BIOTROPICA 40(2): 241–245 2008 10.1111/j.1744-7429.2007.00357.x Density, Distribution, and Attributes of Tree Cavities in an Old-Growth Tropical Rain Forest W. Alice Boyle 1 Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, U.S.A. Carissa N. Ganong 2 Department of Biology, Mansfield University, Mansfield, PA 16933, U.S.A. David B. Clark Department of Biology, University of Missouri-St. Louis, St. Louis, MO 61321, U.S.A. and Marisa A. Hast 3 Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, U.S.A. ABSTRACT Tree cavities are a critical resource for many animals, especially as nesting sites for birds. Patterns of cavity distribution in temperate forests are well studied, yet little is known of cavities in tropical forests, despite a hypothesized decrease in cavity availability with decreasing latitude. We studied cavity density and distribution in a wet lowland tropical forest in Costa Rica and compared our results with estimates from forests around the world. Cavities at our site were common, occurred frequently in living trees, and were often formed by damage or decay rather than by woodpeckers. Most cavities had small openings, and woodpecker-created cavities were nonrandomly oriented. Contrary to prediction, cavity density appears to increase from the poles to the tropics. We suggest potential mechanisms to explain these patterns. Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp. Key words: cavity density; cavity-nesting birds; Costa Rica; La Selva Biological Station; latitudinal gradient; nest-site limitation; snags; tropical rain forest. ELUCIDATING THE GEOGRAPHIC PATTERNS OF SPECIES’ DISTRIBU- TION AND ABUNDANCE requires information on the distribution of critical resources. Microhabitats such as tree cavities can limit ani- mal populations (von Haartman 1957) when that microhabitat is required for survival or reproduction. Cavities provide safe nest- ing sites for mammals, invertebrates, snakes, and especially birds (Hansell 2000). Most data on cavity distribution, abundance, char- acteristics, and the extent to which cavities limit populations of secondary cavity-nesting birds (species utilizing existing cavities) come from temperate regions (e.g., Brawn 1988, Aitken & Martin 2004). Yet the relative safety of cavities compared to open nests appears to hold among latitudes, and may possibly be of even more value in tropical than temperate forests (Skutch 1985). Determining the role cavities play in tropical bird communities requires quanti- fying the abundance and use of cavities. Here we report results of the first step in this process: a descriptive study of cavity abundance and cavity characteristics from an old-growth tropical wet forest. Received 29 September 2006; revision accepted 22 May 2007. 1 Corresponding author; current address: Department of Biology, University of Western Ontario, London, ON N6A 5B7, Canada; e-mail: alboyle@email. arizona.edu 2 Current address: Department of Biology, University of Central Arkansas, Con- way, AR 72035, U.S.A. 3 Current address: 3840 Cloverleaf Dr., Boulder, CO 80304, U.S.A. Forest dynamics differ between tropical and temperate regions in ways that could either decrease or increase cavity densities (Gibbs et al. 1993). Many temperate forest cavities are located in snags (e.g., Raphael & White 1984, Land et al. 1989). Yet most tropical tree deaths are caused by weather events (i.e., strong winds) that do not result in snags, and snag-forming disturbances such as fire or winter storms are rare (Putz et al. 1983, Gale & Barfod 1999). Addition- ally, trees (Losos & Leigh 2004) and possibly snags (D. B. Clark, unpubl. data) persist on average for fewer years in tropical forest relative to temperate forests. Thus, cavity densities may be lower in tropical forests simply because cavities have less opportunity to accrue. Conversely, tropical forests may have more cavities due to: (1) availability of alternative cavity substrates (e.g., arboreal termi- taries; Brightsmith 2005b); (2) high decomposition rates potentially resulting in more decay-caused cavities; and (3) the abundance of arborescent palms. Palms have a unique trunk structure (tough fi- brous exteriors, soft pithy cores) which may speed cavity formation processes. In addition to cavity density, attributes influencing cavity suit- ability such as size (Carlson et al. 1998) and orientation (Butcher et al. 2002) likely vary with latitude. Large cavities with large open- ings may be disproportionately valuable in tropical forests due to the large size of many tropical cavity-nesting species (e.g., Ramphastos toucans, Buceros hornbills, and Ara macaws). Non random cavity orientation in temperate forests is usually attributed to the thermal C  2008 The Author(s) Journal compilation C  2008 by The Association for Tropical Biology and Conservation 241