Though according to Anders Hedenström, who led the research at Lund University in Sweden, it’s not clear why this is.
One possibility is that larger birds are faster fliers than small ones. This might influence overall flock speed because, for purely statistical reasons, larger flocks contain a greater proportion of larger birds.
But surprisingly, despite the advantage of speed in numbers, the terns tend to undertake their epic migrations between the north and south poles in small flocks of about 15 birds.
It may be that the variations in individual speed make large flocks prone to fragmentation. “If the heaviest birds fly at their optimal speed, then the others will have to fly too fast if they’re going to keep up,” Hedenström told BBC Wildlife Magazine.
Main image: An Arctic tern dives down to protect its nest on Inner Farne, England. © Dan Kitwood/Getty