Answer:

nesting

Answer	Crossword 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.
NESTING	Avian activity.
NESTING	Fitting snugly.
NESTING	Fledgling's delight
NESTING	At home, as a hoopoe
NESTING	Nidification
NESTING	Avine activity

NESTING	Fitting one inside the other
NESTING	Laying laying foundations
NESTING	Like some birds or dolls