Measuring woodpecker food: A simple method for comparing wood-boring beetle abundance among fire-killed trees

Measurements of food availability for avian insectivores are desirable, but are complicated by the range of microhabitats insects occupy and the difficulty of comparing non-standard techniques across studies. We describe an efficient method for sampling wood-boring beetle larvae, an important food of many woodpeckers, beneath the bark of fire-killed trees. The method involves taking two 225-cm² bark samples from each tree, and provides enough information to rank most trees by relative wood-borer abundance. At two sites in the northern Rocky Mountains, we tested the reliability of this method by comparing its results to those of a more intensive sampling scheme. For 65 trees of four species, we collected eight bark samples at two different heights, 1.7 m and 6 m, and compared correlations between the smaller, ground-based sampling scheme (two bark samples) and the larger, eight-sample effort. With one exception, correlations for individual tree species ranged from 0.79 to 0.95, indicating that the smaller sample gave rankings of insect abundance comparable to those of the larger sample. Correlation strengths differed between sites, suggesting that the larger sample could be employed at future sites to fine-tune the smaller sampling scheme. Correlations for ponderosa pine (Pinus ponderosa) at one site were very low, suggesting that wood-borers may colonize ponderosa pine in a manner different from the other tree species studied. The strong correlations produced by this test indicate that, with limited effort and training, even small field crews (e.g., two workers) can use this method to rank the foraging resources contained in dead trees.