The Prose Edda 1.16

Then said Gangleri: "What more mighty wonders are to be told of the Ash?" Hárr replied: "Much is to be told of it. An eagle sits in the limbs of the Ash, and he has understanding of many a thing; and between his eyes sits the hawk that is called Vedrfölnir. The squirrel called Ratatöskr runs up and down the length of the Ash, bearing envious words between the eagle and Nídhöggr; and four harts run in the limbs of the Ash and bite the leaves. They are called thus: Dáinn, Dvalinn, Duneyrr, Durathrór. Moreover, so many serpents are in Hvergelmir with Nídhöggr, that no tongue can tell them, as is here said:

Ash Yggdrasill suffers anguish, More than men know of: The stag bites above; on the side it rotteth, And Nídhöggr gnaws from below.

And it is further said:

More serpents lie under Yggdrasill's stock Than every unwise ape can think: Góinn and Móinn (they're Grafvitnir's sons), Grábakr and Grafvölludr; Ófnir and Sváfnir I think shall aye Tear the trunk's twigs.

It is further said that these Norns who dwell by the Well of Urdr take water of the well every day, and with it that clay which lies about the well, and sprinkle it over the Ash, to the end that its limbs shall not wither nor rot; for that water is so holy that all things which come there into the well become as white as the film which lies within the egg-shell,—as is here said:

I know an Ash standing called Yggdrasill, A high tree sprinkled with snow-white clay; Thence come the dews in the dale that fall— It stands ever green above Urdr's Well.

That dew which falls from it onto the earth is called by men honey-dew, and thereon are bees nourished. Two fowls are fed in Urdr's Well: they are called Swans, and from those fowls has come the race of birds which is so called."