Birge works for Khaleth's band of mercenaries and is commanded by Lokath. The band is being paid by locals in Imazi to capture Lord Garson and escort him off his lands, but Gerson has defended his estate by draining the moat, withdrawing the bridge, and dropping the gate, and is holed up inside with his own fighting force. Scouts have seen a TrKaa fly southwest from the manor, evading attempts to shoot it down, doubtless to request reinforcements from Gerson's supporters.
An attempted tunnel under the moat collapsed, and the mercenaries don't have the materials for a bridge. Hiring more TrKaa to carry them across would be both expensive and impractical, and despite local support, they are not equipped to be caught in the middle between Garson's guards and additional troops.
The party can find Birge at Garson's front gate in Imazi, next to the failed tunnel. Birge reports that the band is down to half rations and asks to see what the party brought. Upon receiving the Sealed Note from Lokath, he explains the situation; though the additional rations would carry them through a siege, Garson's expected reinforcements would break it.
When the party returns after talking to Chee, Birge tells them the Montari digging crew has arrived and his band is ready to attack as soon as the tunnel is finished. Instead of keeping "men of your caliber here to mop up," he sends them back to Lokath with another Sealed Note reporting their involvement, sure that Lokath will find another task suitable to their talents.