Sorry not to hijack the thread, just to clarify, are teleconverters the same as extension tubes?
Nope nothing like them at all. Though it is possible that you've confused the Teleconverter with the Canon Extender - the canon extender being their market name for teleconverters (and aside from canons own sites not a single person or shop calls them anything but teleconverters anyway).
Extension tubes reduce the minimum focusing distance of a lens and remove its ability to focus to infinity. The result is a lens that can focus closer and thus get a more magnified image of the subject, whilst also losing the ability to focus on things further off. The specific distances will depend on the lens used and the amount of extension tube added.
There are two primary uses for this macro and birding
The first is on short focal length lenses (eg 50mm f1.8) to get a macro lens out of the setup where focusing really close is the idea. This also extends to some longer lenses for close up macro work such as 70-200mm, 300mm and 400mm lenses primarily focused on capturing larger insects (typically dragonflies in filght).
The second is to allow long lenses like 500mm f4, 600mm f5.6 which have very long minimum focusing distances (several meters) to focus closer; thus aiding the photographer when working in a closer hide or when able to stalk closer to smaller subjects (typically small birds).
Teleconverters on the other hand are setups which use glass (extension tubes have no glass elements inside them) to extend the focal length of the lens. They are typically aimed at being used on the top end telephoto lenses in order to get more reach. This comes at a cost however:
1.4TC - one stop of aperture lost and a minor effect on image quality (sometimes hardly noticable
1.7TC - one and a half stops of aperture lost and a more noticable image quality effect (though from what I've read its still considered quite minor by many)
2*TC - two stops of aperture lost and a very noticeable effect on image quality (this teleconverter really only works on the best of lenses for practical good quality shooting).