Anthropogenic extinctions conceal widespread evolution of flightlessness in birds

Human-driven extinctions can affect our understanding of evolution, through the nonrandom loss of certain types of species. Here, we explore how knowledge of a major evolutionary transition—the evolution of flightlessness in birds—is biased by anthropogenic extinctions. Adding data on 581 known anthropogenic extinctions to the extant avifauna increases the number of species by 5%, but quadruples the number of flightless species. The evolution of flightlessness in birds is a widespread phenomenon, occurring in more than half of bird orders and evolving independently at least 150 times. Thus, we estimate that this major evolutionary transition occurred at a rate four times higher than it would appear based solely on extant species. Our analysis of preanthropogenic avian diversity shows how anthropogenic effects can conceal the frequency of major evolutionary transitions in life forms and highlights that macroevolutionary studies with only small amounts of missing data can still be highly biased.

Methods
To obtain a list of all known bird extinctions during the rise of humans (i.e., from the Late Pleistocene onward), we reviewed the published literature on the topic. First, for historic extinctions (after 1500 CE), we extracted the information from the IUCN Red List of Threatened Species (Accessed on June 2019), which includes 162 species that are categorized as extinct (EX) or extinct in the wild (EW). To search for older extinctions or undescribed species, we carried out a literature search using Google Scholar including the terms “geographical location AND (extinct OR fossil) AND (avian OR bird)” up to August 2019. For the geographical locations, we used all the main island archipelagos in the world as well as the continents. After accessing relevant studies, we also checked extinct species or archaeological sites cited within these papers. We complemented the search with scrutiny of relevant books on the subject. For the prehistoric extinctions, we only included species that were extinct after the last interglacial during the Late Pleistocene (126,000 years B.P.), which is determined by their presence in fossil deposits of later age or by contemporary records of the species.