. The Canadian field-naturalist. 252 The Canadian Field-Natl ralist Vol. 105. FIGURE I. Measuring device for determining the penetrability of soil (not to scale: see text for details). cm diameter base with 1 cm feet and a 10 cm spike. This was fitted to three m wooden rods with telephone wire. The rods were joined in the shape of a tepee above the inverted message spike. A loop fixed to the apex of the rods allowed a spring balance to be attached. When the point of the spike was placed on the soil surface, either vertically or horizontally, and the balance pulled, the balance reading
. The Canadian field-naturalist. 252 The Canadian Field-Natl ralist Vol. 105. FIGURE I. Measuring device for determining the penetrability of soil (not to scale: see text for details). cm diameter base with 1 cm feet and a 10 cm spike. This was fitted to three m wooden rods with telephone wire. The rods were joined in the shape of a tepee above the inverted message spike. A loop fixed to the apex of the rods allowed a spring balance to be attached. When the point of the spike was placed on the soil surface, either vertically or horizontally, and the balance pulled, the balance reading gave a measure of the force required to sink the spike its full length. On all occasions, the spike remained motionless as the force was increased to the point where penetration occurred. Then the spike sank rapidly to or near to its full length. Many trial repetitions within a narrow area gave repeatable readings. I assumed that the measurement was adequately reproduci- ble, at least in the hands of a single operator, to give a gross indication of the differences in soil compaction. I used this equipment to measure penetrability of the soil in areas adjacent to the nest site. Although not an absolute measurement, it reflects the compactness and hence the stability of the soil. Samples of soil for particle size composition were collected at three appropriate sites by digging holes 10 cm deep. Using this technique a total of 53 readings were taken at 6 different locations. Results and Discussion The penetration tests were first done on the surface ol the cliff top immediately above an active nest colon) (Figure 2). The force required to sink the spike was kg (Table 1). The soil from this area was dark brown sand and had the highest proportion of medium and coarse sand (31%) with an organic content of (Table 2). The latter is not surprising as this was a vegetated surface in the early stages of building up a humus layer in the topsoil. The lower force required for pene
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