Birds of a feather
Dickcissels migrate from North to South America down the Isthmus of Panama. They form huge flocks. Flying in an enormous flock like this is the acme of flying skill - coordinating their flight and changing direction as if with one mind. Years ago the flocks would have travelled on south to the plains of South America to feed on the seeds of wild grasses, but in Venezuela they now find fields of cultivated grain, exactly as they found up in the north. So they have no need to disperse and devastate the crops wherever they settle. Some flocks may be half a million strong, and intensive agriculture now allows them to stay and feed together. At night they might roost in sugar cane fields, half a dozen birds to a single stem.