Vancouver Avian Research Centre

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August - is ALL about molt, molt, molt and one of the most exciting times for banders. It can also be a confusing time for inexperienced banders trying to age hatch year birds which have completed their 1st prebasic molt but are retaining very fresh juvenal feathers.

An example of this is illustrated below showing the wing of a hatch year (HY) male Yellow Warbler. On first glance at the photo below left it could be fairly easy to age this bird as an adult given the very fresh appearance of the wing, the edged primary coverts and fairly truncate remiges (primaries and secondaries). However once the wing is examined under magnification (photo below right) it is easy to see the difference between the replaced lesser, median, greater coverts and carpal covert and the retained juvenal primary coverts and alula, the molt limit shown with the red arrow between the outer GC and inner PC. The replaced feathers are better quality feathers, more darkly pigmented with darker rachis and more interlocking barbules and, therefore, tighter barbs, giving them a more solidly structured appearance compared to the adjacent retained juvenal feathers. Notice too the subtle difference between the replaced greater alula covert A1 (indicated with the black arrow) and retained lower alula feathers A2 and A3.

For this reason, especially after adults of these same species begin to show signs of approaching the end of their complete prebasic molt banders should always confirm age by additional criteria such as skull ossification.

One of these additional criteria which is helpful in Wilson's Warblers is the variation in the extent of the crown patch illustrated on the two males below being glossy black on adults (left) and extending further forward and backward than on hatch year birds (right) which are less extensive and show greenish yellow mottling.

This hatch year (HY) male Orange-crowned Warbler (OCWA) had also completed its 1st prebasic molt replacing lesser. median, greater coverts and carpal covert. As in the Yellow Warbler example above the primary coverts, alula and remiges are all retained but still fresh juvenal feathers the molt limit indicated with the red arrow between the outer GC and inner PC.

Molt limits in the making!

Birds in active molt give us an opportunity to see molt as it is happening. This is particularly helpful to new banders who have an opportunity to learn the sequence of events in both complete and less than complete molts.

The photo below left shows a hatch year (HY) Orange-crowned Warbler undergoing its 1st partial prebasic molt with lesser coverts replaced and median and greater coverts in sheath. In many species of warblers, sparrows and vireos, all lesser, median & greater coverts but no tertials, retrices or other flight feathers are replaced.

The photo below right shows an adult Lincoln's Sparrow in the midst of its definitive adult prebasic molt. This bird is replacing all body and flight feathers. This sequence starts with the lesser and median coverts, then tertials and innermost primaries, primary replacement proceeding distally towards the outermost primary (P9 or P10). Primary coverts are molted with the corresponding primary. Greater coverts which are in sheath here commence after the start of the primaries and proceed proximally towards the birds body with the innermost GC10 the last replaced. The carpal covert is replaced when the GCs complete and is not yet molted in the photo below. After the tertials the remainder of the secondaries will be replaced beginning with the outermost and proceeding proximally towards the birds body with secondary 6 (S6 indicated with the red arrow) being the last flight feather replaced.
The alula covert (indicated with the blue arrow) is molted at the same time as the median coverts and is a molted feather in this photo. However the alula feathers are molted during molt of the primaries and are among the last feathers to be replaced.

During heavy prebasic molt like this adult birds can still be aged second year or after second year based on the contrast between the molted alula covert and as yet unmolted alula. Here all are worn but too brightly coloured to be juvenal feathers. This bird was therefore aged after second year (ASY) i.e. an adult bird going through at least its second definitive adult prebasic molt.

Conversely this female Yellow Warbler in the last stages of its adult prebasic molt was aged as a second year (SY) on the basis of the as yet unmolted alula. The molt limit is clearly visible between the darker and more greenish-edged alula covert and retained and now very worn dull brown juvenal middle and lower alula feathers.

Within another week or so the alula will be molted in both of these birds which would then simply be aged as after hatch year (AHY).

This hatch year (HY) Yellow Warbler had completed its 1st prebasic molt which included lesser, median and 9 inner greater coverts (the red arrow pointing to the single retained outer GC 1), carpal covert, greater alula covert A1 (indicated by the blue arrow) and all 3 tertials S7-9 (indicated by the black arrow).

The 1st prebasic molt in Purple Finches (PUFI) normally include 3 to 10 greater coverts but no flight feathers. This hatching year (HY) PUFI was replacing 9 inner GCs showing a molt limit in the making between GC 2 in sheath and the retained outermost GC 1.

Many birds in the midst of prebasic molt look pretty scruffy and that was certainly the case with this male Purple Finch.

PUFI males have delayed plumage maturation meaning they do not acquire male plumage traits until they are more than one year old (i.e., at the time of their definitive prebasic molt, which is their first complete post-breeding molt).

Until this time they resemble the brown-streaked females.  Thus, a male PUFI that is replacing brown juvenal and first basic feathers with new raspberry-colored feathers is an SY, while one replacing old purplish feathers with new ones is an ASY.

Now you should be able to age this bird! (Answer at the end of the blog)


Molt is a protein and energy demanding process so birds do not undertake molt when they are engaged in other energy demanding activities such as breeding or migration. Most species of passerines molt after the breeding season either on their summer (breeding) grounds or after migration on their winter (non-breeding) grounds.

One exception to this is Swainson's Thrushes (SWTH) which overlap their molt with migration in a phenomenon called molt-migration. SWTH are long distance migrants breeding as far north as Alaska and wintering as far south as Argentina.

The term molt-migration is given to individuals that leave their breeding grounds and head south to find a suitable location to undergo their annual prebasic molt before continuing southward migration. Birds may continue to migrate while actively molting or they may initiate and/or complete their molt in an area south of their breeding grounds.

Where arid conditions on the breeding grounds in late summer are not especially conducive to molting, adults routinely migrate substantial distances to special molting areas. In general, their movement away from increasingly drought-stricken breeding habitats is timed for their arrival somewhere in the desert Southwest or Mexico during the period of monsoon rains. The flush of insects associated with these rains constitutes a bumper-crop resource for the energy and protein-demanding molt process.

Our research at Colony Farm has shown that many birds caught for banding after the breeding season are in molt suggesting the site could be a special molting area for this species. Interestingly, following the breeding season we see an influx of unbanded birds and we suspect these are birds leaving their breeding grounds further north and undertaking molt here before continuing southbound migration.

The photos below show a hatch year (HY) SWTH (left) molting inner greater coverts (GCs) as part of its partial 1st prebasic molt, the molt limit is between the 4 retained outer GCs and 6 inner GCs in sheath and an after hatch year (AHY) SWTH (right) molting inner primaries as part of it's complete adult prebasic molt.

This hatching year (HY) SWTH had completed it's 1st prebasic molt showing a clear molt limit (indicated with the red arrow) between the replaced inner 3 greater coverts and the retained 7 outer greater coverts.

Notice the buffy tipping or 'tear-drops' on the retained outer greater coverts and the chocolate brown edging on the replaced inner coverts.

Notice also the replaced feathers are longer than the retained ones producing the visible 'step-in' between replaced and retained coverts typical of Catharus thrushes.

In many species the 1st prebasic molt is incomplete meaning that not only are contour feathers replaced but additional flight feathers can also be replaced. This hatching year (HY) Lincoln's Sparrow had replaced lesser, median, greater coverts, carpal covert and the two inner most tertials (S8 & 9) - molt limits are indicated between the replaced outer greater covert and retained inner primary covert (green arrow), between the replaced greater alula covert (A1) and the main alula feather A2 (blue arrow) and the two tertials S8 and S9 (blue arrow). When tertials are replaced like this central retrices are also sometimes replaced which was the case with this individual.

As many birders know summer and fall can also be a confusing time trying to identify hatch year birds in juvenal and first basic plumages especially when they may only get a fleeting glimpse and don't have the benefit of the bird in the hand as banders do. The two birds below are good examples of this looking very different from their adult counterparts - answers at the end of the blog!

We conduct several studies at Colony Farm involving colour banding birds. One such study in association with Wildlife Rescue Association of BC (WRA) involves releasing some WRA rehabilitated birds in the park to see if survival rates can be determined through recapture and resighting of colour banded birds.

At this time of year WRA are inundated with nestlings and recently fledged birds brought in to the shelter by members of the public and WRA staff and volunteers do an amazing job of hand rearing and caring for these birds until they are ready to be released. We have banded and released 8 fledgling American Robins this year and have posted signs in the park requesting sightings of colour banded birds.

Upon release one baby Robin promptly flew and landed on the VARC truck maybe trying to convince us that our logo should include a Robin and not a Yellow Warbler!

This gorgeous hatching year female Cooper's Hawk was a nice surprise and had everyone running back to the station from net rounds when the radios announced her arrival!

Another 'Traill's' Flycatcher had us scratching our heads again this month and thinking 'Alder'! We band so many Willow Flycatchers which breed in the park that when a bird shows up like this one it just doesn't 'feel' right. These 'odd' birds have much whiter throats and breasts than our WIFLs, much paler lores and narrow, white, albeit faint eyerings.
According to other banders familiar with ALFL they also tend to 'whine' in the hand which our WIFLs never do. This bird constantly whined! This trait is definitely not in 'Pyle' but an interesting characteristic nevertheless!

When the breeding range of Alder Flycatcher is considered it's inconceivable that all of the birds breeding in northern BC migrate eastwards and none return down the coast and we'd be interested to hear the thoughts of any other banders who have experience with this species.

Less confusing was this hatch year (HY) Pacific-slope Flycatcher.

This hatch year male Common Yellowthroat (COYE) was actively undergoing its first prebasic molt with the greater coverts, carpal covert and upper alula (A1), or alula covert, in sheath and flakes of sheathing from pin feathers coming in all over its head and body.
At this time of the year birds like this one with blackish in the lores and auriculars can be sexed male although birds without blackish in the lores and auriculars should not be sexed female as apparently a small proportion of males lack blackish after the 1st prebasic molt.
We have never seen evidence of this with the many COYE banded at Colony Farm with retrap data showing hatch year birds without blackish in the lores and auriculars are indeed females.

With gorgeous weather for most of the month Vancouver was looking at its stunning best and the hot, sunny summer days gave us time for a quick sun bathe between net rounds - well all this banding can wear a guy out!

Another goodbye this month as we bid farewell to British ringer Gabriel Jamie a student from Cambridge University who spent the summer working at the University of British Columbia (UBC) and his spare time at weekends volunteering for VARC. Gabriel is an accomplished ringer, was a fantastic help and although he looks really serious in this photo was also a lot of fun!

He learnt a bit from us on molt (well maybe more than a bit!) and how to use the body grab method for net extractions and despite his tender years was able to teach us a new net furling technique!

This method (photo below right) involves furling as normal where the lower shelves are spun in to the top shelf and furled as normal but then rather than being tied with trail tape or pegs the net is spun by standing in the centre of the net using two hands in a pedaling motion to wind the net tightly on either side of the furler and then simply tied off to a nearby piece of brush. This is difficult to describe and even more difficult to do at first but once mastered really is quite brilliant!

None of us have seen this net furling method at any stations we have ever visited but we would highly recommend it especially if nets are being used in open and/or windy areas. The net ends up very tightly furled avoiding any loose strands and is exceptionally easy to unfurl in the mornings as it requires no trail tape or pegs and even more importantly makes net furling by one person much easier and less time consuming. We may try to video the process and put it up on U-Tube for anyone interested in learning the technique.

I'm not sure Gabriel is going to miss us all that much as his travels now take him on a birding expedition to Peru - We're all very jealous! Thanks for all your help Gabriel - we'll miss you! (P.S. Nice hat!)

Hatch year Cedar Waxwings (CEDW) were much in evidence this month as berries ripened throughout the park. Young CEDWs like these are not only among the cutest of all the baby birds we see they are also among the most confiding often remaining perched on fingers (photo below right) and refusing to leave when released!

This hatch year (HY) male Sharp-shinned Hawk provided an opportunity for us to see the nictitating membrane, the translucent 'third eyelid' present in birds that can be drawn across the eye for protection and to moisten it while maintaining visibility.
Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball. In birds of prey like this Accipiter, it also serves to protect the parents' eyes when feeding nestlings. According to one raptor expert when Peregrine Falcons go into their 200 mph dives, they blink repeatedly with the nictitating membrane to clear debris and spread moisture across the eye.

This hatch year (HY) male Hairy Woodpecker was a nice surprise too. Downy Woodpeckers (DOWOs) are common in the park but HAWOs less so preferring to forage along main branches and trunks of larger trees which are absent in the banding area.

The usual arrangement of toes in passerines is 3 toes in front and 1 in back in a configuration termed anisodactyl. Toes are numbered 1 to 4, 1 being the hind toes or hallux and 2 to 4 are front toes numbered from inner to outer.

In woodpeckers toe 4 has shifted to the back in a configuration termed zygodactyl with 2 toes forward and 2 back to aid in climbing.

We talked last month about the red juvenal crown patch in hatch year male DOWOs and how the red nuchal patch (on the back of the head) characteristic of 'adult' males molts in during the birds 1st prebasic molt. This hatch year male HAWO in the midst of its 1st prebasic molt is showing the last remnants of its red juvenal crown molting in to the red nuchal patch of an 'adult' male.


Thanks to Mark Habdas, Kerry Kenwood, Carol Matthews, Jason Jones, Marg Anderson, Jerry Rolls, Debbie Wheeler, Sarah Gray, Marianne Dawson, Mike Nutter, Marguerite and Chris Sans and Gabriel Jamie for their help with banding this month.


The two birds above in juvenal plumage are White-crowned Sparrow (left) and Savannah Sparrow (right).
- The Purple Finch wing belongs to a second year (SY) male Purple Finch undergoing its first definitive prebasic molt.

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