What strength of zoom lens do I need to photograph birds?

You did not mention whether we are discussing a FILM camera....or a DIGITAL one. You also did not mention the DISTANCE from you to the subject birds. If they are right outside your window (say 10 feet), you don't need much. But if you want to photograph birds in nature from greater distances, you need much more zoom power. For good all-around performance in a FILM camera, I'd get AT LEAST a 35 - 200 zoom, which gives you a WIDE ANGLE (35mm) option in addition to just zooming in. More is better, so a 70 - 350 would zoom more. BUT, in the latter case, 70 mm is a slight zoom from the start, as roughly 50 mm is considered neutral distance in lenses. You can also add a lens doubler to your mix. Lens doublers reduce the available light so you have to consider how bright the subject will be...and whether you can use flash to compensate. But a lens doubler is relatively cheap and can change that 35 - 200 lens into a 70 - 400 lens in a hurry...or make that 70 - 350 lens a really bad boy at 140 - 700.

If you are talking DIGITAL cameras, then you really should insist on a 10X or higher OPTICAL ZOOM. Ignore the digital zoom specs on digital cameras. Digital zoom does enlarge the image...BUT...you lose photo quality. Digital zoom is really worthless, from a quality photo standpoint. That's why cheap digital cameras have ONLY digital zoom...NO optical zoom at all...or something cheap, like 3X optical. Avoid those cameras. Don't settle for less than 8X OPTICAL ZOOM...and try to get 10X or 12X if you can afford it.