tailieunhanh - Stephens & Foraging - Behavior and Ecology - Chapter 9

9 Foraging in the Face of Danger A juvenile coho salmon holds its position in the flow of a brook. To conserve energy, it positions itself in the lee of a small rock. Distinctive blotches of color on its sides, called parr marks, provide effective camouflage. As long as it holds its position, it is virtually impossible to see. | Illllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll 9 Foraging in the Face of Danger Peter A. Bednekoff Prologue A juvenile coho salmon holds its position in the flow of a brook. To conserve energy it positions itself in the lee of a small rock. Distinctive blotches of color on its sides called parr marks provide effective camouflage. As long as it holds its position it is virtually impossible to see. The simple strategy of keeping still hides it from the prying eyes of potential salmon-eaters. Kingfishers and herons threaten from above and cutthroat trout permanent residents of the stream seldom reject a meal of young salmon. The threat posed by these and other predators is ever present. The clear water flowing past the salmon presents a stream of food items midges struggle on the surface mayfly nymphs drift in the current. But here s the rub to capture a prey item the salmon must dash out from its station potentially telegraphing its position to unwelcome observers. When the salmon feels safe it will travel quite a distance to intercept a food item making a leisurely excursion to collect a drifting midge as far as a meter away from its location. Detecting a predator changes the salmon s behavior. Depending on the level of the perceived threat the salmon has several options. It may flush to deep water or another safe location. It may stop feeding altogether but hold its position. It may continue feeding but dramatically lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll 306 Peter A. Bednekoff Figure . Patch residence time increases with travel time between patches as predicted but blue jays stay in patches much longerthan the optimal residence time. Solid squares show observed residence times open squares show the predicted optimal residence times. After Kamil et al. 1993. reduce the distance it will travel to intercept food. This .