The cuckoo. Logan, in his poem To the Cuckoo calls it the "messenger of Spring"; but the Welsh ambassador means that the bird announces the migration of Welsh labourers into England for summer employment.
"Why, thou rogue of universality, do I not know thee? This sound is like the cuckoo, the Welsh ambassador."- Dampet: A Trick to Catch the Old One. iv. 5. Source: Brewer's Dictionary.