Over the next few weeks, into late July and August, you may notice that birds almost seem to be disappearing. You won’t hear as much singing, or see as many birds conspicuously out and about in the woods, fields and towns. Some days you might even be hard pressed to find a Mockingbird sitting up and singing its long-playing song, as they do so reliably much of the rest of the year.

Has there been a plague, or a mass exodus?  Is it a sign of environmental collapse?  No, it’s a perfectly natural thing that happens every year.  The birds are out there, but they are feeling shy and subdued.

There are several reasons for this late summer lull.  The nesting season is winding down for many birds, so the conspicuous singing males quiet down.  Boys that were singing their hearts out a few weeks ago are now sulking in near silence.  A few birds really do begin to head south in July and leave this area; but on the other hand some other early migrants from farther north begin moving into Tennessee as the same time.

But the phenomenon that is actually the major reason for this bird lull in the dog days has a rather odd name:  The pre-basic molt.

A while back I wrote about feathers, and what remarkable things they are: lightweight, water-repellent, tough and versatile. They protect against heat, cold, rain, snow and sun.  But, feathers do wear out.  And once or twice a year, each feather is shed and replaced with a new one.  This process is called molting.

Most birds do this feather replacement in large batches.  They experience a molt during which most or all of their feathers are replaced.  Of course this does not happen all at the same time, which would leave the birds temporarily naked, flightless and vulnerable.  They shed and replace their feathers through a very specific molt sequence, the details of which are unique to each individual bird species.

One of the important aspects of molting is the timing of the replacement of the wing and tail feathers.  This happens in a particular pattern and sequence that insures the birds always have enough functional feathers to be able to fly.  But still, their flight abilities might be a little sub-par while they are missing a couple of these important feathers, which means they have to be more cautious about predators.

Molting also requires a lot of energy.  If you have kept laying hens, you have likely noticed that they often stop laying eggs while they are molting.  Resources that would be used for making eggs are diverted to making new feathers instead.  It’s almost like healing from an injury, where the bird’s body redirects its energy and nutrients into growing new feathers.

This is what is happening to many birds in late summer.  Most of our smaller birds molt twice a year, shifting between their winter (basic) and summer (alternate) plumages.  In some species, these two plumages are nearly identical, but in others the alternate is brightly colored (especially in males) while the basic is drab and inconspicuous. The “pre-basic molt” occurs in these birds in late summer, when they shed their alternate plumage and grow their basic plumage.  It also includes a complete molt of the flight feathers in many species.

As I have mentioned before, most of the birds you see out and about in late summer and fall are actually youngsters who were just hatched in the previous months.  These juveniles, wearing the first set of feathers they have ever had, also go through the pre-basic molt and replace their juvenile plumage with their first, fully adult, basic plumage.

You can often see evidence of the pre-basic molt looking at the males of many species, which are shifting between the brightly-colored alternate and drab basic plumage. Male tanagers are often a patchwork of summer red and winter yellow.  Male Indigo Buntings are a blotchy mix of brown and blue.  In many other species the color changes are not so obvious, but the molt is happening just the same.

One result of all this feather replacement is that for the next month or so, a large portion of our local birds will be feeling lethargic, shy and vulnerable.  They will be more inclined to skulk in the bushes than to jump up onto the fences and power lines.  They do not want to attract attention to themselves, so they often remain almost entirely silent.  And, I should not need to tell you that small, silent birds lurking in the bushes are extremely hard to find! It really can seem like they all just vanished.

Towards the end of August, though, things shift again. Most of these birds have finished their molt, and now they are focused on fattening up for their autumn migration and the coming winter.  All of a sudden the birds reappear, and they are joined by the beginning of the mass invasion of migrants from farther north.  Soon the woods, hedges and yards wil once again be filled with noisy, active, hungry birds.

So when the bird world gets quiet in the next few weeks, don’t worry.  The birds are out there, taking a break from the hullabaloo, and preparing themselves for the challenges and drama of autumn and winter with a new set of fresh feathers.  They will be back in force around the end of August, just in time for the autumn migration and some of the most challenging and exciting birding of the year.