Yeah, yeah, it’s molting. So what…right?

Right guys?  Right?...Guys?... 

As the title implies a molting bird, in and of itself is not of much interest to a birder let alone anyone with any sense of natural history or levelheadedness.  The regular occurrence of the growing of new feathers is a simple fact of nature; a necessity for the physical laws of flight.  As feathers age, they become worn and brittle.  They no longer provide the bird with streamlining or the perfect airfoil wing-shape needed for dynamic flight, and instead produce extra turbulence and leave an otherwise healthy bird aeronautically indisposed.  So yes, the fact that feather molt evolved is interesting.  The physics of flight is interesting.  But a molting bird is just, well, a consequence of breaking though an amniotic eggshell with a horny, toothless beak.

A Mangrove Swallow visibly molting primaries.  The difference between the outer juvenile primaries (9 and 10) and the fresh basic plumage flight feathers is blatant.  Given that this bird is replacing very worn juvenile flight feathers, what age is this bird?  Try to use some of the information in this post to tell this bird's story.

So why is calling out or talking about a bird’s molt de rigueur amongst "good" birders?  What’s the freaking point? Yeah it’s inner primaries are shorter.  They’re growing.  It's outer primaries are browner.  They're old.  That’s what feathers inevitably do.  But can we learn something from a molting bird relative to, say, identifying or aging it?  Well, I am by no means an expert on molt, but I can tell you this: you can learn a lot about a bird from the timing and extent of its molt, from identifying its age, to distinguishing similar species in the field.  In these contexts, molt is both relevant to the average birder seeking to identify birds in the field, and to biologists (e.g. studying survivorship or reproductive success) looking at birds in the hand.  In some cases, being certain of a bird's age, sex, or species may be contingent on the timing and extent of its current molt.

We have touched on molt and especially with regards to molt limits (an outcome of molt) in the past, and bringing it up again merely stresses its importance.  I will not give you a detailed description of all molt cycles, stages, types, etc, nor the patterns or timing associated with specific families or groups of birds.  However, to start your own investigation of molt, it is helpful to get a little of the basics.  Molt, by definition, is the periodic growing of new feathers not the replacement of feathers.  Therefore anytime I refer to molting feathers, I do not mean feathers that are falling off, I refer to feathers that are growing.  

Here is an extreme example of a bird that critically needs to molt.  This Pigeon Guillemot's  remiges are worn to a point in which the feather shafts are all that remain.  This bird cannot fly, and chances are it cannot maneuver well underwater either since alcids use their wings to help them "fly" subsurface and catch food.  In fact we watched this bird for most of fall 2012 as it swam in the waters just off of Southeast Farallon Island.  The bird began molting quite late, but unfortunately never was able to finish.  The bird was eventually found dead on the island mid-molt.  Thus I imagine birds may often face the dilemma, molt or die.  Reason's why a bird might forgo molt include malnutrition, parasites, or disease among others.  I imagine this bird was struggling with at least one of these issues.

General Sequence for birds showing Complex Basic Strategy for molt (I suggest you read Howell's 2003 essays from Birding and his book, Molt in North American Birds, to really begin to understand the complexities in detail):

Pre-juvenal Molt (now referred to as first pre-basic, and includes all body and flight feathers, and is generally grown in the nest) --> Juvenal plumage (more often than not, much browner, streakier, and of poorer quality) --> Pre-formative Molt (generally within a few months of leaving the nest replacing most or all body feathers and no or some flight feathers) --> formative plumage (birds in this plumage are generally referred to as immature or "first year" rather than juvenal) --> pre-basic molt (the term definitive PB molt refers to the molt that results in the adult plumage: generally the following calendar year replacing all  feathers) --> basic plumage --> pre-basic molt (generally the following calender year all feathers)...

This molt pattern, in my opinion is the most important to get down.  Keep in mind that generally if you insert pre-alternate molts into the life cycle (complex alternate strategy) or remove the pre-formative (simple basic strategy) you have then pretty much addressed every species North American landbird.  Also keep in mind that, generally birds only have one complete molt (i.e. all feathers including flight feathers) per year.  So if we are looking at molting flight feathers, which is generally the case, we know it's probably a pre-basic molt or probably not a pre-formative molt (some exceptions to this rule).  This will almost never be pre-alternate molt (if ever?)

Knowledge of molt and their subsequent plumages can be helpful even in birds you've had no experience with.  This bird is bizarre in that during it's preformative molt, it replaced tertials, outer primaries, and the first secondary.  I had no experience with Garyish Saltators prior to this bird in my hand, but the contrast in colors resulting from two generations of feathers is glaringly obvious.  With no prior knowledge of how this species molts or looks as a juvenal, I can tell that the greener, worn feathers are juvenal (first basic).  Thus, the bird must be in formative plumage since during it's next basic molt (should happen soon) it should replace all flight feathers.  The eccentric molt seen here is further complicated by that middle replaced feather, but that may be a feather that was lost and replaced later (adventitious molt).  Keep in mind tropical birds may behave differently than their temperate counterparts. 

Here are some things to think about when looking at a bird showing flight-feather molt:

1) What time of year is it and where are you?  Birds typically molt their costly flight feathers after major life-cycle events like breeding or migration.  This is because many of these events require a lot of resources.  During the breeding season, birds put all their energy into creating and raising offspring. During migration birds put all their energy into...migrating.  So depending on when and where you are, molt can tell you something about a bird's age or even species.  For example, some species molt their feathers on the wintering grounds, some on the breeding grounds, and less commonly some during migration.  Knowing where species molt can sometimes help you rule out similar species.

I can think of two classic examples: 1) Jaegers molt on their wintering waters (I believe this to be universally true off the top of my head, but if anyone has a correction I would really like to know). During the fall in California, it may be possible to see all three jaegers in migration, but only Pomarine Jaeger winters along the CA coast. Therefore a molting jaeger in CA waters is almost certainly a Pomarine on its wintering grounds.  2) Similarly, most Empidonax flycatchers molt on the wintering grounds.  However, Hammond's Flycatcher molts on the breeding grounds.  Therefore any Empidonax with freshly molted plumage in fall in Western North America is likely Hammond's.  There are many other examples that I won't get into here (frankly I don't know a lot of it).  They include separating shorebirds in migration and winter, separating nighthawks in southern California (Common molts on the wintering grounds), and swallows in TX.

2) What feathers are molting? Tertials? Secondaries? Primaries? Coverts?  For example, it is very common for birds to molt tertials and secondary coverts, but no other wing and flight feathers during pre-formative molt. Thus you can generally age a bird in pre-formative molt as a hatching year.

3) What do other feathers in the wing look like compared to the feathers that are growing.  Are other feathers similar in color or much browner?  Browner, and often more tapered, feathers can often be attributed to juvenal plumage.  Feathers grown in the nest are grown rapidly and are of much poorer quality than adult feathers.  The difference is often quite clear, although not always.  The photo below shows very clearly three generations of feathers.)

Clay-colored Thrush with three generations of feathers in the wing.  The brown and very worn outer primaries and middle secondaries are retained juvenal feathers.  The olive green and moderately worn tertials (inner secondaries) were replaced during the pre-formative molt.  This bird was symmetrically molting the first few primaries and secondaries.  Notice how the juvenile feathers contrast in both color and wear with other feathers, but the formative feathers only contrast in wear (i.e. not color) to the brand new adult feathers.  Understanding the complex basic molt pattern (above) helped us to age this bird as a second year and understand the contrasts we were seeing in the wing.

4) What is the bird doing regarding it's life cycle?  Is the bird on it's breeding grounds? Is the bird on it's wintering grounds?  Is the bird migrating?  As I pointed out earlier, molting is energetically costly. As it turns out so is migration.  Interestingly, some species of birds have been identified as molt migrants, birds that molt at migration stopover sites.  Understanding where a bird molts has important implications for conservation.  Traditionally we like to think that looking at breeding and wintering grounds pretty much represents the whole picture.  If you protect those two areas, you are protecting the birds. Critically, however, molt stopover sites (e.g. Mono Lake for Wilson's Phalaropes and Eared Grebes) represent another consideration for successful conservation.

Practice looking at molt at your feeder in mid-late summer, and by taking or looking at photos.  Or by banding birds, but not everyone has that luxury.  While you're practicing, go back to that first photo of the Mangrove Swallow here.  Tell its story.  How old is it? Try to map out its life cycle thus far.  

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.

Compare-A-Sharpie

In addition to exciting birding, the new year has also ushered in a time for possible confusion. That is because birds are aged on a calendar system, so a hatch year bird on December 31st is now a second year bird on January 1st. We are comparing three different ages of Sharp-shinned Hawks (SSHA) in this post, but all three birds just passed their "birthday" and became a different age along with the new year. To reduce confusion I will try to break down how these birds are aged and how that relates to plumage differences before I get into the rest of the post.

From hatching until December 31st of that calendar year, these SSHA are considered hatch year (HY). The subsequent year they are considered second year (SY), and so on. Simple enough. But, these birds don't molt on December 31st and January 1st, so the different plumage types don't correlate exactly with their calendar age.  These birds generally molt in the fall; therefore, a SY SSHA early in the year (before molt) looks different than a SY SSHA late in the year (after molt). Also, SSHA that are past their 3rd calendar year don't show enough unique characteristics to age any farther than third year (TY) in December or after third year (ATY) in January. So, even if a SSHA is in its 5th calendar year, it would still be called an ATY.

So, to avoid confusion I will call our three Sharp-shinned Hawks juvenile, sub-adult, and adult. The juvenile hatched in 2011 and was a HY in December but a SY now, the sub-adult hatched in 2010 and just became an third year (TY) bird, and the adult hatched in 2009 or earlier and is now considered an after third year (ATY).
 
Now we will compare our two new SSHA to the one from last month in the post Birds in Hand: A Couple of Recent Favorites.

This is how we weigh birds. There is no harm done by putting the bird in a can or cup. [Photo by Luke Musher]

Here is a juvenile male Sharp-shinned Hawk. Note the yellow iris and thick, brown streaking (running the length of
 the body) as these are characteristic of juvenile SSHA. [Photo by Luke Musher]

Another look at the juvenile male Sharp-shinned Hawk. [Photo by Luke Musher]

This is the sub-adult male Sharp-shinned Hawk that we posted about last month. Note the orange iris characteristic of
 sub-adult male SSHA and the rufous barring (running across the body vs. along the length of the body) characteristic 
of older (sub-adult and adult) Sharp-shinned Hawks. [Photo by Cory Ritter]

A closer look at the sub-adult SSHA (SY last week, ASY today). Again, note the orange iris and rufous barring. [Photo
 by Cory Ritter]

This is a third male Sharp-shinned Hawk that was caught last Saturday. This SSHA is an adult based on the deep red 
iris. Therefore, this bird hatched in 2009 or earlier. So, these three Sharpies hatched in three different years. The top 
bird hatched in 2011, the middle in 2010, and this one in 2009 or earlier. Note how the rufous barring is pretty much
 the same as the middle bird, but the iris is a deeper red. [Photo by Cory Ritter]

We have caught some other pretty sweet birds lately. Below are some photos of some recent captures:

AHY Male Anna's Hummingbird. We feed hummingbirds sugar water, the same that goes into hummingbird feeders, to 
ensure they don't run low on energy during processing. This is a photo of Frenchie feeding this Anna's Hummingbird
 some sugar water. [Photo by Dan Lipp]

Here is another look at the AHY male Anna's Hummingbird. [Photo by Dan Lipp]

Cory measuring the wing-chord of a Bewick's Wren. [Photo by Dan Lipp]

Bewick's Wren posing for the camera. [Photo by Dan Lipp]

Posing, once more. [Photo by Dan Lipp]

This is Red-breasted Sapsucker was caught on New Year's Eve, and was a SY bird (now, in 2012, a third year [TY]
 bird). This photo shows the primary coverts that are uniform in wear and abraded--a trait of SY/ASY Red-breasted
 Sapsuckers. [Photo by Cory Ritter]

This photo shows more evidence of this being a SY/TY bird. There is a block of retained feathers, which are duller
 black and more worn, in the secondaries. Secondaries 1-4 are retained and 5-9 are replaced--this was the same on
 both wings.  HY/SY birds will show all secondaries of the same generation (no contrast) and ASY/ATY birds will show
a mixture of retained and replaced feathers that are neither in a block nor symmetrical on both wings. [Photo by Cory
 Ritter]

SY/TY Red-breasted Sapsucker [Photo by Cory Ritter]

So, we hope we haven't confused you by this post. There are lots of ages being thrown around in this post, and if anything doesn't make sense, please ask us and we can explain it further. Thanks!

By Cory Ritter and Luke Musher

Find the Molt Limit

A quick and dirty molt lesson that will focus on a few aspects of molt that are necessary to know if you want to understand how and why we age birds the way we do:

All birds (for our intents and purposes) undergo one main molt, or growing-in of new feathers, each year. This molt is called the prebasic molt in AHY (after hatch year) birds, and the preformative molt in HY (hatch year) birds. Molts can be complete, incomplete, or partial. During a complete molt, birds grow in all new feathers--this includes all body feathers, and all flight (wing and tail) feathers. Birds that undergo an incomplete molt will grow in new body feathers and some, but not all, flight feathers. Finally, during a partial molt, birds grow in new body feathers and a varying number of wing coverts, but generally no flight feathers.

Now, why is this important? It is important because almost every adult (AHY) bird will undergo a complete prebasic molt, but the HY birds that have a prefomative molt will usually have a partial or incomplete molt. And it is specifically this partial or incomplete molt that will allow us to see a molt limit!

Molt limits are the "boundaries between replaced and retained feathers, resulting from partial or incomplete molts" (Pyle 1997). These boundaries are visible because the newer, replaced, feathers are generally less worn and less faded than older, retained, feathers. So, if all (minus an exception or two) adult birds have complete prebasic molts, then a bird showing a molt limit must be a........................hatch year bird!

Bird Topography.

We had a typical but interesting day at Palo today, with two Varied Thrush and a brand new hatching year female Spotted Towhee.  Check out some photos and descriptions of how we age and sex these species.

HY male Varied Thrush.  Females would generally not have such dark auriculars (cheek band) and breast band. [Photo by Cory Ritter]

HY male Varied Thrush.  Most of the rectrices (tail feathers) look fairly tapered rather than truncate, which is a good sign that this bird is a HY.  HY birds of most species retain many of their juvenal flight feathers.  These feathers are grown in the nest and quickly, so the quality is much lower than those of adults. [Photo by Cory Ritter]

HY Varied Thrush.  Note the overall ratty appearance and brown wash to the head and back (brown on the back is hard to see in the photo).  The wear and overall messiness is not a definitive indicator of age, but it certainly is supportive of a first year bird.  Again, this is because juvenal flight feathers are grown rapidly, and so are of poor quality, and become worn quickly in the nest. [Photo by Cory Ritter]

Male Varied Thrush. Deeper orange and blacker black than the previous HY above. [Photo by Luke Musher]

Male Varied Thrush.  Note neater, better quality flight feathers, broad and rather truncate primary coverts, indicative of an after hatching-year bird. [Photo by Luke Musher]

These rectrices appear more tapered than those of the previous Varied Thrush.  The inner edges of the outer rectrices form a gradual curve.  After-hatch-year birds will show rectrices whose inner edges form abrupt angles.  This ambiguity kept us from aging the bird. [Photo by Luke Musher]

HY female Spotted Towhee.  Its brownish rather than blackish head, wings and back indicate that it's a female.

HY female Spotted Towhee.  This Is textbook HY rectrix shape, narrow and tapered. [Photo by Dan Lipp]

Can you find the molt limit in this photo? [Photo by Dan Lipp]

Okay okay, not the best photo, but enlarging the photo, and putting in a nice red arrow, should help you spot the molt limit. The replaced alula covert on top is darker (less faded) than the retained alula feather.  This is another clue that the towhee is a HY. [Photo by Dan Lipp]

By Cory Ritter, Luke Musher, and Dan Lipp