The role of Magellanic Woodpeckers as a keystone species on Navarino Island, Chile
Magellanic Woodpeckers (Campephilus magellanicus), the largest extant woodpeckers in South America, are listed as Endangered or Vulnerable throughout their Chilean distribution and are a species of conservation interest throughout their range. They are endemic to old-growth southern beech (Nothofagus spp.) forests of southern Chile and Argentina. As primary cavity-nesters, Magellanic Woodpeckers excavate cavities in trees, which provide nest and roost sites for themselves and secondary cavity-nesters unable to excavate their own cavities. Since they provide a habitat component that may otherwise be unavailable, Magellanic Woodpeckers are hypothesized to be a keystone species and are likely vital for maintaining the diversity of taxa in their ecosystems. However, to our knowledge, this hypothesis has not been tested. Therefore, as a first step towards testing this hypothesis, we examined the contents of excavated and natural cavities on Navarino Island, Chile (55°04’60”S, 67°40’00”W) to determine cavity use by secondary cavity-nesters. As part of a pilot study from January-March 2015, we examined the contents of 90 excavated and 11 natural cavities using a wireless camera and monitor and extendable poles. Of the excavated cavities, 73 were located via known coordinates from a previous woodpecker study, while 17 excavated and all natural cavities were located opportunistically.
Several excavated cavities indicated use by other avian species including a down-feathered nest (n = 1), grass nests (n = 2), avian excrement (n = 1), non-woodpecker feathers (n = 3), and twigs (n = 1). Cavities also contained evidence of woodpecker use, including fresh woodchips (n = 34) and woodpecker feathers (n = 8). Others were empty (n = 13), contained water (n = 1), and unknown contents (n = 8). Eighteen woodpecker excavations were shallow and incomplete; therefore, they were considered platforms and not cavities. Similarly, nine natural cavities were platforms, but two natural cavities contained Austral Parakeet (Enicognathus ferrugineus) nests. Although sample sizes are small, 8.9% of excavated and 18.2% of natural cavities were used by secondary cavity-nesters, indicating Magellanic Woodpeckers may not serve as a keystone species on Navarino. Further, there was a significant difference in the proportion of natural and excavated platforms (z = 4.3732, P = 0.00), indicating that researchers should verify cavity existence. If occupants are not observed, potential cavities should be further examined to reduce overestimating cavity availability. Further research will be conducted earlier in the breeding season to increase sample size and verify cavity use (nest or roost) and species occupancy.