Merging Birds

In the last several years I have become a really big fan of singing mice.  It seems that unbeknownst to us, these white little fur balls have been plunging from aria to aria while gorging on food pellets and simultaneously training their ever-vigilant grad student minders to react appropriately whenever they pressed a bar.  Their songs sound birdish though at a higher pitch. Now it seems that many kinds of mice sing, not only those complaining of incarceration. I was delighted and amazed (though as my daughter pointed out, we’ve known since the first Feival film that mice are great singers). 

I don’t know how extensively rodent operettas have been studied, but recently there has been a lot of research on the structure of bird song and interesting speculation about what it may tell us about the species specificity of the kind of hierarchical recursion we find in natural language (NL). Berwick, Beckers, Okanoya and Bolhuis (BBOB; hmm, kind of a stuttering version of Berwick’s first name) provide an extensive linguist friendly review of the relevant literature which I recommend to the ornithophile with interests in UG. 

BBOB’s review is especially relevant to anyone interested in the evolution of the faculty of language (FL) (ahem, I’m talking to all you minimalists out there!). They note “many striking parallels between speech and vocal production and learning in birds and humans” but also note qualitative differences “when one compares language syntax and birdsong more generally (5/1).” The value of the review, however, is not in these broad conclusions but in the detailed comparisons between phonological vs syntactic vs birdsong structure that it outlines. In particular, both birdsong and the human sound system display precedence based dependencies (1^st order markov), adjacency-based dependencies, some (limited) non-adjacent dependencies, and the grouping of elements into “chunks” (“phrases,” “syllables”).  In effect, birdsongs seem restricted to linear precedence relations alone, just what Heinz and Idsardi propose suffices to represent the essentials of the human sound system. Importantly, there is no evidence that birdsong allows for the kind of hierarchical recursion that is typical of syntactic structures:

Birdsong does not admit such extended self-nested structures, even in the nightingale song chunks are not contained within other song chunks, or song packets within other song packets or contexts within contexts (5/6) (my emphasis).

Nor do they provide any evidence for unbounded dependencies, unboundedly hierarchical asymmetric “phrases,” or displacement relations (aka movement), all characteristic features of NLs.

The BBOB paper also contains an interesting comparison of songbird and human brains remarking on various possible shared vocalization homologies in human and bird brain architecture. Even FoxP2, (that ubiquitous rascal) makes a cameo appearance, with BBOB briefly reviewing the current speculations concerning how “this system may be part of a “molecular toolkit that is essential for sensory-guided motor learning” in the relevant regions of songbirds and humans (5/9).”

All in all then I found this a very useful guide to the current state of the art, especially for those with minimalist interests.

Why minimalists in particular? Because it has possible bearing on a currently active speculation regarding the species specificity and domain specificity of Merge.  Merge, recall, is the minimalist replacement for phrase structure rules (and movement). It’s the operation responsible both for unbounded hierarchical embedding and displacement.  So if birdsong displays context free patterns one source for this could be the presence of Merge as a basic operation in the songbird brain. BBOB carefully review the evidence that birdsong patterns exceed the descriptive power of finite transition networks and demand the resources of context free grammars. They conclude that there is currently “no compelling evidence” that they do (5/14). Furthermore, BBOB note that there is no evidence for displacement-like operations in birdsong, the second product of a merge-like operation. Thus, at this time, NLs alone provide clear evidence of context free and displacement structures. So, if Merge is the operation that generates such structures, there is currently no evidence that Merge has arisen in any species other than humans or in any domain other than syntax.

Why is this important for minimalists? The minimalist Genesis story goes as follows: Some “miracle” occurred in the last 100,000 years that allowed for NLs to arise in humans. Following Chomsky, let’s call this miracle “Merge.” By hypothesis, Merge is a very “simple” addition to the cognitive repertoire. Conceptually, there are (at least) two ways it might have been added: (i) Merge is a linguistically specific miracle or (ii) it is a more general cognitive one. If (ii), then we might expect Merge to have arisen before in other species and to be expressed in other cognitive domains, e.g. birdsong.  This is where BBOB’s conclusions are important for they indicate that there is currently no evidence in birdsong for the kind of structures (i.e. ones displaying unbounded nested dependencies and displacement) Merge would generate. Thus, at present, the only cognitive products of Merge we have found occur in species that have NLs, i.e. us.

Moreover, as BBOB emphasize the impact of Merge is only visible in a subpart of our linguistic products. It is a property of syntactic structures not phonological ones.  Indeed, as BBOB show, human sound systems and birdsong systems look very similar.  This suggests that Miracle Merge is quite a picky operation, exercising its powers in just a restricted part of FL (widely construed).  So not only is Merge not cognitively general, it’s not even linguistically general. Its signature properties are restricted to syntactic structures.

If this is correct, then it suggests (to me at least) that Merge is a linguistically local miracle and so proprietary to FL and so part of UG. This, I believe, comports more with Chomsky’s earlier conception of Merge, than his current one.  The former sees the capacity to build bigger and bigger hierarchically embedded structures (and movement) as resting on being able to spread “edge features” (EF) from lexical items to the complexes of lexical items that Merge forms.  So given two lexical items (LI) (each with an inherent EF), a complex inherits an EF (presumably from one of its participants) and this inherited EF is what licenses the further merging of the created complex with other EF bearing elements (LIs and earlier products of Merge). Inherited EFs then are essentially the products of labeling (full disclosure: I confess to liking this idea as I outlined/adopted a version of it here (Btw, it makes a wonderful stocking stuffer so buy early buy often!) and labeling is the miracle primarily responsible for the e(/I)mergence (like that?) of both phrase structure and displacement.

Chomsky’s more current view seems to be that labeling (and so EFs) are dispensable and that Merge alone is the source of phrase structure and movement. There is no need for EFs as Merge is defined as being able to apply to any cognitive objects at all, primitive or constructed.  In particular, both lexical items and complexes of lexical items formed by prior applications of Merge are in the domain of Merge. EFs are unnecessary and so, with a hat tip to Ockham, should be dispensed with. 

And this brings us back to birds, their songs and their brains.  It would have been a powerful piece of evidence in favor of this latter conception were a signature of merge attested in the cognitive products of some other species for it would have been evidence that the operation isn’t FL/UG peculiar.  Birdsong was a plausible place to look and it appears that it isn’t there.  BBOB’s review locates the effects of Merge exclusively to the syntax of NL.  Were Merge more domain general and less species specific we might have expected other dogs to bark (or sing more complex songs). And though absence of evidence should not be mistaken for evidence of absence, at least right now, it looks like Merge is very domain specific, something more compatible with Chomsky’s first version of Merge than his second.