website gives information about indicator species.
Of course the classic indicator, as recorded in the letters pages of The Times, is the first cuckoo of the year. I like this from 1951:
"I think we can claim to have heard the first cuckoo of spring." A bold claim this, for the date of hearing was March 29 (at about 10 minutes to 7am) and there are not a few naturalists, in ordinary circumstances kindly and tolerant persons, to whom the mere suggestion that a cuckoo can be heard before April is as a red rag to a bull.
As per his recent entry, Duncan has heard one already in Somerset, which would indicate that spring down there is progressing apace. No cuckoos here, but the blossom is a delight!