100–400 or 150–600 telephoto
$$$$Birds and most wildlife are small in the frame until you have 400mm and up. A 150–600 f/5–6.3 is the honest enthusiast lens: enough reach, carryable, and sharp enough stopped a little. Super-tele primes are a different budget.
What to look for: At least 400mm on the long end, image stabilization, and a tripod foot. Weather sealing if you sit in marshes.
Skip it if: you mostly shoot mammals from a vehicle at 70–200mm. Rent a long lens for the birding trip instead of owning it.