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Bird dialects and migration (long!)

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Subject: Bird dialects and migration (long!)
From: "Dean Portelli" <>
Date: Thu, 08 May 2003 15:23:09 +1000
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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