Birds are omnipresent and their twittering accompanies us even in the big city. But how many of them there are worldwide has been unclear until now. Now, with the help of a Citizen Science project, researchers have determined the number of individuals for 9700 bird species – that is 92 percent of all known bird species. According to this, there are a good 50 billion birds worldwide and there are around six birdies for every person on earth. The distribution of birds among the species is, however, very uneven: only four species have over a billion specimens each, 1180 species, on the other hand, with fewer than 5000 specimens each, are rather rare.
The question of how many specimens there are of a species is of great importance in several respects. Because it provides information about the population dynamics and evolution of the species, the structure of ecosystems and communities and, last but not least, the threat status of a species. “The frequency distribution of species is fundamental for many long-standing ecological questions,” explains Corey Callaghan from the University of New South Wales in Sydney and its colleagues. “Nevertheless, the distribution pattern on a global level is usually hardly quantified. The abundance of just a few species is well known, but most of the others have not been researched. ”One of the reasons for this is that on-site counting is extremely time-consuming and that there are many hardly populated and inaccessible areas around the world.
Six birds for every human
However, Callaghan and his team have now combined several methods to create the most comprehensive inventory of global bird life to date. Your count is based on data from the global eBird database – a citizen science project in which more than 600,000 bird watchers from around the world recorded almost a billion bird sightings. For 724 bird species, the researchers compared these data with the results of scientific studies in order to determine to what extent the easy visibility of a bird species falsifies the count values and how the actual density can be determined from this. “To do this, we took into account characteristics such as body size, color, threat status or swarm size that influence the detectability of a species,” the scientists explain.
Using this data, they developed an algorithm that extrapolates the bird numbers for 9700 species on the basis of the eBird data and the known uncertainty factors. That corresponds to 92 percent of the known bird species. However, the remaining eight percent are so rare that they are of little consequence, according to the research team. The evaluation showed: “We have determined that there are currently around 50 billion birds in the world – around six for every person on the planet,” report Callaghan and his team. Although the census was subject to greater uncertainty in some areas, it represented the best available data for many species.
Frequencies unevenly distributed
The results also show that the numbers are very unevenly distributed among the 9700 species: “There are very few common species and many rare species on a global level,” the researchers said. Only four bird species make the leap into the “Billionaire’s Club”: the house sparrow with 1.6 billion specimens, the star with 1.3 billion, the North American ring-billed gull (Larus delawarensis) with 1.2 billion and the barn swallow with 1.1 billion copies. “It was surprising that so few species dominate bird numbers so much,” says Callaghan. This raises the question of what is so special about these species from an evolutionary point of view that they have become so hyper-successful. Conversely, 1180 bird species are represented, each with fewer than 5000 specimens. “That is around 200 rare species more than expected,” the scientists say. These include the rare amber tern (Thalasseus bernsteini), which only occurs on some islands off Taiwan, and the drum rail (Habroptila wallacii), a non-flightable bird from Indonesia.
Overall, the frequency distribution also showed some overarching trends in terms of habitat, way of life and phylogenetic classification: According to this, most birds live in the subtropical to arctic latitudes of the northern hemisphere – around 18 billion in Eurasia and 16 billion in North America and Greenland. The most individual bird orders are the sparrows with 28 billion specimens, followed by waders and water birds, the rarest order are the kiwis with only 3000 specimens. As the scientists emphasize, such data on global abundance and the associated characteristics are also important in order to be able to better protect rare species in particular. “Quantifying the abundance of a species is a critical first step in conservation,” says Callaghan. “By counting what’s out there, we can see which species are endangered and see how this changes over time.”
According to the researchers, their method is not only suitable for birds, but could also be transferred to other animal groups. “Our large-scale data integration can serve as a blueprint to calculate species-specific frequencies for you too,” says Callaghan. This could help clarify some fundamental questions in ecology and evolutionary biology.
Source: Corey Callaghan (University of New South Wales, Sydney) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2023170118