A brief history on the link between birds and dinosaurs...
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In a nutshell - the relationship between birds and dinosaurs: 

Are birds actually living dinosaurs?

Most workers today say yes (and is the popular view put forward in movies and documentaries in the last few decades), though some workers still disagree. Those that work in the field and dino-enthusiasts (believe me there's a few of those) will be familiar with this situation, so the following is a basic unbiased explanation for those who are unfamiliar.

As far as I am aware all workers agree that dinosaurs, pterosaurs (flying reptiles often referred to as 'pterodactyls' in popular media) crocodilians and birds are grouped as archosaurs, which sets them aside from all other reptiles (e.g. turtles, lizards, snakes, extinct marine reptiles etc.). However, there has been some controversy on the nature of the relationships between the archosaurs, specifically on where birds fit in. For the major part of the 20th century most workers regarded birds and dinosaurs were 'cousins', in that both were archosaurs, but evolved separately from common ancestral stock. Other workers suggested that the relationship was closer than that: birds evolved directly from dinosaur ancestors, but this was an unpopular view at that time.

This latter idea was revived with the discoveries of the small predatory dinosaurs in the 1960's now known as 'maniraptorans' or 'dromaeosaurs' (depending on how specific you want to be). Non-specialists these days will probably know these by their nickname 'raptor', given to them by Michael Crichton for 'Jurassic Park' (NB: prior to JP the term 'raptor' was used by ornithologists for centuries to denote living predatory birds such as eagles and hawks etc., but that's another matter...). In the movie these are the smallish dinosaurs that chase the main characters through the computer labs etc.

One of these was Deinonychus, which was described by John Ostrom in 1969. He recognised that to make use of the large sickle like claw on the second toe, these small dinosaurs must have been nimble and had a good sense of balance. Ostrom also had the luck of discovering a specimen of Archaeopteryx (often nicknamed 'first bird') in an old fossil collection and struck by the many anatomical similarities between it and Deinonychus. This pretty much kick started what has been called the 'dinosaur renaissance'; which essentially involved a reassessment of dinosaur physiology - Ostrom's student Robert Bakker being a notable proponent. This brought into question the previous tendency to view dinosaurs as slow lumbering giants, with sluggish 'cold-blooded' physiologies and that they had been superseded by 'better' 'warm-blooded' birds and mammals. During the 1970's and 1980's many ideas started oming from the other extreme suggested that dinosaurs were active, smart, and 'warm/hot-blooded'. Although not all these ideas have been excepted in their original form, they have reshaped the modern view of what a dinosaur is. Compare illustrations between dinosaur books of the 1950's and 60's to those of the late 80's and 90's and you'll get the picture.

With dinosaurs reconceptualised as successful active animals, and the increasing number of recognised anatomical similarities between them and birds (e.g. the bones that evolved into the wishbones of birds), the debate of bird origins (was the ancestors dinosaurs or some other archosaurian group?) and the related debate the origins of bird flight (i.e. how did flight evolve? Did it originate from leaping in tree dwelling ancestors, or from something fast running ground predators were doing with their forelimbs?) heated up during the 1980's.

By the 1990s a considerable number of dinosaur specialists had accepted the view that birds had evolved directly from dinosaurs. Most of the opponents were non-dinosaur specialists, including quite a few bird/fossil-bird specialists. 

Something else interesting had happened during this period, a new way of mathematically working out the relationships between species had arrived in dinosaur research. It's called 'cladistic phylogenetics', and requires the researcher to put a number to each anatomical feature they could see (like a rating or a score: 0 or 1 for present/absent in some cases; or a range of numbers for cases when differing variation is studied). Once all the features have been scored for all the species the researcher is looking at,  a computer program then works out the simplest 'family tree' that fits all this data. This method was another hotly debated point, many in this field disliked the new method and there have been serious critiques. It is now accepted by most, though there are still those that point out it's not infallible, but if this is kept in mind and applied carefully it can be a very useful tool.

Interestingly, the new 'family trees' generated by this method have favoured the direct links between dinosaurs and birds.

to be continued...