鼠巣 is read as そそう (sosou) and means rat's nest.
そそう
sosou
ねず
mouse
す
(spider's) web
あきす
empty nest
ふるす
old haunts
らんそう
ovary
えいそう
building a nest
びょうそう
focus
げんぱつそう
primary lesion
てんいそう
metastatic lesion
ていりゅうせいそう
cryptorchidism
きすう
homing (instinct)
すだち
leaving the nest