High Density of Tree-Cavities and Snags in Tropical Dry Forest of Western Mexico Raises Questions for a Latitudinal Gradient

Abstract

It has been suggested that a latitudinal gradient exists of a low density of snags and high density of naturally-formed tree-cavities in tropical vs. temperate forests, though few cavities may have characteristics suitable for nesting by birds. We determined snag and cavity density, characteristics, and suitability for birds in a tropical dry forest biome of western Mexico, and evaluated whether our data fits the trend of snag and cavity density typically found in tropical moist and wet forests. We established five 0.25-ha transects to survey and measure tree-cavities and snags in each of three vegetation types of deciduous, semi-deciduous, and mono-dominant Piranhea mexicana forest, comprising a total of 3.75 ha. We found a high density of 77 cavities/ha, with 37 cavities suitable for birds/ha, where density, and characteristics of cavities varied significantly among vegetation types. Lowest abundance of cavities occurred in deciduous forest, and these were in smaller trees, at a lower height, and with a narrower entrance diameter. Only 8.6% of cavities were excavated by woodpeckers, and only 11% of cavities were occupied, mainly by arthropods, though 52% of all cavities were unsuitable for birds. We also found a high density of 56 snags/ha, with greatest density in deciduous forest (70 snags/ha), though these were of significantly smaller diameter, and snags of larger diameter were more likely to contain cavities. The Chamela-Cuixmala tropical dry forest had the highest density of snags recorded for any tropical or temperate forest, and while snag density was significantly correlated with mean snag dbh, neither latitude nor mean dbh predicted snag density in ten forest sites. The high spatial aggregation of snag and cavity resources in tropical dry forest may limit their availability, particularly for large-bodied cavity adopters, and highlights the importance of habitat heterogeneity in providing resources for primary and secondary cavity-nesters.

Do tree cavity density and characteristics vary across topographical habitats in the tropics? A case study from Xishuangbanna, southwest China

“…This may have been influenced by the prevailing south-westerly winds in our study area. The majority of cavities in the study area were rot holes rather than woodpecker breeding cavities, which is similar to that found in other tropical forests (Boyle et al 2008;Cockle et al 2011b;Vázquez and Renton 2015). Therefore, it appears as if the mean orientation of cavities was mainly influenced by wind direction and may be an indirect consequence of the angle of the limbs or branches exposed to wind.…”

“…However, the variations in cavity density and characteristics across habitat types have not been well studied in tropical areas. To our knowledge, the only study on cavity characteristics in the tropics is by Vázquez and Renton (2015), who found that compared with semi-deciduous and Piranhea forests, cavities in deciduous forests were fewer and had lower heights and smaller entrance widths, i.e. cavity density and characteristics varied significantly among vegetation types.…”

The breeding biology and nest success of the Short-tailed Antthrush Chamaeza campanisona (Aves: Formicariidae) in the Atlantic rainforest of northeastern Brazil

“…However, C. campanisona birds may not always be able to choose their cavities due to limited availability (Vazquez and Renton 2015). As tree cavities are rare in forests, they become a limiting factor for cavity-nesting birds and couples may try to breed in cavities that are unsuited for the purpose (Martin andLi 1992, Martin 1995).…”

Resource requirements of parrots: nest site selectivity and dietary plasticity of Psittaciformes

“…Furthermore, in contrast with temperate forests where avian primary excavators create 77 % of cavities used by secondary-cavity nesters (Martin and Eadie 1999;Martin et al 2004;Aitken and Martin 2007), the majority of cavities in tropical forests are formed naturally through decay processes (Boyle et al 2008;Cockle et al 2011a). Hence, there would appear to be a high density of naturally formed cavities in the tropical and subtropical forests inhabited by many parrot populations Snyder et al 1987;Cockle et al 2008Cockle et al , 2010Váz-quez and Renton 2015), although only a low proportion of these cavities have characteristics that make them suitable for use by secondary cavity-nesting birds (Cockle et al 2010(Cockle et al , 2011bStojanovic et al 2012;Vázquez and Renton 2015).…”