Low forest bird migration proportions in sub-Antarctic and temperate forests of southwestern South America

Background/Question/Methods

The UNESCO Cape Horn Biosphere Reserve (CHBR) at the austral end of the Americas is a landmark for global biodiversity conservation hosting the world’s southernmost forests. These Magellanic sub-Antarctic forests are dominated by a mix of evergreen broadleaf Nothofagus betuloides, deciduous N. pumilio, N. antarctica, and a mosaic of moorland, high-Andean, coastal, and wetland habitats. Birds of the sub-Antarctic forests of South America exhibit features of island settings, such as high endemism, low diversity, genetic isolation, and on-going specialization processes. Compared to North American temperate and boreal forest bird assemblages, South American ones are subjected to a less contrasting weather patterns characterized by moderate winters and mild summers, which we hypothesize that is associated with a lower proportion of migratory birds. Here we report the proportion of migratory birds in South American temperate and sub-Antarctic forests along 25 degrees of latitude, and compared it with the proportions described for North American equivalent latitudinal regions. Using 13 years of standardized monthly mist-netting at Omora Park on Navarino Island (55^oS), combined with bird censuses in 61 sites distributed throughout the CHBR, and censuses conducted on Chiloe Island (42^oS) and Fray Jorge National Park (30^oS), we analyzed the patterns of seasonal migration.  

Results/Conclusions

In the CHBR we captured 22 forest bird species. More than half of the species (63.7%) are residents, 13.6% are partial migrants, and only 22.7% were migrants. Furthermore, only 3 species are longer distance migrants; Elaenia albiceps is the only long-distant migrant, wintering in tropical forests of the Amazon. On Chiloe Island, out of 33 forest bird species, 15.2% are migrants, whereas in Fray Jorge National Park only 11.1% of the 27 forest bird species are migratory. These proportions of migratory species are ca. three times lower than those reported for North America where approximately 30%, 51%, and 70% of the bird species migrate south in autumn at 30^oN, 42^oN, and 55^oN, respectively. Regarding climate, annual temperature range recorded in Alaska at 64ºN for the years 2009-2012 was 47.9ºC, whereas for the same period, the annual temperature range recorded at Omora Park (55^oS) was 8.9ºC; i.e.,  annual temperature range is 5 times lower in Magellanic sub-Antarctic than in boreal forests. Therefore, compared to North America, the seasonality of temperature regimes and the proportion of migratory birds are much less pronounced in South American high-latitude forests, highlighting major climatic and ecological contrasts between temperate and subpolar forests of both hemispheres.