Say you’re a Heron. And say it’s July or August somewhere in North America. The year has been good, and you’ve gotten your fledglings out of the nest and more or less self-sufficient; or the year has been bad, and they’re all dead but it’s far too late to start on another batch. Or you just fledged yourself, and you have nothing to do but not die for the next little while.
There’s no real point to starting to head south. Compared to those strange short birds that are just now trickling down from the Arctic to cluster around your feet in the mud at Jamaica Bay, you haven’t got very far to go. The bodies of water you favor haven’t even started to think about icing over yet. There are still fish, there are still insects, there are still young, callow amphibians and crustaceans wandering around. But, as we’ve said, another brood of young isn’t a possibility. It’s as though nature itself is giving you an enforced summer vacation!
Thus, the post-breeding dispersal. Every year about this time, wading birds (some other species have post-breeding dispersals too, but on their own schedule and outside the scope of this post) spread out from their rookeries and explore the rest of the country. With large wings that let them take advantage of warm rising air, they can often end up quite a long way from home, to the delight of birders. This is the season that brings Little Blue Herons almost to the Canadian border, peaks the number of Tricolored Herons at Jamaica Bay, and sometimes brings even more exotic goodies.
(Western M%^*&^&F$%*ing Reef Heron results not typical. Or even probable.)
But even gliding, this takes energy, and birds do not expend energy to delight – or frustrate – birders, as much as it may sometimes seem so. I’ve been able to find little on the whys and wherefores of post-breeding dispersal in herons and their kin, although a popular – and seemingly reasonable – theory is that it allows the birds to spread the pressure of feeding their still-growing and/or soon-to-be-migrating appetites over a wider area than just the immediate vicinity of the rookery, exploiting smaller patches of more varied habitat.