Karakteristieken van in 1985 in Nederland gestrande Middelste Jagers Stercorarius pomarinus

In late autumn 1985, some 160 Pomarine Skuas washed ashore in the Netherlands, following the most massive influx in the southern North Sea since 1879. Some 54 specimens were collected for detailed investigations considering age, sex, condition, plumage, causes of death, and stomach contents. Most wrecked birds were juveniles (83.3%, n= 54), including 32 barred phase, 1 dark phase, and 1 light phase. Of the 9 remaining birds, 3 were considered immature and 6 ’adult’ using plumage characteristics (table 1). Few corpses were fresh and complete and suitable for dissection. One ’adult’ proved to be a sexually mature male, 8 juveniles were sexed as 4 males and 4 females, and 2 immatures as a male and a female (table 2). Juveniles were all in fresh plumage, not showing any signs of moult. Wing moult war found in 1 immature, tail moult in an adult, body moult (towards winter plumage) was noted in six of nine non-juveniles (table 1). The biometrics of juveniles and non-juveniles are presented separately (table 3, 4), ignoring any sexual dimorphism because of the small sample of sexed birds. In 8 juveniles, females showed on average larger wings (♀♀ avg 357.7, ♂♂ avg 351.7 mm), males on average longer central tail feather projections (♀♀ avg 13.7, ♂♂ avg 19.0 mm). All dissected birds were in poor condition, indicating starvation as the proximate cause of death. A notable, very fat, exception was a juvenile which was the only specimen collected in October. Three skuas were clearly oiled, but after careful examination, 37 (68.5%, n= 54) appeared to have small specks of oil in the feathers. Stomach contents are listed in table 5. The presence of Ruffe Gymnocephalus cernuus indicate that some skuas had been foraging in fresh water, probably in the IJsselmeer. The ’golden hairs’ were probably of the worm Pectinaria koreni, which sometimes washes ashore in large numbers. The feathers found in the stomach, including those of Pomarine Skuas, were probably from corpses on which birds had been scavenging on the beach. Cannibalism was not unusual (field observations). It is that most birds died in the first week of the influx (5-10 November), during two storms following a prolonged period of remarkable quiet weather. It is concluded that the birds must have entered the North Sea in poor condition.