On that topic, why does it make sense to kill a lot of birds to get eggs back?
Same reason that people send soldiers to die for the freedom of millions, I suppose.
Ignoring the first game, most of the franchise seems to imply that there is one of each type of bird (Blue aside), and that whenever you use a bird more than once per round, it's one of the previous birds coming back for more. Angry Birds 2 seems to go by this theory, since you always get each bird once per level, and any bonus birds you get are not of a type still in the queue.
The pigs stole your eggs. So you use white birds to drop more eggs on them?!
Unfertilised eggs. Think about what kind of juice the birds need to be hopped up on to do the kind of things they do, then imagine using that for bombs.
That leads to: Why aren't the birds selling unfertilised eggs to the pigs?
It's Personal. The way I see it, the pigs stole the birds' eggs once (then moved them around a bunch of times to let the game have more chapters), and ever since the birds have been unwilling to "give" the pigs their unfertilized eggs (in exchange for... money, or something) and the pigs have kept stealing their eggs back as vengeance for all the destruction the birds wrought and pigs they killed.
Egg Bombs. That's their name from the wiki, and I swear it was officially mentioned at least once before.
Why do the birds need slingshots when they're birds. They can fly. They don't look like no flightless birds to me!
For the momentum.
Also, do you see wings on them? They're probably FLIGHTLESS birds.
Probably cook or eat them? Some of the official animations and ads show the pigs trying to cook the eggs.
On level 3-21 (the last level in poached eggs), you can see the eggs plain in sight. However, if you hit the eggs, they don't break. If the eggs don't break, then what are the birds so worried about? If they can't break the eggs, then the pigs probably can't either.
I sure hope the birdies inside them can.
Obviously the eggs can withstand the obstacles. And to answer your question, it could be possible that the birds weaken with age.
They hop around to walk. Also, they proabaly build them by putting the supplies on their heads and placing them in the designated areas.
In the Angry Birds Stella episode "The Prankster" why does Stella fall so easily for Poppy's feather prank? She was perfectly aware that she going to be her next victim, suspected something when Poppy invited her for a drink of water, but then blindly decided that there was no way it could be a prank.