What does Tennyson mean by "by the bird's song ye may learn the nest"?

Question

In Tennyson's The Marriage of Geraint, the protagonist, Geraint, has just heard the song of Enid, the daughter of his host, Yniol. When Enid's song concludes, Yniol says the following to Geraint:

'Hark, by the bird's song ye may learn the nest,'
Said Yniol; 'enter quickly.' Entering then,...

What is meant by the expression, "by the bird's song ye may learn the nest"? Is the nest referring to Enid? This would be supported by the fact that Tennyson has just compared Enid to a bird. And yet, the phrase would be a strange way to describe her; one would rather say, "By the bird's song you may learn its beauty/form," not its nest/home.

On the other hand, if the phrase is supposed to describe the poor condition of Yniol's and Enid's home, then it seems strange that the subjunctive is used ("ye may learn"), rather than the indicative (e.g., "you will find [the condition of]").

In sum, what does Tennyson mean by this phrase?

Answer 1

Yniol has just invited Geraint to

                “Enter therefore and partake
The slender entertainment of a house
Once rich, now poor, but ever open-doored.”

and Geraint has waited in the courtyard of this half-ruined castle to listen to Enid, whose voice is described as “the sweet voice of a bird”. In this context, when Yniol says, “by the bird’s song ye may learn the nest”, he is using a metaphor where Enid is the bird and their house is the nest, and he means that if Geraint listens to the words of Enid’s song, he may learn about Yniol’s household. (The subjunctive implies the conditional here—if Geraint listens, he may learn.)

Enid’s song was about the wheel of fortune. This was a popular medieval image representing the capriciousness of fate: the goddess Fortuna spins her wheel at random, elevating the lowly and bringing low the high.

“Turn, Fortune, turn thy wheel with smile or frown;
With that wild wheel we go not up or down;
Our hoard is little, but our hearts are great.”

This corresponds to Yniol’s description “once rich, now poor” that I quoted above; and shortly we learn that Yniol has been ruined by his nephew Edyrn, nicknamed the “sparrow-hawk”, who

“Bribed with large promises the men who served
About my [Yniol’s] person, the more easily
Because my means were somewhat broken into
Through open doors and hospitality;
Raised my own town against me in the night
Before my Enid’s birthday, sacked my house;
From mine own earldom foully ousted me”