Young male Sparrowhawk Accipiter nisus, Epervier d'Europe.
WIV 3 3 19 p How now, my eyas-musket, what news with you?
The sparrowhawk must have been more numerous before systematic clearance began, but it prefers broken rather than continuous forest, tending to leave mature forest to one of its few predators, the Goshawk (see ESTRIDGE). Leslie BROWN, quoted by HARRISON in The History of The Birds of Britain (1988), suggests that the medieval population might have been between a quarter to a half a million adult sparrowhawks. In the mid-1980s it has been estimated at 50 000. The first figure claims no scientific value but gives an idea of what "nature" must have been like even a bit later in Shakespeare's time. Given the contrast between these figures, we can conclude that the sparrowhawk must have been common and known and seen by all.
Male sparrowhawks, which are a third smaller than females, were probably used by children or during early training sessions, since the sparrowhawk was not praised much for falconry as it is too small.
"Eyas" was a term of falconry used to designate the young and unfledged birds of prey still in their eyrie (or aery) and making a lot of noise. In Hamlet, when talking about the troupe of young actors, Rosencrantz says:
there is, sir, an eyrie of children, little eyases, that
cry out on the top of question, and are most tyran-
nically clapped for't. These are now the fashion, and
so berattle the common stages
(2 2 336-40)
All the details are here: the eyrie, the noise of the young birds, etc
This quotation became famous as this "striking topical allusion would seem to place Hamlet firmly in 1601. [...it] is universally recognized as an account of the boy actors who from Michaelmas 1600 were established at the Blackfriars" (Harold JENKINS, editor of the Arden Hamlet in the introduction, p. 1).