Preliminary Analysis of the Ecology and Geography of the Asian Nuthatches (Aves: Sittidae)

—We explored distributions of Asian nuthatch species in ecological and geographic space using ecological niche modeling based on occurrence data associated with specimens and observations. Nuthatches represent a well-defined clade occurring throughout the Northern Hemisphere, but are most diverse in southern Asia where 15 of the 24 species occur and where the lineage is believed to have evolved. Species richness was focused in a narrow east–west band corresponding to the forested parts of the Himalayas with a maximum number of nine species predicted present in these foci. The distributional predictions have a mid-elevation focus with highest species diversity between 1,000 and 2,000 m. Niche breadth and volume were positively related, but accumulation of distributional area (niche volume) decreased with additional environmental combinations (niche breadth). The extent of potential range filling, a measure of distributional disequilibrium, was connected with montane habit (R2 0.422) indicating that montane situations limit the distributional potential of species. Received 13 September 2007. Accepted 1 February 2008. The Sittidae consists of 25 species in two genera: Sitta with 24 species (nuthatches) and Tichodroma with a single species (Wallcreeper [T. muraria]). The family was conceived much more broadly (Mayr and Amadon 1951) to contain other, superficially similar groups, such as Daphoenositta (the sittellas of New Guinea and Australia), now known to have converged on a similar feeding niche and morphology from different ancestry (Sibley and Ahlquist 1990). True nuthatches occur throughout the Northern Hemisphere, but are most diverse in southern Asia where 15 of the 24 species occur (Harrap and Quinn 1995), and where the lineage probably evolved (Matthysen 1998). The co-occurrence of many closely-related species was noted by Ripley (1959) and Lack (1971). Matthysen (1998) observed that many nuthatch species have small ranges, although 5–7 nuthatch species occur at sites across southeast Asia, generally segregated by elevation, habitat, or both (Lack 1971, Matthyssen 1998). Nuthatch distributions range from narrow endemism to broad distributions crossing con1 Biology Department, Grand Valley State University, Allendale, MI 49401, USA. 2 Bombay Natural History Society, Mumbai, India. 3 Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, KS 66045, USA. 4 Current address: National Wildlife Research Center, Taif, Saudi Arabia. 5 Corresponding author; e-mail: [email protected] tinents. For example, the White-browed Nuthatch (Sitta victoriae) is microendemic, restricted to Mount Victoria in western Burma; in contrast, Eurasian Nuthatch (S. europaea) ranges across much of Eurasia. Nuthatches are typical in temperate and subtropical areas of the Northern Hemisphere with two species (S. europaea and S. canadensis) occurring well into the subarctic, almost to the northern edge of the boreal forest (Matthysen 1998). Fewer nuthatch species occur in tropical regions of southern Asia and only two (S. frontalis and S. azurea) reach the Equator. New informatic approaches offer novel insights into the interaction between ecology and geography in evolving lineages (Soberón and Peterson 2004, 2005). In particular, diverse hypotheses relevant to distributional ecology and biogeography can be tested: conservatism of ecological niche characteristics (Peterson et al. 1999), ecological innovation (Peterson and Holt 2003, Graham et al. 2004), distributional equilibrium with climate features (Svenning and Skov 2004), identification of barriers to dispersal (Peterson 2003), the role of interspecific competition in shaping species’ distributions (Anderson et al. 2002), and others. The objective of this paper is to analyze and explore the ecology and distributions of Asian nuthatch species as a first step toward a more integrative view of nuthatch evolution and biogeography.