Hi All (esp Penn Gwynne),
Keith Brandwood made earlier comment regarding bird dialects in commenting
on Val Curtis' comments on geographical variation in Pied Currawong song.
Firstly, the definition of dialects can be somewhat confusing as the term
has been used to describe different forms of variation in vocalisations. One
definition is that dialects are "population specific features in acquired
vocal patterns" (Mundinger 1982, pp. 147-208 in Acoustic Communication in
Birds Vol 2). The use of the term "population" in this definition further
complicates the issue as population can be a difficult term to define in
this context. It may not be surprising that there is much confusion and
controversy over the definition of dialects. Variation in vocalisations when
viewed across all different species probably represents a gradient (not
necessarily a smooth one!), and the question becomes where do you place a
line along this gradient that delineates what is and what is not regarded as
a dialect. At one end of this gradient variation in vocalisations can be
across large geographical areas where individuals within a geographical area
sound remarkably similar but dramatically different from individuals of
another geographical area (i.e. variation within < variation between). At
the other end of the gradient local variation may be greater than any
variation over a large geographical area; for example within a population
there is a large amount of variation in the structure of a particular
vocalisation (e.g. advertisement song) and when you take an individual from
that population and analyse its songs you may not be able to identify which
geographic locality it came from as the variation within a locality 'swamps'
the variation that exists between localities (i.e. variation within = or >
variation between). I think that the use of the term dialect is least
confusing at the first end of the gradient that is described above (i.e.
where variation within < variation between).
Now, moving on (phew!). Keith asked if dialects would show up on a
spectrogram (aka sonagram) - the answer is generally yes. If there is
sufficient variation in the structure of the vocalisation between sites it
will show up on a sonagram as a different looking picture of lines,
squiggles, blurs etc that represent the particular sounds that comprise the
vocalisation in a frequency vs time display (with relative amplitude
indicated within the display). In fact, this is usually how the possible
existence of dialects is investigated.
Keith also suggested the possibility of using dialects to track migration
and suggested that this may be much easier than banding. I agree that
recording songs would be much much easier than the many hours that go into
erecting nets in the hope of catching an already banded bird (hopefully from
somewhere else other than the banding site!!!). However, what should be
realised from the discussion above is that variation in vocalisations can be
very complex, and may not be as simple as different geographic areas having
a distinct vocalisation 'signature'. To attempt this with a migratory
species much information on the vocal behaviour of the species must first be
known, I will take the Yellow-Faced Honeyeater Keith mentioned as an
example. Obviously the first question is to examine the pattern of variation
in song across the range of the YFHE when it is not migrating so we can
identify the 'source' of migrating birds (one possible problem we will
encounter here is understanding what happens to the non-breeding YFHE, do
they always return to the breeding grounds or do they move elsewhere? - this
may be a complication), is there a distinct pattern of variation in
vocalisations that is associated with geographical variation? is this
variation great enough that we can confidently identify which locality any
individual chosen at random came from if we can record its vocalisations?
Another important aspect is an understanding of the ontogeny of the
variation in vocalisations within the species, i.e. when do YFHE's acquire
their song? is it within the first few months of life at the nesting site?,
or is it during subsequent migration?, or at the non-breeding grounds?, or
is the YFHE an open-ended learning species that retains the capacity to
modify the structure of it's song throughout life? (as opposed to species
with a restricted time window in which the structure of vocalisations is
learnt and then crystallised for the rest of the individuals life, accepting
possibly some slight modification). For example, it is known that for some
species when birds move populations they adopt the distinct songs of the new
population and drop their old songs, so if you were using vocalisations to
monitor movements in these species you wouldn't detect the movement!! While
this particular scenario may not apply to YFHE a similarly confounding
scenario may. This last question basically addresses where the song came
from and how constant are songs across an individual's life. If the YFHE can
modify its song through life it may complicate our interpretation of results
of possible geographical variation in song when trying to establish
population movement (i.e. migration patterns). In addition, we need to
examine the singing behaviour of the species at different times of the year.
Do YFHE's sing the same song they do at their breeding grounds when they are
on migration or at their non-breeding grounds? Or do they have different
songs (if they sing throughout the year!) used at different times of the
year? Undoubtedly there are many other questions of importance, but I think
it is clear that there must be a strong knowledge base of the vocal
behaviour of the species in question before there is any real possibility of
using vocalisations to track movements. Perhaps, not much easier than
setting up nets to catch birds!?!? It certainly would make an interesting
long-term study! One that may even involve banding birds for individual
identification.
One last point: Penn Gwynne wrote "Keith, I'm worried about you my good
friend? ask a bander to think? ". Well, Penn I myself am a bander. And I ask
you this question: do you think the above discussion required any thought on
my part?
Cheers, Dean
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