Only a handful of units can defend themselves against aerial attacks: archers, mages, destroyers and their orcish counterparts. Because destroyers are weak in sea battles, they are never built in large numbers; the same applies to archers and axethrowers. Especially orcish players tend to mass-produce ogres, neglecting axethrowers and sometimes even death knights! And on sea maps, birds can reach the targets far more quickly than land units who have to be loaded into a transport and unloaded at the target area.
That's where birds kick in - they can attack juggs and ogres safely. Even though the idea as such sounds appealing, these birds have major weaknesses:
Orcish players have a huge advantage due to haste and bloodlust: Haste makes the dragons attack twice as fast, and bloodlust doubles their power. In fact, you'll have a dragon that is four times more powerful afterwards! (At least in terms of damage; the dragon will still have the same amount of hit points.) By the way, many players think, quite mistakenly, that dragons become three times stronger with bloodlust (like ogres); obviously, they haven't done their homework. Check out the combat equation page to find out why.
In the rest of this section, a gryphon means a gryphon without spells on it and an "enchanted" dragon means a dragon powered with bloodlust and hasted.
There are three non-magical units that can defend themselves against birds:
Orcish players have bloodlust to power up dragons, but axethrowers kick but as well when bloodlusted. You will not have much of a chance against enchanted dragons with bloodlusted axethrowers only (unless you have a lot of them, at least twice as many as there are dragons). That is where death knights are needed: their spell death coil sucks 50 health points from any unit; a death knight with full mana can therefore kill a dragon from a save distance. Use them together with axethrowers: cast one death coil on each dragon and send an axethrower with bloodlust to finish the bird off. If you have any mana left, cast as many death coils as you can. If you are in trouble, cast bloodlust on your death knights as well and attack the dragons with your dks. Orcs have a better defense against the dragons than humans due to death coil.
Even though elven rangers are more powerful than axethrowers, they cannot be bloodlusted. Human players should use paladins with healing. A paladin with full mana can restore 42 hit points, doubling the use of your archers almost like bloodlust. However, you should also take advantage of your mages. Unless you have enough mana, slow is more powerful than polymorph. Polymorph prevents all damage and can be used without other attackers (like archers), so you should use it if possible. I use polymorph if few dragons attack me and slow, combined with archers and sometimes Blizzard, against a mass attack. Make sure you stop the blizzard early (three or four casts are enough for 100 damage), or your buildings will suffer, too. Of course any of your units under the Blizzard will die. So be careful when choosing the target area.
Until now, it seems that birds are pretty useless - you have a wide choice of means to stop them. In fact, on land maps you'll rarely encounter birds unless you have no air defense. On sea maps, aerial units have the distinct advantage that they can reach any spot on the map - unlike sea or land units - and they get anywhere more quickly than other units. This makes them a good support if you don't have enough transports to carry all enough units.
Control your birds well. Don't send more than four or five units to the same spot or they will kill each other. Always make sure that no bird is on the path of another one's attack line. Be aware that your birds react very slowly so if you have to take them out of the battle or move on, make it early enough or they will die. Evade the destroyers or your attack will be a short one. Good scouting will be rewarded.
Take out mages, archers, guard towers first! After this is done, head for the lumbermill(s) and mage towers/temples of the damned. If your attack is successful up to this point, your opponent will be without air defense. Of course you should also use your land units to destroy these key buildings.
Because birds cost so much gold, you must make sure that you mine more gold than anyone else before using them. You should also possess a fairly high number of other units already (unless you go for dragons only). In that situation, more mages or ogres will no longer make you much more powerful while a few birds will.
You should never use birds as a last resort. Some players use a dragon attack if they have lost the sea battle and cannot get more gold. Unless you have a huge mine left, you will not be able to inflict enough damage and will end up with all your gold wasted. Concentrate on other units then.
The threat of an aerial attack is worse than the attack itself! If your opponent knows that you have a couple of dragons ready, he will be forced to defend all his expansions and his home town against aerial attacks, using up resources for axethrowers and guard towers that would otherwise go into juggs or ogre mages. You do not need many birds to get that effect, and you do not need to attack with them often. The threat does the work for you. Just announce the presence of your birds with a hit and run attack and retreat afterwards. While your opponent is building up an air defense, you go for mages or ogres, having an advantage for your land attack afterwards. The only way to counter this is a good recon.
This is especially effective for human players. Scout the map for weakly defended expansions and send a few gryphons (four or five) there. Try to kill as many peons as you can. As soon as the reinforcements approach, turn back to your base. Human players have the advantage of healing; if you rescue your gryphons early enough (head home if their health drops below 40), you can heal them and use them for another attack. On the other hand, dragons with their "quad damage" can kill units easily. You only need about 11000 gold for that (one gryphon aviary is enough) and some time to train them.
"Dragon rushing" can be effective if you suprise your enemy. What you do is to get at least four dragon roosts and train dragons like crazy. Of course you will need a lot of gold for that, so make sure you can get it! You should be mining at least two mines with 15+ peons for each mine to support the roosts. The key for this strategy is that your enemy should not know of your plans. Otherwise, attack immediately because your advantage will be gone soon.
After you have 20+ dragons trained, send all of them to the target area (making groups of four or five dragons and leaving some free space between the groups - you will not want your birds to finish off each other). One ogre and one death knight with full mana should enchant all the dragons in a group before they "take off". Humans cannot power up their gryphons, so they should build 25+ of them.
If you surprise your enemy, this strategy is devastatingly effective. However, it will only work on maps with large mines, and you will be very vulnerable to conventional attacks while training your dragons. Personally, I do not use this strategy unless if I have already subdued my opponents and have to wipe them out or if I want to have fun. You must wall in your forces to be able to defend your town with less units than usual (sparing gold for your dragons).
This strategy created by Skyren does not use birds only, but also land attackers. What you do is to make about 15 gryphons, dividing them into three groups. One group will attack the enemy at his weakest spot. The point of this attack is to make the enemy use all his archers and mages to defend against it. When all archers have moved to the first group and the mages are finishing them off, you take the two other groups to the part of the town that is more heavily defended (i. e. the guard towers).
If everything goes well, you can take out the guard towers before the (probably already weakened) archers and mages/death knights arrive. If your attack is successful (build reinforcements during the first battle), you can take out his whole aerial defense. Then kill as many land units (ogres or knights) as you can before invading with your land attack force. Because you have already weakened the defenses very much, your land attackers have now a good chance to win where they would have been outnumbered before.
The fact that the strongest units in combat (juggs and ogres) are helpless against dragons makes them interesting. However, the dragons are too costly to be efficient if your enemy is warned. So any viable strategy involving them uses a surprise momentum.