Parent birds should take greater risks defending nests that have a higher probability of success. Given high rates of mammalian nest predation, therefore, parents should risk more for nests in areas with a lower risk of mammalian predation. We tested this hypothesis using nest defence data from over 1300 nests of six species of dabbling ducks studied in an area where predation risk had been reduced through removal of mammalian predators. When predator removal reduced nest predation, the ducks increased risk taking as predicted. Also as predicted, risk taking varied inversely with body size, an index of annual survival, among species. For ducks to vary nest defence in response to variation in predation risk they must be able to assess the risk of nest predation. Because ducks modified nest defence in the breeding season immediately following predator removal, ducks may be able to assess predator abundance indirectly (e.g. by UV reflection from urine) rather than by seeing or interacting directly with the predators.
Methods were Investigated for determining total volatile base and tertiary volatile base in fish flesh as an index of spoilage. Sampling methods tested included use of press juice, protein-free press juice, 60% ethanol-leached samples, samples "liquidized" with 60% ethanol, and samples of ground fish suspended directly in solution. Volatile base was removed by microdiffusion, distillation, and aeration. Most precise results were obtained for total volatile base by extracting the fish flesh with 60% ethanol and removing the volatile base by distillation from the solution made alkaline with borax. Tertiary volatile base could best be determined by a slight modification of the microdiffusion method of Beatty and Gibbons (3) whereby a sample extracted with 60% ethanol was used in place of press juice.
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