Largest bird count in 24 hours (worldwide)

The most bird species identified in one day as part of a global effort is 7,234, logged by 51,816 people in 192 different countries on 8 May 2021. The birdwatching bonanza, known as Global Big Day, was organized by the online ornithology hub eBird, a birdwatching community and tracker that is managed by the Cornell Lab of Ornithology.

Among birders, a "Big Day" is a challenge to identify as many bird species as possible during a single calendar day, usually conducted as a small team. As with most aspects of birding, accumulating a list of species seen or heard fundamentally depends on an honour system. Whether birding only casually for an hour or two, or participating in officially sanctioned birding competitions, experienced birders recognize that it is vital to tally only those species that they identify positively and without doubt. To encourage adherence to the honour system, and to standardize practices that ensure comparability among birding practices of different teams, the American Birding Association has developed official rules by which an officially registered Big Day Count must be conducted.

eBird's annual Global Big Day was established in 2015, with some 14,000 people taking part in the inaugural count. As well as the number of bird species logged, the 2021 edition set records on three other fronts: the number of participants (51,816), the number of countries involved (192) and the most checklists submitted (133,887). Over the course of the day, 69,311 photographs and 2,391 audio recordings were also added to the Macaulay Library, a natural history archive (focused on avifauna but not exclusively) overseen by the Cornell Lab of Ornithology that can be accessed and contributed to by all.

The top five countries participating in Global Big Day 2021 were all in South America: Peru (1,357 species), Colombia (1,228 species), Ecuador (1,125 species), Brazil (994 species) and Bolivia (899 species).

The largest bird count in 24 hours by a single team is 431 by professional guides Dušan Brinkhuizen, Rudy Gelis (both Netherlands), Mitch Lysinger (USA) and Tuomas Seimola (Finland), who conducted their count in Ecuador on 8 October 2015.