The Half-Hags are tiny, poisonous bird-like creatures that live in the feathers of hagsfiends. Their beaks are said to drip a kind of poison that hagsfiends themselves are inmune to. It is a poison that kills the mites that live in hagsfiends' feathers. The half-hags feed on these mites and are therefore depent on the hagsfiends for their sustance. Half-hags have also woeful eysight, some say it because they are part bat.
The half-hags can communicate with other half-hags through a strange language that only they understand.
In the books there are two missing phases:
Meebla yeben yip"-Ygrik to her half-hags, The Coming of Hoole. This appears to be a command to attack the enemy
"H'blen b'shrieek micht garnish schmoot" -Grank to Ygrik's half-hags in The Coming of Hoole. (Unknown what it means.).