Generally speaking, the word thanks is considered to be plural, so we would say "thanks are due," not "thanks is due".
That said, this Ngram provided shows a modest number of hits for thanks is due. So what's going on there?
Fact is, the word thanks is a little flexible, and can be used in various ways that would make it hard to provide a black-and-white answer to this question.
For example, if we put the indefinite article in front of thanks, we notice that "a thanks" is an informal way of saying "a thank-you", particularly when there is an adjective between them. So, neither of these would be considered incorrect:
A special thanks is due to FumbleFingers, who pointed out that Ngrams can be misleading.
A huge thanks is reserved for Linda and Penelope.
Moreover, the word thanks may be the object in a prepositional phrase, just before a verb. In those cases, the verb would agree with the subject of the sentence, not the object in the prepositional phrase. For example:
After all the work he has done for us, I think a token of thanks is due to Mr. Smith.
She worked her fingers to the bone; a word of thanks is due here.
A warm expression of thanks is due to those on both sides of the Atlantic.
(That last one is quoted from the book entitled Europe and America: The Prospects for Partnership by John Peterson, 1996)
I managed to find a few instances where neither one of these were true, like this one:
Thanks is due to my parents, Charles and Susan, for their encouragement, help, and support.
Personally, I think "thanks are due" would have read better here, but this author may have deliberately opted to treat the word thanks as a mass noun – which is probably justifiable. After all, it's not like I can give you "one thank" (or, if I'm especially grateful, "two thanks").
Best Answer
When each is used as a pronoun, it is always singular:
However, here, it looks like each is an adverb that modifies tell:
This makes it clear that each person tells a story. Without each, maybe they are telling one story as a group. Consider the difference between the two sentences:
In the first case, we get one song. In the second case, we get many songs (one song per student).
Some people might argue that this use of each is grammatically an adjective modifying they, indicating that each member of the subject ("they") should be considered separately. I'm not sure if an adjective can supply that kind of semantic meaning to a sentence, though. Consider the word together, when used an adverb:
It is very clear to me that together does not modify they as an adjective; it modifies sing as an adverb. It makes sense to me that the opposite sentence uses each an adverb as well. However, I might be wrong and welcome clarification from a more knowledgeable grammarian.