per clutch started, on average 2.02 birds left the nest (N=490). Main cause of nest loss was predation; 35% of predation events occurred during the egg stage, 29% during the nestling stage and 36% at an unknown stage.

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