HULSCHER JB (1985) Habitat choice of juvenile and adult Black-headed Gulls Larus ridibundus: a choice between wet and dry. LIMOSA 58 (2): 49-56.

Black-headed Gull Chroicocephalus ridibundus = Larus ridibundus The ratio of juvenile and adult Black-headed Gulls was regularly determined in three habitats: rural pasture land, the built-up area of a village, and village greens. The percentage of young birds was highest in the built-up area, intermediate on the village greens, and lowest in the pastures (fig. 1,2). The total number of gulls present in the built-up area was negatively correlated with rainfall (fig. 3, 4). When the number of gulls in the built-up area increased, the percentage of young birds decreased (fig. 5). The number of gulls was particularly large and the percentage of young birds low during periods of frost and snowfall (fig. 6). It is postulated that a high percentage of young birds in a particular habitat reflects a low food quality for that habitat. Changes in habitat use are largely determined by changes in the availability of earthworms on the pastures. During wet weather, worm availability is high and most gulls feed on the pastures where their intake rate is supposed to be high. When the rains hold off and the availability of the worms consequently decreases, the food intake of the gulls drops too. Then birds start to leave the pastures for the built-up area of the village where the food quality might be better. As young birds are less efficient feeders than older birds and moreover subdominant in food quarrels, their food intake rate drops more steeply and they will leave the pastures sooner. Consequently the percentage of young birds in the built-up area will always be higher than that on the pastures.

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