I use a Nikon Scoutmaster 2 zoom. I bought it at least 7 years ago . It is 7-15 X 35 zoom. I love the zoom function and it has very sharp optics. All in all it has been wonderful. Out in the field I noticed several other bird watchers were using super zoom lenses on their cameras to view the action. Hmmm...
I am partial to Nikon and the least expensive Nikon I found now, just looking for the heck of it on LLBean-- was $130.00. If I were to buy a new one I probably would go for the Trailblazer model, but it is another $30. There are some other less expensive brands, but make sure, as a minimum factor, it has weatherproof properties.