http://www.youtube.com/watch?v=B_TxzMUKwyc&mode=related&search=
Please take a look at the video again. The object clearly comes from behind WTC 1 (North Tower) travels in a straight line and has a exhaust trail. Set the video on large screen and do moment by moment clicks and you will see this.
Okay, there you go. You're basing your approximate measurement of its speed ("too fast for a bird") on the assumption that it has passed behind the tower, which is what accounts for its sudden appearance to the right of the tower in the image.
That's a plausible (though not yet proven) assumption, so let's go with it. I agree with you that
if it did indeed emerge from behind the tower, it would have to be moving too quickly to be a flying bird.
However, I can also show that under the same assumption, the object is much too large to be an air-to-air missile. The height of each story of the north tower is between 2 and 3 pixels in the youTube rendering of the video (and much closer to 2 than to 3). We have to allow for some foreshortening because we're looking upward at the towers, but even if we allowed for an upward angle of 45 degrees (actually it's much less) and for an overestimated 3 pixels per tower story, our vertical scale at the minimum possible behind-the-tower distance is not more than 3 * sqrt(2), or about 4.25, pixels per story, or about 1.25 pixels per meter. The diamater of an air to air missile would be somewhere between 1/8 to less than 1/2 meter, so it should be less than a pixel wide in the image.
Blurring cannot account for it appearing as wide as it does. The camera is focused at a distance; under the assumption that the object has passed behind the tower, it cannot be out of focus. And the object's motion would not cause blurring in the direction perpendicular to the direction it's moving.
Furthermore, given that the true image size of a missile at that distance should be less than 1 pixel wide and 12 pixels long, no amount of blurring of any sort can account for it making a much larger smudge (bwtween 5x and 10x larger, linearly) while retaining that much contrast. A bright object against a darker background can (if it starts out intensely bright) can blur into a much larger image and still stand out, but a dark object cannot start out darker than black, and so must fade away as it gets blurred. I urge you to try this out with any kind of camera (motion or still, digital or film) at your disposal. Image a matte black object against a bright background, blur (not enlarge) the object using motion or focus or any other means or any combination to make its outline larger in your image by at least a factor of 5, and see how visible it remains.
Then there is the question of why the object appears black in the first place. Continuing to assume that the object appears from behind the tower, it is in direct sunlight (notice the glare on the nearest sides of the towers, the position of the impact fireball's shadow, and the brightness of the debris object moving right of the north tower about a second later, not as high up) so it would only appear black if it were painted black. I suppose that's possible, but I've never seen AAMs surfaced like that in any photo. There's also no reason for any exahust (if any part of the visible trail of the object were missile exhaust) to appear black, nor for it to disappear completely in the fractions of a second between video frames.
My conclusion from the video is that if the object did indeed pass behind the north tower, then it cannot be an air-to-air missile.
What is it, then? I don't know. I could give you my best guess, but guesses (that is, conjectures without conclusive confirming evidence) are pretty useless, aren't they?
Respectfully,
Myriad