An Australian study has set out to measure the number of birds worldwide, returning an estimate of 50 billion – although the uncertainty on that number is large.
“Humans have spent a great deal of effort counting the members of our own species – all 7.8 billion of us,” says Will Cornwell, an associate professor of ecology and co-author on a paper describing the research, published in Proceedings of the National Academy of Science.
“This is the first comprehensive effort to count a suite of other species.”
Top 10 most abundant birds, according to Callaghan et al’s study:
- House sparrow (1.6 billion)
- European starling (1.3 billion)
- Ring-billed gull (1.2 billion)
- Barn swallow (1.1 billion)
- Glaucous gull (949 million)
- Alder flycatcher (896 million)
- Black-legged kittiwake (815 million)
- Horned lark (771 million)
- Sooty tern (711 million)
- Savannah sparrow (599 million)
The researchers, who are all based at the University of New South Wales, used the database eBird for their estimate. Run by the US-based Cornell Lab of Ornithology, eBird allows birdwatchers to record their observations, making the data publicly available for researchers – or keen citizen scientists – to use.
The researchers used an eBird dataset totalling nearly a billion sightings to develop an algorithm that could estimate total population for each species recorded. In total, 9,700 species of bird were examined – 92% of all living recorded species. The remaining 8% of species were considered rare enough that they were unlikely to have an effect on the overall estimate.
An important part of the algorithm was bird ‘detectability’ – that is, how likely it is that a birdwatcher will see any specific species, making sightings more prominent in the data. Detectability includes factors like proximity to cities, size, colour, and flocking behaviour of birds. The researchers acknowledge that birdwatching behaviours (like seeking out and recording rarer species) may also have an effect on the data.
“A range of uncertainty is necessary when making global-level estimates,” says co-author Shinichi Nakagawa, a professor of ecology and statistics at UNSW.
“Our findings, while rough in some areas, represent the best-available data we currently have for many species.”
The midpoint (or median) of the researchers’ estimates was 50 billion individual birds in the world, which they believe to be the most accurate guess. There was considerable uncertainty in this number, and it could be lower or much higher – the mean estimate was 428 billion birds, for instance.
“Quantifying the abundance of a species is a crucial first step in conservation,” says Corey Callaghan, lead author on the study, who completed the research as a postdoctoral researcher at UNSW.
“By properly counting what’s out there, we learn what species might be vulnerable and can track how these patterns change over time – in other words, we can better understand our baselines.”
Callaghan says that this estimate can be used as a starting point for future analyses. “We will need to repeat and refine this effort to really keep tabs on biodiversity – especially as human-caused changes to the world continue and intensify.”
The study noted that several Australian bird species number in the millions, including the rainbow lorikeet (at 19 million), sulphur-crested cockatoo (10 million) and laughing kookaburra (3.4 million). Globally, the most abundant orders of birds were perching birds, shorebirds, and waterfowl, while the least abundant were kiwis and mesites.
The researchers hope that more people will get involved in citizen science projects like eBird. “Large global citizen science databases such as eBird are revolutionising our ability to study macroecology,” says Cornwell.
“This type of data simply wasn’t available a decade ago.”
“Birding is a hobby that just keeps on giving,” adds Callaghan.
“You can usually find a bird or two to identify and watch anywhere you go, anytime of the day, anywhere in the world.”
“A great starting point is to learn a handful of birds that come to your local area, like rainbow lorikeets, sulphur-crested cockatoo, and Australian white ibis,” says Cornwell.
“It can be as simple as seeing if you can spot any out the window while you’re drinking your coffee in the morning.”
Ellen Phiddian is a science journalist at The Royal Institution of Australia.
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