The sights and sounds of nearby birds performing courtship displays or constructing nests seems to have a stimulating effect that brings other birds into breeding readiness
This social facilitation may have spelled doom for the extinct Passenger Pigeon
Once nested in colonies that numbered in the billions of birds, and stretched over several square miles
So numerous that in April 1873 in Saginaw MI a single flock took from 7:30 AM to 4:00 PM to pass overhead
Flocks were commonly observed over a mile wide during migration
Flocks were so dense that one shot could bring down 30-40 birds!
Their roosts covered up to 5x12 mi2, with 90 nests or more in a single tree
Branches frequently broke under the weight of the birds - droppings covered the ground several inches deep!
In a single hunt in 1896 the last surviving flock was destroyed
Over 200,000 birds were slaughtered and 40,000 mutilated and left for dead, with tens of thousands of chicks left to starve to death
By the time the slaughter of these birds was stopped, colonies were reduced to the point where birds were insufficiently stimulated to breed together, and may have been picked off individually by predators
Coloniality probably evolved independently in several taxa