Late to the thread, but the 90D is a solid choice for birding due to the high resolution and crop factor. With 350 dollars left, prioritizing the technical essentials is more important than aesthetics.

• Power: Wildlife photography involves heavy use of continuous autofocus and high-speed bursts. These drain the battery faster than standard shooting. Picking up a Canon LP-E6NH Lithium-Ion Battery is a smart move. It provides slightly better capacity than the older E6N version and is fully compatible.
• Storage: Since you will be taking hundreds of shots per outing, card speed is critical. If the card is too slow, the buffer will hang and youll miss shots. The SanDisk Extreme PRO SDXC UHS-I 128GB 200MB/s is a reliable standard that handles the 90D burst rates well.
• Stability: Rather than a heavy tripod, a monopod is often better for walking around nature preserves. It carries the weight of the lens while letting you pivot quickly for birds in flight. The Manfrotto Element MII Aluminum Monopod is sturdy enough for a 90D setup.
• Camo covers: These are mostly for protecting the lens barrel from scuffs and providing a bit of weather padding. Birds are generally more sensitive to movement and noise than the color of your gear. Better to spend that money on a comfortable strap or more storage for now.

Be careful with slow cards. Make sure to get the SanDisk Extreme Pro 128GB SDXC UHS-I so you dont lag during those high speed bird bursts.