iCloud & iCloud Photos · Troubleshooting iCloud space
What are bird and cloudd in Activity Monitor, and why are they busy?
Short answer: They're macOS's own iCloud sync processes. They get busy when there's lots to sync — after enabling iCloud Drive, adding many files or setting up a new Mac — and settle when it's done. If they stay busy for days, check iCloud isn't full and restart.
They're part of macOS itself: bird and cloudd are the background processes that sync iCloud Drive and other iCloud data. They get busy when there's a lot to sync — after turning on iCloud Drive or Desktop & Documents, adding many files, or signing in on a new Mac — and settle down once syncing finishes.
If they stay busy for days, check that iCloud isn't full, that you're signed in, and restart the Mac. Don't delete iCloud's system folders to stop them — see is it safe to delete Mobile Documents.