Sigma now makes some lenses that are unequaled in performance. Their 35mm f1.4 is not to be beaten. Their 17-55mm f2.8 is an excellent performer. The 18-35mm f1.8 is an outstanding performer. Fortunately for us, Sigma has gotten serious about higher end lenses in the past few years.
In the Canon world, for wildlife, the best two long lenses I have found are by Canon: 55-250mm IS STM (amazing performance for money at $125 used), and the EF 100-400 IS II USM at around $1500. Both lenses produce images at the edge of the resolution of a 24 Mpixel APS-C sensor.
Focal length doublers cost you 2 f stops and give marginal results on full frame cameras and poor results on APS-C cameras.
If you're thinking super zoom out to 600, listen carefully to Frost's review.
For around $350, you can get a T6i body, and for another $125 you can get the 55-250. That is a very respectable starting point.
Add in the Neewer fiberglass tripod and you're on the air. Once you get a taste of it, plan on hocking a couple of nice rifles to get additional lenses.
