Molt and Aging Birds

By Dan Lipp

Here at Palomarin, we like to keep track of how old our birds are.  Since we can’t ask a bird when it hatched, we resort to slightly more direct measures of age.  One of the best ways to determine when a bird hatched is to look to its flight feathers and wing coverts for molt limits.  You may already know that a freshly hatched bird closer resembles an amorphous blob with mold growing on it than the svelte adults we've come to know.  This is due in no small part to the hatchling’s lack of non-downy feathers.

Anna's Hummingbirds, freshly hatched.  If you squint, it looks like the inside of a 70-year-old man's ear.

During the next few weeks, while the hatchling wallows in its own filth, it will undergo its first molt, the pre-juvenal molt, and grow a complete set of feathers, termed the juvenal plumage.  These feathers actually have shafts, allowing the bird to fly.  However, they are of poorer quality than adult feathers, and tend to make the bird look like it just rolled out of bed after a 3-day meth bender.  The juvenal flight feathers tend to be narrower and more tapered than adult feathers; presumably because they need to grow at such a rapid rate to allow the nestlings to fledge in a short period of time.  Because they grow while the bird is in the nest, they  become worn and tattered rather quickly.  Juvenal feathers are often easily distinguished, by shape and wear, from the higher quality feathers that are molted later in the bird's life.

The same Anna's Hummingbirds as above 10 days later, with most of their juvenal plumage.  That'll comb right out.

The second molt that many birds go through is the pre-formative molt, which produces the formative plumage.  It isn’t until several months after the bird has fledged that it begins to molt these higher quality feathers.  However, when it replaces the flight feathers and coverts, it tends not to replace them all.  This leaves it with a somewhat predictable mix of crappy juvenal feathers and sleek adult feathers.  When this mix occurs within one feather tract (e.g. greater coverts or primaries), it is called a molt limit.  At this time of year, we can surmise that birds showing molt limits were born during the last breeding season since all adults would have gone through a complete molt--replacing off of their feathers (this only applies to birds on a complex basic strategy--see bottom for additional information on molt strategies).

Take a look at this Red-breasted Sapsucker caught in November of 2011.  Note how the brown, tattered, and tapered retained juvenal primary coverts contrast with the newer, replaced greater coverts.  Also, there is a molt limit in the secondaries (tertials AKA the three innermost secondaries have been replaced, whereas a block of worn juvenal feathers remain on the outside).  Woodpeckers are actually a little more complex with regards to age, but this example shows what a molt limit looks like.  This individual is actually a second year (pre Jan 1) or a third year (post Jan 1) bird.

He was whining a lot and acting like an immature in general.  Unfortunately, we can't use that to age them.

Another way we might come upon a molt limit is if a bird shows an eccentric replacement pattern.  Normally birds molt their flight feathers starting at P1 (primary #1) moving outwards and S1 (secondary #1) moving inwards.  However, in birds exhibiting an eccentric molt pattern, the bird will begin molting not at P1 but at a middle primary such as P3 or P4 and continue distally until its outermost wing feathers are replaced while the inner feathers are retained.

Occasionally, we see abnormalities in molt limits. We caught a Wrentit in November that essentially displayed this pattern, except that it retained its outermost primary, P10.  The difference in color is evident in the photo below.  Older feathers are a light caramel color, bleached with sun, while newer feathers are more of a darker chocolate.  Mmmm, suddenly I'm hungry.

P4 through P9 and the tertials have been replaced, while all the secondaries, 
primaries 1-3, and primary 10 were retained.

Anyway, because of the eccentric molt, we can deduce that this bird (caught in late 2011) is a hatch-year.  What's interesting is that in nearly every documented case, birds that undergo eccentric molt will complete by molting the outermost flight feather; in the case of Wrentits, that would be primary number 10.  However, if we look at the photo, we can see that P10, the outermost flight feather, is light caramel meaning that it was retained.  We asked friend of Palomarin and all-around molt wizard Steve Howell why this might be, and we were told that, likely, some environmental pressure or genetic impetus forced it to stop before it had completed its molt.  "Well, that's a data point," he said.

So what the hell do we do with this guy?  Oh yeah, I forgot; we don't catch pelicans.

Special thanks to Luke Musher and Cory Ritter for sharing their soapbox.  They run a helluva bird blog.

Additional information on molt strategies:

Since most of the birds we encounter at Palomarin are on the complex basic molt strategy, this post almost exclusively applies to birds on this system; however, keep in mind that a strong understanding of basic strategies is critical to understanding other such strategies.  The basic strategy is divided into two types, simple and complex.  The simple basic strategy means that every molt the bird goes through is the same, and thus always involves molting all feathers.  The complex basic strategy is the same except that the second molt, the pre-formative molt, is almost always partial (only molting some body feathers) to incomplete (some to all body feathers and some flight feathers), yielding different generations of feathers in the same plumage.