Best lens for bird photography on a budget?

Here is what the site says about that.

From Your first camera: How to split the budget: Spend roughly as much on lenses as on the body, or more. A modest body with a great lens takes better pictures than a flagship body with a cheap zoom. A sensible first kit: 1. The body with its kit zoom, for everything. 2. A cheap fast prime, a 50mm f/1.8 or its APS-C equivalent, for people and low light. See lenses. 3. A spare battery and a decent memory ...

From Sensor sizes: Is full frame worth it?: For low light, events, and portraits with very shallow depth of field, yes. For travel, street, wildlife on a budget, and most hobby photography, a good APS-C camera with good lenses is enough, and you'll carry it more. Lenses and light matter more than the last stop of sensor performance.

Quoted from this site's pages.