Who would you repopulate the earth with?

Around 50 billion birds populate the earth

Australian scientists have extrapolated how many birds currently populate the earth: It should therefore be around 50 billion, but only four species come to more than a billion specimens. Numerous species, however, are rare or very rare. The birds that are still quite common include the sparrow and the barn swallow, while the rare ones include the kiwifruit. Knowing about the frequency of a species is important for timely conservation measures, say the researchers.

Comprehensive database

The study by the team led by William Cornwell from the University of New South Wales (UNSW) in Sydney (Australia) was presented in the journal "Pnas". The researchers combined scientific surveys of individual species in certain distribution areas with the almost one billion entries in the Internet database "eBird". Around 600,000 citizen scientists enter bird sightings in it.

Cornwell and colleagues used the research data to determine how many of them are estimated to occur per unit area for 724 species. They compared this with the frequency of sightings of these species on "eBird". They also took into account how often a bird species is likely to be discovered by humans based on its appearance and lifestyle. Birds that breed near settlements are spotted more often than those that are mostly in remote areas.

92 percent of the known species

From these specifications, the researchers developed an estimate for the global frequency of 9,700 bird species - around 92 percent of all known bird species. The remaining eight percent were not included in the estimate due to the uncertain data situation. However, since they are all rare species, their number would hardly add to the total number of birds, the researchers explain.

According to their calculations, there are only four species, of which more than a billion individuals exist: the house sparrow or sparrow (Passer domesticus; 1.6 billion), the starling (Sturnus vulgaris; 1.3 billion), the ring-billed gull (Larus delawarensis; 1.2 billion) and the barn swallow (Hirundo rustica; 1.1 billion).

About twelve percent of the species, on the other hand, only have a population size of less than 5,000 animals, of many only a few individuals exist. The Madagascar bog duck (Aythya innotata), for example, is poised with an estimated 25 wild animals on the verge of extinction. The amber tern (Thalasseus bernsteini), the brown-bellied thicket bird (Atrichornis clamosus) and the flightless drum rail (Habroptila wallacii) are similar.

No general pattern

The researchers write that quantifying the abundance of a species is a crucial first step in its conservation. "By correctly counting what's out there, we learn which species might be susceptible and we can see how those patterns change over time," explains first author Corey Callaghan of UNSW. The researchers investigated whether a rare occurrence is typical of certain groups of birds, but could not find a general pattern. The birds from the kiwifruit (3,000) and stilt-claw families (154,000) were the least common.

Diet data were also collected for the study: Most of the birds currently alive therefore eat invertebrates such as worms (15 billion), but many are also omnivores (13 billion). The rarest are scavengers (194 million) and birds that live on flower nectar (479 million).

Indicator for the health of ecosystems

How these and other numbers develop could be recorded every five to ten years, the researchers write. "If their population falls, it could be a real alarm bell for the health of our ecosystem," Cornwell points out. The researchers assume that their method could also be used for other animal species. They also advocate that as many interested laypeople as possible work as citizen scientists. "It can be as simple as checking to see if you can see something out the window while you have your morning coffee," says Cornwell. (red, APA, May 19, 2021)