Vancouver Avian Research Centre

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Our winter banding program at Burnaby Lake is very different from the banding we do the rest of the year at Colony Farm. For a start we only run 3 nets and 2 ground traps next to our feeder station and diversity is reduced to our few resident species who tough out Vancouver winters rather than head south with Neotropical migrants.

A bird we get to know intimately in the winter is the Black-capped Chickadee – everyone’s favourite bird from a ‘cuteness’ factor point of view but in many ways the bander’s nightmare!

Chickadees are notoriously difficult to extract from mist nets. This is one of the reasons we use the winter banding program for one-on-one net extraction training – if you can extract Chickadees you can extract just about anything!

 

But it’s not just their tendency to get tangled and grip the netting tightly with their strong feet while all the time biting the extractor that makes these birds the bander's nightmare - once they are extracted they present another challenge in determining both age and sex.
 

Unlike birds such as Spotted Towhees which are sexually dimorphic (meaning we can separate males from females by plumage differences) Chickadees are monomorphic meaning males and females look identical. It’s only when they develop different breeding characteristics during the breeding season that they can be separated  So during the winter months Chickadees are all sexed as 'sex unknown'.
 

The next challenge comes with ageing Chickadees. They are among the most difficult species to age because there is frequently a pseudo molt limit among the outer greater coverts. ( see below).

Chickadees, like many NA passerines, have a partial first prebasic molt. This means that juveniles replace only body and some contour feathers - unlike adults which replace all body and flight feathers in their annual prebasic molt. (The prebasic molt occurs after the breeding season each year.)

In Chickadees this first prebasic molt normally includes a number but not all greater coverts allowing us to differentiate between retained and replaced feathers and creating a molt limit. So if we catch a Chickadee now that  is showing this 'molt limit' we know that its last prebasic molt ( i.e. the one that occurred after the breeding season in 2010) was a partial molt - meaning it was the bird's first prebasic molt so it must have been born in 2010.  So today it is in it's second calendar year and is classified as a second year (or SY) bird.
 

The photo on the right shows this  molt limit within the greater coverts. We must be careful though when looking for this molt limit not to be misled by the pseudo limit mentioned above which is the result of a natural colour contrast within the inner greater coverts and can simulate a molt limit but is NOT the result of feather replacement. However, in the case of this bird, the molt limit among the outer greater coverts is real with the two outermost feathers being retained juvenile feathers and the inner four visible feathers being replaced feathers.

While the colour does fade a bit distally (outwards) on the wing, the outer two greater coverts, and for that matter the retained carpal, aula, and primary coverts, are identified as retained feathers because of their much more washed out, worn, and lightly pigmented appearance.  Furthermore, retained feathers on members of the tit family (Paridae) will sometimes be longer than the replaced ones - the step down (or out) visible in this photo.
 

Contrast the above photo with the one on the right which shows an adult bird. Here there is no difference in shape, wear, sheen, gloss or length between the greater coverts, all of these feathers having been replaced in the bird's 'complete' adult prebasic molt last year.

If we catch this bird today it means it must have been born in 2009 or earlier and thus allows us to age it as an after second year bird (or ASY) i.e. a bird which is alive in at least it's third calendar year.

A complete understanding of the timing, sequence and extent of molt is essential to accurate ageing and sexing of birds in the hand and is the focus of our banding studies in determining population dynamics.
 

Tail shape is another helpful clue to ageing birds in the hand. It is important not to use tail shape alone as it is generally not very reliable because of a) individual variation and b) the possibility of accidental loss and replacement. (This is called adventitious molt and is another way in which Chickadees confuse as they often lose and replace tail feathers!)

The retrices on the right show our second year bird. They are more tapered (pointed),  have little or no white edging on the outer retrices and the white doesn't wrap around on to the inner web of those feathers. Notice also the amount of wear on these feathers - retained juvenile feathers which are poor quality and weaker structured are more prone to wear than adult feathers.

The retrices in this photo show a quintessential adult Black-capped Chickadee tail - notice the shape of the retrices are broader and more truncate than the juvenile retrices of our second year bird above.

In addition the extensive white edging on the outer web of the outer retrices wraps around on to the inner web.

Unfortunately, not many Chickadee tails look like this providing such a definite clue to age. Often there are mixed feathers to confuse even more although retained juvenile feathers always equals a hatch year (HY) / second year (SY) bird.

An example of mixed retrices can be seen in this final photograph.

The eight rectrices on the left side of the photo (the bird's right rectrices 1-6, numbered from the center, and left rectrices 1 & 2) are adult-like replaced rectrices. The four on the right (the bird's left rectrices 3-6) are retained juvenile feathers.  Note again the more worn and pointed appearance of the juvenile rectrices, as well as the much more extensive white edging on the adult-like replaced feathers. 

The very fact that tail feathers are frequently lost and replaced between normal molts is one of the reasons why tail feather shape must be used with great caution by banders, and only with other supporting characters (e.g., molt limits), when determining the ages of birds in hand.


So there you have it - Ageing Black-capped Chickadees 101! Even then sometimes we get stumped and simply age the bird unknown although at this time of the year we can always age them as after hatch year (or AHY) on the basis they must have been born last year or earlier.
 
They are of course among our favourite birds in the hand. Who could not be charmed by their inquisitive nature and feisty attitude and they prove that even among our most common birds there is always something new to learn!

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