Use by Bats of Old-Growth Redwood Hollows on the North Coast of California

Abstract

Use by bats of basal hollows in old-growth redwoods was indexed using the weight of guano collected on water-permeable screens (guano traps) installed inside the trees. Twenty-six trees were systematically selected from a stand that flanks both sides of an interstate highway in Del Norte Co., California. Traps were checked for guano once a month for 18 months. The quantity of guano collected each month and that every tree was used suggests that the trees served either as day roosts by small numbers of bats or as temporary night roosts. Although bats could not be identified on the basis of characteristics of guano, three fecal morphotypes were identified. Guano was found in all 26 trees during most of the sample periods ; the greatest deposition occurred May through August. Surprisingly, there was a substantial amount of fresh guano collected from most trees during winter months. When all trees were considered, weight of guano was not significantly related to diameter at breast height (dbh) of the tree during summer, winter, or annually, but was related to the internal volume of the hollow during summer and annually. Trees closer to perennial streams had greater weight of guano, but only during summer. The most frequently used trees had greater hollow volumes, greater diameters, and were closer to water than the least-frequently used trees. Although other potential roost sites on redwood trees were not sampled, our work indicates that large hollows are important roost sites. The relationship between weight of guano and size of hollow suggests that forests lacking trees large enough to contain these hollows will provide fewer roosting opportunities that could affect the abundance and diversity of bats.