The Sigma 150-600 seems very well thought of but, as with most 600mm lenses, it's a bit of a large and heavy thing. If working from a hide or other static location and you can walk there with your kit easily enough, then no problem. If mobility and weight is more of an issue, could you manage with a 100-400mm lens?
If so, Canon's 100-400 L IS Mk II is reputed to be a very good lens, but it's quite expensive, even on the used market. If the price puts you off then perhaps have a look at the Sigma 100-400 and the Tamron 100-400. If looking at the reviews, there's been a firmware update on the Sigma version since a lot of them were written. However, the Tamron has weatherproofing and the Sigma only has that round the lens mount. Hope this is useful.