Saying "Humans are mammals so birds are dinosaurs" shows a basic lack of understanding of taxonomy. Humans belong to the class Mammalia (all mammals). Dinosaurs belonged to the class Reptilia (all reptiles). Birds belong to a class of their own, Aves.
I am not disputing that birds evolved from dinosaurs, but that does not MAKE them dinosaurs - saying it does is essentially saying evolution never happened. Mammals evolved from reptiles too (though not dinosaurs, of course) - stating that birds are dinosaurs is like saying dogs are Dimetrodon. You could go one step further and say all vertebrates are fish, since we are all descended from them originally!
Also, to count as a bird it must have forelimbs longer than its hind limbs? By that definition ostriches, rheas, emus, kiwis, etc. are not birds!
Modern birds can trace their origins to theropods, a branch of mostly meat-eaters on the dinosaur family tree.
I have heard this before as well. This type of analysis of bone structures etc has now been overtaken the last five years with genetics. In fact few people now believe that life evolved as Darwin and his followers believed. Since it was discovered about five years ago that mechanisms in the genes prevent mutations from occurring, there is now no known mechanisms by which Darwin's idea of a tree of life could have developed whereby everything evolved from a simpler life form. All human female DNA has now also been traced back to a single individual. In the same way the only way of being able to confirm that birds are related to dinosaurs would be to trace markers in their DNA, tell tale unique indicators that confirm inheritance. This has never been done as far as I know. Not sure if you have any other information that could shed light on this.
Actually, in all due respect...Several legitimate cladistic analyses do not support the "birds are dinosaurs" notion (e.g., see Pourtless and James, 2009 https://www.bio.fsu.edu/James/Ornithological Monographs 2009.pdf). As I said before, this is sort of making a mountain out of a molehill. Birds are archosaurs, dinosaurs and archosaurs, but WHICH GROUP of archosaurs each descended from is the critical component. Both chronology and morphology are in direct disagreement on the theropodan status of birds (in my opinion). While these arguments may seem trivial to most casual observers, for those of us that work in this field they are key. I can list dozens of key characters that don't support the "birds are dinosaur" hypothesis...When you roll them up according to cladistics, of course birds *are* dinosaurs. However, when you look at their anatomy and physiology in detail, birds are most definitely...BIRDS. The bony modifications of the tail and wings along with the loss of teeth are obvious significant diffences. However THE major difference between birds and theropod dinosaurs - that nobody ever seems to mention in all these discussions of birds and dinosaurs - is the arrangement of the pubic bones: theropods had joined pubic bones that pointed away from the hip region, while birds have bones that do not join and instead are parallel to each other as well as parallel to the sacral vertebrae. This difference is MAJOR: the joined pubic bones of the theropods (including Archeopteryx) restricted the size of their eggs while the parallel pubic bones of birds allow for an egg which is much larger per body mass. That's why - for as huge as dinosaurs got - the biggest vertebrate egg known is from the Elephant Bird/"Aepyornis". That's also why it's impossible to genetically alter a chicken into a dinosaur: while turning on a "finger" or "tooth" gene may seem simple (it's not - but that's a different discussion), to alter a chicken's reproductive system so that it even remotely resembles a theropod's is the equivalent of undoing some 90+ million years of evolution (the estimated origin of the earliest Neornithines).
When I was in high school (BTW I'm 76) we had these disussions on dinosaurs. At that time, all birds were considered actual dinosaurs, not some mutated descendants of dinosaurs. The reason being, back then, was that modern birds hip structure is identical to the fossils of birds found in, I believe, in China and Germany!