How to trace the path of an object in the video editor? Suppose I have a movie showing an object (such as a bird) moving around.  I would like to use the video editor to add a line behind the object showing the trajectory.  Does anyone have suggestions on how to accomplish this?  I've tried using the UV/Image editor to add a dynamically changing mask but am unable to figure out how to apply a color to this dynamic mask.  Perhaps this is the wrong approach?  Any help is appreciated, thank you!
Combining colored elements is easiest in the compositor.

I would also suggest using 3D to create the colored line as Blender has good visual tools in 3D. Just use a Shadeless or emitting material to make it monochromatic.

Then mask out the part of the bird, which shouldn't be covered by the line.



Render it with transparency and use a mix node in the compositor. Check the invert button in the mask options (in the image editor) to invert the mask.