A clip-on “zoom lens” for a tablet is unlikely to give satisfying results. Tablet cameras and add-on clip lenses generally don’t provide the reach or image quality needed for birdwatching, especially if ease of use is important.

A better low-cost option is a spotting scope on a small tripod. That gives much more real magnification than a clip-on tablet lens, and the tripod means it can stay aimed at the feeder without needing to be held. A tabletop setup by the window would likely be simpler and more comfortable.

Many inexpensive spotting scopes include a small tripod, and some also include a phone adapter. Even if the adapter isn’t useful with a Kindle Fire, the scope itself is still the main benefit.

So the simplest recommendation is: skip the Kindle clip-on lens and look for a basic spotting scope with a tabletop tripod for indoor birdwatching.