Meet the flockers

For many years, Charlotte Reid wrote a column for The Lakeville Journal called “Have You Noticed …,” in which she pointed out seasonal changes in the natural world. The column was put to rest after Charlotte died in 2011, but readers have missed it. We are bringing it back occasionally with observations from our many local nature experts. This week, former Nature’s Notebook columnist Fred Baumgarten returns with an update on seasonal changes among area birds. 

— The Editors

As summer slides toward its soft transition, crickets sing and katydids serenade in the evenings, many birds are in the “Great Gathering” — prelude to fall migration.

Tree swallows and barn swallows can be seen in growing flocks around ponds and lakes, swooping in to fatten up on insects in preparation for their coming journey south. At dusk, battalions of them fly silently overhead on their way to gathering in marsh grasses to roost.

Chimney swifts — sleek like the swallows but with shallow, fluttery wingbeats and a constant chatter — also come together in the evenings around their roosts, usually chimneys of old buildings and houses. They swirl around like water circling a drain before plummeting into the chimney to settle down for the evening.

And then will come the common nighthawks, larger birds with a buoyant flight, feeding in silent groups over open fields, later making for warmer climes in steady southward streams.

What makes so many birds want to throw in their lot together and huddle up in the waning days leading up to fall? Remember that in breeding season most birds are fiercely territorial, defending the family unit against all enemies, foreign (other species) and especially domestic (the same species). Scientists say they do this to have the best shot at claiming the best resources — the most productive feeding area, the optimal piece of real estate, the best mates. (The aim of evolution, put in the simplest terms, is to produce the most, healthiest, and longest-lived young.)

So why drop their competitive guard now? There are a number of theories, all of which probably play some role.

In migration, most birds travel in flocks. This offers certain advantages toward survival: greater protection from predators (just as with terrestrial animals in herds), efficiency (easier to follow the group than find your way alone), and location of resources, which is discussed next.

Both in migration and in “pre-migration” season, if we can call it that, food sources begin to thin out. They become, in ecological jargon, more “patchy” in their distribution. While in nesting season insects may be all around you, by late August they may be more numerous over fields and ponds, for instance. Birds that flock together can home in on rich feeding grounds as soon as a single individual spots them. You can see this when a flock of nighthawks swirls around a single spot in a frenzy of feeding. 

Flocking can provide warmth during the cooling nights of August and early autumn. Roosting in close quarters almost certainly helps species such as swallows and swifts.

Sometimes the best bird watching can be done with the least effort. All you have to do is look skyward and see the gathering bird-storm.