Orientation, latitude, and elevation of 1501 cavities excavated by 25 avian species across 12 terrestrial ecoregions (15°S – 55°S) in South America.

In the Northern Hemisphere, several avian cavity excavators (e.g., woodpeckers) orient their cavities increasingly toward the equator as latitude increases (i.e., farther north), and it is proposed that they do so to take advantage of incident solar radiation at their nests. If latitude is a key driver of cavity orientations globally, this pattern should extend to the Southern Hemisphere. Here, we test the prediction that cavities are oriented increasingly northward at higher (i.e., colder) latitudes in the Southern Hemisphere and describe the preferred entrance direction(s) of 1501 cavities excavated by 25 avian species (n = 22 Picidae, 2 Trogonidae, 1 Furnariidae) across 12 terrestrial ecoregions (15°S – 55°S) in South America. We used Bayesian projected normal mixed-effects models for circular data to examine the influence of latitude, and potential confounding factors, on cavity orientation. Also, a probability model-selection procedure was used to simultaneously examine multiple orientation hypotheses in each ecoregion to explore underlying cavity-orientation patterns. Contrary to predictions, and patterns from the Northern Hemisphere, birds did not orient their cavities more toward the equator with increasing latitude, suggesting that latitude may not be an important underlying selective force shaping excavation behavior in South America. Moreover, unimodal cavity-entrance orientations were not frequent among the ecoregions analyzed (only in four ecoregions), whereas bimodal (in five ecoregions) or uniform (in three ecoregions) orientations were also present, although many of these patterns were not very clear. Our results highlight the need to include data from under-studied biotas and regions to improve inferences at macroecological scales. Furthermore, we suggest a re-analysis of Northern Hemisphere cavity orientation patterns using a multi-model approach, and a more comprehensive assessment of the role of environmental factors as drivers of cavity orientation at different spatial scales in both hemispheres.

Methods
The present study combines data on excavated cavities from short- and long-term field studies of avian cavity excavators and their nest and roost sites collected in Argentina, Brazil, and Chile. Our data were obtained at 21 field sites that correspond to 12 South American ecoregions, as defined by Olson et al. (2001, https://doi.org/10.1641/0006-3568(2001)051[0933:TEOTWA]2.0).

We included data collected by a variety of methods (e.g., cavity surveys, nest searching, and radio-telemetry) that we considered unbiased in terms of cavity orientation. We included avian-excavated cavities in substrates where all orientations were available (i.e., trees, posts, and free-standing termitaria), and excluded cavities in directional substrates (earth banks or termitaria attached to trees). Non-excavated cavities, although occasionally used by excavators, were excluded from our datasets.