Possibly a Diver Gavia sp., Plongeon.
In American English, this genus which is also found in America, is still called Loon;
or the Great-Crested Grebe Podiceps cristatus, Grèbe huppé.
MAC 5 3 11 The devil damm thee black, thou cream-fac'd loon!
Sir Archibald GEIKIE, author of The Birds of Shakespeare gives a detailed account on the "loon" (p. 52):
The word "loon" or "lown" is employed by the Poet to denote a rogue or a low fellow. A messenger of evil tidings is called by Macbeth "a cream-faced loon". In the play of Pericles we hear of a company that would include "both lord and lown" [4 6 17] and in Othello Iago sings part of a north-county ballad in which the same word occurs [2 3 82].
The name of loon is a popular appellation which includes two distinct families of water-birds, all remarkable for their clumsy gait on land.
Whether this name was applied to them after it had first been in use as an uncomplimentary epithet for a man, or originally their own common designation which came eventually to acquire a human application, remains in doubt. More probably the bird was first owner, and the word may belong to the group of bird-names like goose, snipe, kite, hawk and others which had become disparaging epithet for human subjects.
The bird interpretation is, especially in Macbeth, reinforced by the presence of "goose" and "geese" in the following lines:
The devil damn thee black, thou cream-fac'd loon!
Where gott'st thou that goose look?
There is ten thousand -
(MAC 5 3 11-3)
Both grebes and divers share a black and white contrast, a contrast which is found in "thee black" and "cream fac'd". ARMSTRONG (p. 60) thinks that "the black and white imagery was almost certainly aroused by the thought of writing with a goose-quill pen on white paper."
The presence of birds in this context is not surprising as birds are often associated with omens: here with a messenger bringing news. The tell-tale bird is important for Shakespeare, especially in the Macbeth context where there already was:
Augures, and understood relations, have
By magot-pies, and choughs, and rooks, brought forth
The secret'st man of blood.
(MAC 3 4 123-5)