The biting their shields part stuck out to me, so I double-checked, and believed you may have heard that from the ynglinga saga. Full text here:
THE YNGLINGA SAGA
Here's the quote in question describing berserker's from this saga:
"Odin could
make his enemies in battle blind, or deaf, or terror-struck, and
their weapons so blunt that they could no more but than a willow
wand; on the other hand, his men rushed forwards without armour,
were as mad as dogs or wolves, bit their shields, and were strong
as bears or wild bulls, and killed people at a blow, but neither
fire nor iron told upon themselves. These were called Berserker."