Museum Specimens Help Solve a Century-Old Mystery in Hawaiian Bird Evolution

A new genomic study from Smithsonian scientists traces the honeycreepers’ explosive diversification to the formation of O’ahu millions of years ago, offering fresh insight into a group of birds now facing one of the steepest extinction rates in the world.

Smithsonian researchers have identified the genetic relationships among all the Hawaiian honeycreeper species that were known to exist—and have subsequently faced major losses—since the arrival of Europeans to the islands. The study, published recently in the Proceedings of the National Academy of Sciences by scientists at the Smithsonian’s National Zoo and Conservation Biology Institute (NZCBI) and the National Museum of Natural History, places extinct and extant species of this famously diverse group of birds in their evolutionary context, informing diversity loss studies and potential conservation priorities.

More than 60 Hawaiian honeycreeper species evolved from a single ancestor on the Hawaiian Islands across a relatively small time window, making them a prime example of adaptive radiation, the rapid diversification of one species into many new ones. When a species diversifies this quickly, it can be challenging to build a comprehensive family tree and accurately measure subsequent biodiversity loss. This problem is coming to a head today as only 17 Hawaiian honeycreeper species are still alive, with most facing extinction due to habitat loss, introduced diseases, invasive predators and climate change. To address this challenge, the Smithsonian team used genetic methods to resolve the relationships among the 17 living species, plus an additional 18 that existed when Europeans first arrived at the Hawaiian Islands in 1778 and two that are only known from fossil bones.

“The findings are bittersweet because while we have a much better understanding of the relationships between the Hawaiian honeycreepers and the role hybridization played in their evolution, they also show we have lost more Hawaiian honeycreeper lineages than we previously knew,” said Michael Campana, genomicist at NZCBI and lead author of the study.

The research team presents the first comprehensive map of the Hawaiian honeycreeper radiation. They found that most species originated during a “big bang” of diversification that took place approximately 2.5–3.5 million years ago, around the same time that the island of O’ahu formed. The team speculates this period facilitated a burst of diversification among honeycreepers because the new island afforded them unoccupied habitats with room to form multiple, distinct populations that then contributed their new genetic material back to other lineages on older islands. While breeding between two animals of different species is often a genetic dead end, the team confirmed genetic mixing between two honeycreeper species, ‘ō‘ū (Psittirostra psittacea) and Lāna‘i hookbill (Dysmorodrepanis munroi), and that a group of closely related honeycreepers called ‘amakihi (Chlorodrepanis spp.), which occupied different islands, continue to share genetic material today.

Researchers were able to gain a fuller understanding of the honeycreeper radiation by using minimally invasive techniques to collect samples from extant and extinct species held within museum collections in the United States and Europe. The work demonstrates the crucial nature of museum and paleontological specimens for genomics research.

“I marvel at the amount of genetic data that my colleagues recovered from tiny bits of epidermis cut from old museum specimens of extinct honeycreepers,” said Helen James, curator of birds and the chair of vertebrate zoology at the National Museum of Natural History. “They even succeeded at getting genetic data out of small fossil bones of the honeycreepers. This really highlights how improvements in genetic sequencing have transformed traditional natural history museum collections into vast repositories of historical and comparative genetic information. This is especially true for birds and mammals, which were often preserved in the form of dried skins and bones, and we are now good at getting DNA from those types of specimens.”

As the team looks ahead, they are focused on investigating the impacts, evolution and control of introduced diseases that have decimated honeycreeper populations, including avipoxvirus and avian malaria. Specifically, they are seeking to understand how both the birds and the pathogens have evolved since the diseases’ introductions.

“The genomic data we generated for this study should also be useful for understanding the morphological evolution of the honeycreepers, as well as to offer clues about how the genomic variation of some honeycreeper species may have influenced their ability to survive disease,” said Robert Fleischer, scientist emeritus at NZCBI and senior author of the study.