Spring (March through May) and autumn (September through November) are good times to watch the migratory flocks.

With migratory flocks containing hundreds of birds, they can create clear outlines of the normally invisible rising columns of air (thermals) that they ride.

It lived in enormous migratory flocks - sometimes containing more than two billion birds - that could stretch one mile (1.6 km) wide and 300 miles (500 km) long across the sky, sometimes taking several hours to pass.

Experts trying to establish a second migratory flock face the politically sensitive matter of choosing the breeding and wintering places.

The species lived in enormous migratory flocks until the early 20th century, when hunting and habitat destruction led to its demise.

A flock of slow birds, migratory, made their way parallel with the mountains.

ICF is a founding partner of the Whooping Crane Eastern Partnership, dedicated to the breeding and release of the endangered whooping cranes into a new migratory flock in the eastern United States.

The last remaining natural migratory flock of whooping cranes in the world nest in and around Wood Buffalo National Park.

Birds let us get closer; they stared at us suspiciously from trees or bushes, or flew overhead in vast migratory flocks.

Such geese, he said, do not migrate - not even to tag along with a migratory flock passing through.