Attack his pride. Introduce reputation and honor rules. He is known as a coward to the locales. Many players want to be a badass, not just in ability but in reputation. They want to cultivate an image. Perhaps he is thinking of himself as Stormcrow or Deathwing or some other vain title. Have the group come upon a wanted poster offering 500gp for the capture of the infamous “Chicken Witch” or the “Yellow Druid” or some name to illustrate his actions.
Building upon my first suggestion, have the locales perhaps launch an inquisition against “witches” (witches, druids, all the same to a peasant). They could use falconry (trained predator birds to attack). Have orcs train hawks, and giants train rocs.
Introduce a cult of druids of opposing alignments to basically employ a similar tactic.
The wingclipper feat would cause him to plummet down. As would a dispel magic, or an arrow imbued with dispel magic.
Rangers with species enemy “animals.” See the “Ranger Creature” template to give ranger qualities to monsters.
Start awarding bonus experience points to other players for creative tactics and their ability to handle monsters in unique ways. Then if he is discontent just shrug your shoulders and tell him that there isn’t anything creative about doing the same thing over and over again.
Since his spells come from nature or a being of some sort, perhaps such a being may not approve of its follower using its abilities to rain death at every encounter like an apache helicopter.
I’m not sure if you can adjust the XP for an easy encounter but if you can then go for it.
Introduce a much higher level NPC druid of similar alignment that could wipe the whole party out if he wanted but don’t. Just have him be a part of an adventure hook. Basically the embodiment of what your player wishes to be. Your player will expect a connection but perhaps this much older and wiser druid scoffs at the way your player carries himself. An old school playa disgusted with this generation of druids. “The gifts of mother nature are a shield, not a sword young druidling.”
As for his argument. As others have pointed out spellcasting is noticeable whether coming from a bird or human. Building upon this, if his logic is sound in that an animal wouldn’t be perceived as a threat then watch how he reacts when you introduce the same tactic. If you describe a bird flying overhead and he immediately casts a stoneskin before you are done reading the description then call him out on his hypocritical argument. Remind him he is roleplaying poorly.
I had a character act in this manner. As the DM you aren’t going to describe every scene in which the players encounter. They will see birds, they may even hear a noise that turns out to be just a mouse. Prepping for battle every time you open your mouth is poor role playing. The players may know that every time you describe something an encounter is likely to occur. But their characters do not and you cannot act on player knowledge if your character is ignorant. I had a player do something like this. Whether I was describing the winter chill before a frost giant emerged, or the smell of fungus and blood before a troll came out of hiding he would cast stoneskin before I could even finish the description. So I simply changed the encounter on the fly. I continued with my description which ended with nothing more than flavor text and him out 100gp material component. Do the same to him. Have him waste his wildshape abilities on minor encounters. And remind players what poor roleplaying entails.
Time to introduce a subterranean adventure. Or travel through a forest where he is forced to fly low in order to actually see the enemies.
My friend. I feel for you. Players like this take the charm out of the game. Druids are supposed to be mystical and mysterious. Not ultrapractical 100% efficient military scientists. This is a tricky issue to handle though. The rules are so robust that weaknesses exist and players love to exploit them. But they don’t look at it as exploitation. You don’t want to give him the impression you are waiving the DM wand. The trick is to slowly implement these changes so subtly that you don’t appear as though you’ve done your homework and plan to single him out. Try to have all these suggestions effect another player before him. Perhaps in 2 or 3 sessions the dynamics of your campaign will change such that your new implements won’t make you seem like an A-hole DM.