The Destructiometer is located in the top right corner. Fill it to get an extra bird in the current level. It can be filled by causing a lot of damage or by smashing a golden pig.
Gems
Watch advertising videos to double your gem rewards. The number of videos you can watch daily is limited. You may want to save some in the event that you fail a level. Complete the daily missions. They are usually very simple and give gems as rewards.
Power-ups
Power-ups are very useful, but have a limited amount. They cannot be used during Boss battles. Save them in later levels when they are really needed.
Restarts
Avoid restarting a level because of a bad start. You have a limited number of failures available. Instead, try to continue the level with your remaining bird cards.
Shooting birds
Before shooting, take note of any environmental objects that can help you, such as spitting plants and fans. After shooting a bird, wait until everything stops moving before shooting again. Sometimes an item will fall into place that will be useful for your next shot. If you pulled back with the wrong bird in the sling, bring it back to its resting position to avoid shooting it. Always determine a structure's weak point and take advantage of it.
Special abilities
The following birds have the listed special abilities.
Ability
How to unlock
Blues
Splits into three and passes through ice.
Bomb
Explodes upon request after a tap or after he lands in a certain area.
Chuck
Moves in a straight line that can remove wooden pieces.
Matilda
Tap to drop an egg bomb. After laying the egg, Matilda will get a flight boost.
Red
Can move strong stone blocks. Tap to have Red release a gust of energy that can break stone blocks.
Silver
Tap to start a loop that ends up downwards at the point where the loop started. She can take out blocks during the loops, and can reach out of place pigs hiding under blocks.
Terence
No special ability, but can that destroy anything in his way.
Special powers
Use a bird's special power at the correct moment. Try to plan your moves and activate special powers when they can result in maximum destruction.
Spells
Spells are occasionally given for free, but are normally obtained through an in-app purchase. New spells become available at levels 11, 12, 20, 31, and 61. Do not use spells until you go through a level at least once in order to learn which ones will be most useful. You can only use spells a limited number of times. They cannot be used during Boss battles.
Spell
How to unlock
Golden Duck
Causes Golden Ducks to rain down. They are heavy and will fill the entire level.
Frost
Freezes all blocks into ice. Consider using this when you have at least one Blues card remaining and have to deal with difficult stone blocks.
Hot Chili
Turns a random pig into a bomb. Consider using this on a single remaining difficult pig, or when all the pigs are grouped together.
Destructiometer...