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