How Big Can a Flock of Birds Get?

Looking up in the sky and seeing a flock of birds temporarily obscuring the Sun can be an awe-inspiring sight. But while you may have seen flocks containing hundreds or thousands of birds – or even a million in a murmuration of starlings – that's nothing compared to one particular flock that darkened the skies over southern Ontario in 1866.

Once the most common bird in North America, passenger pigeons gathered in such large numbers that one flock is thought to have contained 3.5 billion birds. The size of that flock was truly mind-boggling, stretching over a mile wide and 300 miles long, and taking 14 hours to pass overhead. Incredibly, the passenger pigeon would be extinct by 1900, due to over-hunting and habitat loss.

Believe it or not, another 19th-century event dwarfs the 1866 passenger pigeon flock. In 1875, physician and meteorologist Albert Child witnessed a swarm of Rocky Mountain locusts that he estimated contained around 3.5 trillion insects (though some estimates say 12.5 trillion). "Albert's Swarm" covered some 198,000 square miles (512,800 square km), darkening much of the western United States. Incredibly, just like the passenger pigeon, the Rocky Mountain locust is now extinct, despite these inconceivably large numbers.

Strength in numbers: