Official journal of the Netherlands Ornithologists' Union
|Daan S. & Tinbergen J.M. (1979) Young Guillemots (Uria lomvia) leaving their arctic breeding cliffs: a daily rhythm in numbers and risk. ARDEA 67 (3-4): 96-100|
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At three weeks of age young Brunnichs' Guillemots (Uria lomvia) jump from their breeding ledges to reach the sea (Fig. I). Some don't glide far enough and drop on the slopes under the colony (Fig. 2). These birds struggle on down the slope on foot and are especially vulnerable to predation at +this time. The circadian rhythm of jumping behaviour and predation was quantified at Ingeborgfjellet (Spitsbergen) under continuous daylight conditions. The jumping activity has a distinct peak between 20-24 hr (Figs. 3 and 4). The predation of Glaucous Gull (Larus hyperboreus), an important predator, varied considerably during the night. The combined results of jumping activity and predation show that the predation risk for young Guillemots is smallest at the time that most jumpers occur (Fig. 5). Thus predation as a selection pressure favours synchrony in jumping activity. The question of why the late night period has been selected for must remain open at present.