When do blackbirds build their nest?
Your answer relies on asking WHICH birds nest WHERE: Some birds
nest out in the open, right on the ground, such as ducks, gooneys
and penguins. Some nest on desolate islands, such as the albatross
and the blue-footed booby. Some nest INSIDE of trees, such as the
woodpecker, and some nest HIGH in the trees, such as falcons,
hawks, and eagles.
The rainforest (like the Amazon) has three layers of canopy in
which various birds nest.
Ptarmigans nest right on the tundra. You could step on them if
they didn't jump up and fly away right before you do so.