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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	[Great Northern Diver Gavia immer eats small fish: A drowned bird reveals mysteries] (2000)
Auteurs :	Mardik F. Leopold, Auteur ; Piet Wim van Leeuwen, Auteur ; Harry Horn, Auteur ; Kees Camphuysen, Auteur
Type de document :	Article
Dans :	Limosa (73(4) 2000)
Article en page(s) :	135-143
Résumé :	Some 6000 Great Northern Divers in NW-Europe. They are elusive birds, living dispersed along the rocky shores of Iceland, Norway, the British Isles and Brittany. What the birds actually do there in winter is largely unknown. There are very few direct observations on feeding and these may be biased towards prey items that need long and awkward handling, such as broad-bodied flatfish, twisting Eel Anguila anguila, spiny large-headed fish such as Bull-rout Myoxocephalus scorpius or crabs, rather than more regularly eaten prey that may be swallowed under water. Indeed, a few stomachs of shot birds around Denmark revealed a much wider prey spectrum, of mainly bottom-dwelling fish (Madsen 1957). Stomachs of tens of Great Northern Divers found in Shetland after two major oiling incidents were basically empty (Heubeck et al. 1993, Furness 1994). In contrast, a single stomach of an apparently drowned bird found at Terschelling, Netherlands at 28 November 1999, contained large numbers of fish otoliths that allowed a detailed reconstruction of its last meal. A total of 305 otoliths of gobies, mainly of Sand Goby Pomatoschistus minutus (identification based on the size and the length/width ratios of otoliths; Fig. 4) were retrieved from the stomach and gut. Also, bones and otoliths were found of one Brill Scophthalmus rhombus (total length 8.5 cm, 8.45 gram) and of five 0-group Flounders Platichtys flesus, (5.9-8.5 cm and 2.1-8.5 gram) and 4 shrimp Crangon crangon (0.5 gram each). The shrimp were undigested and apparently the last food items ingested. The large number of goby otoliths and their relatively undamaged state allowed a good reconstruction of size and mass of the fish taken. Otoliths fell into two groups when wear was considered: apparently pristine otoliths and slightly worn ones (Tab. 1). The large sample sizes of both groups allowed for a correction for wear in the latter group. Median lengths and widths of the worn otoliths were compared to the medians of pristine ones, and sizes of worn otoliths were accordingly corrected before calculating fish size. Fish mass was then calculated from these fish lengths. Fish taken were small, on average 5 cm long and 1.23 gram (Fig. 5), but such small prey are not unusual for Great Northern Divers or indeed other large piscivorous birds in Dutch coastal waters. The bird was in good health and quite fat when it drowned and, based on an estimate of total daily needs and day length in November, it is calculated that the total represented prey mass of 220 gram could have been taken in about two hours of foraging.

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