AUTHOR BLOG: Tracking the Japanese Bush-Warbler Invasion of Hawaii

Jeff Foster

Linked paper: Population genetics of an island invasion by Japanese Bush-Warblers in Hawaii, USA by J.T. Foster, F.M. Walker, B.D. Rannals, and D.E. Sanchez, The Auk: Ornithological Advances 135:2, April 2018.

JABW (c) Jim Denny
A Japanese Bush-Warbler in Hawaii. Photo credit: J. Denny

Over the past several centuries, Hawaii’s native bird populations have been decimated due to an array of factors, including introduced diseases (avian malaria and pox), introduced rats, habitat change, and hunting. As a result, most live near the tops of the mountains and have small populations. Few birds and remote locations make studying many of these native populations incredibly challenging.

In contrast, Hawaii is also home to many introduced birds that can be seen everywhere, from Brazilian Cardinals and Common Mynas on the beaches to Japanese White-eyes and various game birds at the mountaintops. Various organizations in Hawaii introduced these birds from elsewhere in the world to have birdsong fill the air again and occasionally to serve as pest control for crops. Over 170 species have been brought to Hawaii and released into the wild. Of these releases, at least 54 species now have breeding populations, and most seem destined to stay for the long haul. Many species, such as the Japanese White-eye, Northern Cardinal, Zebra Dove, and Common Myna, have robust populations and can be found in a variety of habitats.

One introduced species, the Japanese Bush-Warbler, is perhaps the coolest of them all. However, despite its prominent place as the iconic harbinger of spring in Japan, few people in Hawaii think much of this species—perhaps because it is often heard but rarely seen, or perhaps because when one does finally spy a bush-warbler, it is a drab olive-brown with few prominent markings. Whatever the reason for overlooking it, bush-warblers have successfully colonized most brushy habitats on all of the main Hawaiian Islands. They were released on the island of Oahu in the 1920s, and after decades of population growth on Oahu, they naturally spread to the remaining main Hawaiian Islands by 1997.

Birds on islands have provided some of the best historical examples of the evolutionary process—think Charles Darwin in the Galapagos and Alfred Russel Wallace in the Malay Archipelago. Capturing cases of evolution “in action” is difficult. However, introductions of non-native birds into the Hawaiian Islands provide numerous opportunities for research, particularly in assessing potential evolutionary changes over a relatively short time frame. In this study, we were afforded a unique opportunity to look at the evolution of the Japanese Bush-Warbler within the past several decades by combining population genetic analyses of this species with a detailed invasion timeline on each island. As a result, we were able to see how rapidly genetic changes can occur during an invasion. We found both expected patterns, such as a decline in genetic diversity on the most recently invaded island, and an unexpected pattern, potential assortative mating on each island. These findings suggesting substantial room for future work in a system and setting that is pretty hard to match.