Birds : Leaving the nest
The breeding season is winding down. All the single-brooded species are just finishing their nesting chores, and others are well into round two. Birds, the most mobile and migratory of animals, are at their most vulnerable while nesting. Tied to one spot, not only are the adults easy targets for predators, but the eggs and nestlings have no defense at all. However, over the course of time and constant evolution, the birds that nest successfully must be doing something right, and this has been passed on to the succeeding generations.
This is why bird nests are so hard to find. They are placed as to be inaccessible to four-legged predators or defensible from winged predators by nesting colonially - the colony mobs would-be predators and drives them off. At any rate, the moment the young can leave the nest and become mobile, the parent birds get them away from the nest site. Instantly, they are safer from terrestrial predators that no longer have the scent clues and fixed targets to hunt.
Terns that nest on nearby islands come to congregate on Vineyard shores, starting from now until late summer. These birds with young that are just now able to take their first flights move away from the nesting colony and closer to better sources of food. This is post-breeding dispersal of adults and young away from traditional colonial nesting sites.
The nesting site was of primary importance for the species' safety. It was predator free - the most important criteria for high-strung tern colonies. The adults may have had to fly more than 300 miles a day to provide food - in this case, small live fish - to feed ever-hungry nestlings. So as soon as the young can fly, the adults move them closer to the best fishing areas. The young will be dependent on the adults for quite a while, even during the upcoming southbound migration. Fishing, especially as performed by terns in a plunge-dive, is very hard to learn and takes years to master. The oldest terns are the most successful at nesting, because they are the best at catching fish.
This could be called post-breeding dispersal, but it is not what ornithologists or birders mean when they use this term. True post-breeding dispersal is a phenomenon that has been increasingly documented and debated in the behavior exhibited by many species of herons, terns, raptors, and sub-adult birds of many other species. The Vineyard is a very good place to find it: in southern species that have wandered or purposely chosen to venture far north of their species' normal or expected range. Maybe it is normal for them to occur, late in the breeding season, mid to late summer, in some or most years. We learn more every year.