Exotic birds provide unique insight into species invasions.

For most readers, their exposure to invasive species will come from media accounts of “killer” algae in the Mediterranean Sea, Burmese pythons prowling through Florida’s Everglades, or mosquitoes vectoring Zika virus in Brazil. What these observers typically miss, however, are the thousands of species that each year are moved and released well outside their native range (labeled as exotics) but never make the headlines. A best-guess estimate is that nearly of 4% of the Earth’s vascular plant species are considered as exotics somewhere in the world, a number of species that is roughly equivalent to the size of the entire European flora (1). Others estimate that the rate at which new exotic species are appearing in written records has risen to 1.5 species per day (2). This rate of exotic species introduction has risen dramatically over the last couple of decades, mirroring nearly perfectly the rise in global international trade, with no sign that this rate is letting up (2). As a result, there is no biological realm today, including nearly all oceans and the poles, that has been left clear of exotic species (2, 3). This global reorganization of the Earth’s biodiversity has become a signature of the Anthropocene (2). However, we lack a clear understanding of how many of these exotic species will go on to catch attention as harmful to economies, human health, or natural ecosystems. The quest to find answers to that question has motivated an all-out research push from biologists over the last two decades (4). In PNAS, Abellán et al. (5) make headway in that effort using a remarkably complete contemporary dataset on exotic birds introduced into the Iberian Peninsula. There are two aspects of the work by Abellán et al. (5) that give it gravitas within invasion ecology: the uniqueness of the underlying dataset and the conceptual clarity of their analysis. Although birds are not often on the list of species that grab headlines for their harmful impacts, they have been moved as exotics for centuries (6). Add to this history the equally longlasting fervor with which many people, scientists and citizens alike, record the numbers and whereabouts of birds and the result is an extended and detailed accounting of which bird species have been introduced where, and their fate. For precisely this reason this record has generated insights into biological invasions for years (7). However, the dataset the majority of these analyses are based upon reflects a bygone era in terms of the reasons why birds are transported and introduced, leaving it unclear how useful it is for informing today’s invasion problems. A recently updated global accounting of bird invasions shows two distinct peaks in the rate at which birds have been introduced worldwide, one occurring at the end of the 19th century and another occurring in the past couple of decades (6). The first peak came from the actions of acclimatization societies and other groups that purposefully transported and released birds to augment local hunting opportunities or in an effort to enhance the natural aesthetics of their local avifauna (6, 7). That is the data source that fueled past research insights using exotic birds. The recent peak in bird invasions stems almost exclusively from the Fig. 1. The invasion process has four distinct stages: transport, release, establishment, and spread (9). For a species to transit any one stage it must successfully overcome ecological, climatic, and stochastic factors that pose barriers to its success (ovals). Depicted here is the gathering of a fraction of the species native to North American and transporting them to Australia where they are released as exotics. These initial barriers are substantial, leading to far fewer species presented with the opportunity to establish viable populations and then increase in abundance and geographical extent (spread) than entered the process. This continued narrowing in the number of species across every invasion stage means many fewer species are considered invasive than were originally transported (funneling). It is as important to track the species that ‟dropped out” of this process at each stage as it is to follow those that successfully transited them if we are to gain a clear understanding of what factors influence the invasion process (10).