The most common species, with a score of over one billion, are the house sparrow (1.6 billion), which is found in many parts of the world, the starling (1.3 billion), the barn swallow (1.1 billion) and ring-billed gull (1.2 billion). The latter resembles “our” common gull and is common in North America.
Scientists, those published their results in the American scientific journal PNAS, arrive at this estimate by combining the professional and scientific counts with that of the so-called citizen science on the site eBird – an international equivalent of Dutch Waarneming.nl – on which bird watchers have entered hundreds of millions of observations. For example, they developed an algorithm that should estimate the population sizes of almost all bird species, in part based on the estimated density of those populations. The result of these counts led researchers to estimate that there are about six times as many birds as humans in the world.
The researchers also found that there are relatively few species present in large numbers, while a relatively large number of species are rare.
In their publication, the researchers acknowledge that their tally contains uncertainties and is not complete. For example, they inventoried about 9,700 bird species, or about 92 percent of the total number of extant bird species. This number is estimated to be around 11,000, depending on the taxonomic classification you use.
They also realize that citizen science is not always accurate. For example, researchers take into account that birders are primarily interested in rarities and special sightings, which can lead to overrepresentation in this research. Some bird species are also seen or reported so little that they are also under-represented in this study.
The researchers believe that the quality and reliability of “citizen science” will only improve in the future, which will also allow their future estimates to become more precise.
Evolution, ecology and conservation
Their research, scientists say, may prove useful for fundamental questions about ecology and evolution. Such as the influence of humans on bird populations and the question of which bird species deserve the most protection. “Fully understanding how and why bird species reached their current population size will be of paramount importance for the future study of evolution, ecology and conservation,” the researchers write.
This is not the first time that scientists have attempted to map the number of birds in the world. The results are quite different, which indicates how difficult it is. In 1997, two British researchers came estimate of between 200 and 400 billion individual birds. Australian researchers attribute this large difference to the use of a different method. British researchers reportedly estimated the population density of all bird species equally, while Australian scientists looked at it separately for each bird species.