Answer: nesting

AnswerCrossword Clue
NESTING

Fitting one within another

NESTING

Up a tree, perhaps

NESTING

Avian activity

NESTING
Building a tree house?
NESTING
Spring activity in the avian world.
NESTINGAvian activity.
NESTINGFitting snugly.
NESTINGFledgling's delight
NESTINGAt home, as a hoopoe
NESTINGNidification
NESTINGAvine activity
NESTINGFitting one inside the other
NESTINGLaying laying foundations
NESTINGLike some birds or dolls