I hope this hasn't been asked yet, but is there concensus on "trees down" or "ground up"?

The very short answer is no and no. It hasn't been asked before and I don't think there is a real consesnus on this yet (though others might disagree which would be quite ironic really). The problem is that there is plenty of evidence on both sides for the two primary models of bird flight (there are a couple of others like the 'pouncing pro-avis' model which I actually rather like) which either conflict or compliment each other, and certainly nothing definitive that everyone can flock to and try to support (or disprove). I think in time we will reach a consensus as more information comes in and more researhc is done, but I think you could probably find the best part of a hundred researchers to support either side if you tried. Most probably favour 'ground up' right now, but a true consensus? Not really.

The difficulty with this question is that it is probably unknowable. Every new fossil can swing the balance of the evidence either way.

Evolution works in a branching pattern, not a linear one - we're playing a game of join the dots, when most of the dots are missing. If evolution was a linear process it would be possible to draw a hypothetical line through those dots to get an approximation of the trajectory of evolution (which is more or less where hypothetical "transitional species" arise), but since the pattern of evolution is not linear, there is a strong risk of joining dots that result in spurious trajectory. However, evolutionary theory provides us with a means of filling gaps in our knowledge with a hypothesis that can be tested (even if it is prone to error). In 1915 Beebe postulated a tetrapteryx (four-winged) stage in the ancestry of birds, identifying it as a transition between an arboreal gliding form and a two-winged form with powered flight. Microraptor was later discovered, providing strong support for the trees-down hypothesis - except it was far too young to be an avian ancestor.

I'm not sure about the current state of the debates. The ground-up hypothesis was originally a tongue-in cheek suggestion that has gained credence. It is good science to consider alternative hypotheses and there is also a need to minimise unsupported assumptions. As it is, we can only look at the dots we have and hope they provide a good representation of avian ancestors from which to reconstruct the evolution of bird flight. We can also extrapolate ancestors that can be tested by comparison with new fossil finds. Common sense, aerodynamics and energetic hypotheses are rather redundant as evidence, since they demonstrate what could happen rather than what did, although they at least provide an idea of what might be required from a hypothetical ancestor. Coming from an aerodynamics and energetics background I tend to favour a trees-down hypothesis.

Well put, Paolo. Evolution is neither linear nor is it progressive...

Just to add a bit to this, there is the added confusion that these arguments have also been intimately linked with evolutionary hypotheses regarding bird origins. The 'trees-down' proponents have historically been closely associated with opponents of the 'dinosaur origin hypothesis'. Thus, 'grounds-up' proponents used to clash with 'trees-down' proponents not just on the discussions on acquisition of characters and aerodynamics and so forth but crucially on the fundamental starting point of what the ancestral bird looked like. So you could have been arguing for a trees-down approach based on an arboreal 'thecodont' ancestor, or you could have been arguing for a trees-down approach based on an arboreal dinosaurian ancestor (albeit not many people argued this in the past?), or conversely, you could have been arguing for a 'ground-up' approach based on a cursorial dinosaurian ancestor running around flapping its arms until it started flying. Relatively more recently, this issue became a little bit more simple in that 'trees-down' arguments have been focused primarily on a Mircroraptor-like dinosaurian ancestor of birds, thus limiting the argument to dinosaurian ancestors either arboreal or cursorial.

On the other hand, despite it's basal position within the Dromaeosauridae, Microraptor is still nested within the Deinonychosauria, a clade comprised of Troodontidae and Dromaeosauridae. According to most recent studies birds comprise a clade that is in a sister-relation with Deinonychosauria; both the bird group and troodontids+dromaeosaurs group share a common ancestor. So if you have Microraptor as a member of the Dromaeosauridae, which itself is a member of a bigger group Deinonychosauria, then I don't think it is strictly a good candidate for a 'missing-link' or a 'transitional form'. We don't know what the primitive condition of the last common ancestor between troodontids and dromaeosaurs was, neither do we know what the primitive condition of the last common ancestor between birds and troodontids+dromaeosaurs was. In a way, Microraptor is just one dinosaur with an interesting characteristic, that may or may not have retained a primitive condition from the last common ancestor between it and birds. But there are also other troodontids and dromaeosaurs in similar positions.

As Paolo has so eloquently described, evolution is not a linear process and you can't just join the dots directly. If you have two species that are clearly closely related, then you'd need to figure out what its common ancestor would have looked like, thus introducing a third species. If you join the dots now, it wouldn't be a single line connecting the two species but instead, the two species connected indirectly with two lines diverging away from their last common ancestor.

So, even though Microraptor provides strong support for a possible arboreal ancestry of birds, it is far from proving it.

BTW, I like the 'trees-down' hypothesis more than the 'ground-up', but I just thought it is necessary to bring up the question, 'Is it reasonable to assume the Microraptor-like condition to be the primitive condition in the ancestral bird?'

As an indication of how very much there is no consensus, at the meeting for the society of vertebrate palaeontology this year, I attended a session in which a talk arguing that it was almost impossible flight evolved trees-down was directly followed by a talk arguing the opposite.  Both were good talks with good points.

Personally, I always thought Dial's incline running gave strong support to ground-up.  But there are more than enough good arguments to counter that.

Playing devil's advocate, what does it matter? There are more than enough unknown things/areas we can study and hopefully come up with sensible if not definitive answers, as compared to those where we have no way of ever retrospectively proving a yes or no (in the absence of a time machine).

At least part of the answer to that lies i think in the enormous long term interest humans have had in flight, and a similar level of interest in birds as objects of beauty. At a more biological level, birds are incredibly important research model (huge amounts of ecological and behavioural word is doen on birds) and modern birds (even the flightless ones) are largely defined as having sprung from a flying ancestor and it was flight that triggered the radiation of birds. If we can work out how and why they took to the skies, it provides an evolutionary basis for working out much of how they have evolved. Flight is such an amazing thing to have evolved and is a complete game changer for any clade that manages it, so it is fascianting to see how such an enormous evolutionary shift occured.

Many thanks Dave that makes perfect sense and I have learned something new!