Friday, May 24, 2013

By E. Vernon Laux
Published: July 9, 2009

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.

A mother osprey swooped above as her young were banded and returned safely to their nest. Photo by E. Vernon Laux

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.

The list of variables is so long that it is impossible to isolate a single trigger that would make a species move north of its range. Changes in environmental conditions, severe drought, prospecting for future nest sites, and loss of formerly good habitat are possible reasons or triggers.

Many Arctic and boreal species periodically invade south, which has everything to do with available food supplies. Birds are able to withstand the coldest temperatures on earth, provided they have enough food. Lack of food forces these northern-living species to head south in order to find it.

In the case of southern species it does not seem that food is the usual or primary factor. Certainly, lack of rainfall, drought, and the associated lack of food are important triggers in many parts of the world, particularly in the tropics and especially in Australia. Many nomadic Australian species follow the rains around the desert interior.

This is the season - it is already occurring - when the Island gets visited by southern species. Perhaps they are just doing a fly-about. Or this dispersive behavior is at the forefront of future expansion. Time, as always, will tell. The possibilities are many for different birds to appear at this season. It is more the rule than the exception. While not truly migrating (or migration as we normally think of it) they still perform a seasonal movement. They do appear to be checking out someone else's neighborhood for possible future use.

So while perhaps not the most interesting time of year for birders, things are far from dull. These most mobile of creatures can turn up in far-flung locations, far from where they are "supposed" to be, even during the nesting season.

Until next time-keep your eyes to the sky.