If Merlin is grumpy, it's usually for a small selection of reasons:
- There's something in the environment that he's afraid of (common culprits are: a stranger with big hair, something dark and new on the ground, big shoes, any time I put on a hat)
- I've done something he didn't expect (including reaching past him for something, not coming home when I usually do, walking up to him when he's engaged with a toy or eating). This happens less frequently now that we know each other more.
- He's upset because one of his needs isn't being met and I'm not paying attention to what that is (i.e., hungry, he did a trick and I didn't notice, he's lonely and I wasn't able to spend time with him, he wants a shower, he's got a pinfeather on his neck he can't get to, etc.)
- He's territorial (I minimize this by only cleaning his cage when he's outside of it, taking him out using a perch instead of my hand, and never offering him a place to claim as a nest, such as the closet, a box, or the corner under the cabinets)
That's pretty much it. Merlin has always been very gentle taking treats, and I've gotten him over his fear of new objects by heavily rewarding him with a click and a treat whenever he touches something
gently with his beak. If I notice him being aggressive/fearful towards a specific object, that's what we focus on when we train. For instance, my roommate, an enormous toy that I made him (which he has since happily obliterated), and the handheld broom I use to sweep off the counter. Positive reinforcement has made it super easy to motivate him to accept novelty!
Now to get him over his grudges every time I leave for work...
P.S., if your Meyer's has been conditioned to lunge at non-food objects, that's probably what he'll continue to do for a while until you've proven to him that it's waaaay more rewarding to be gentle. Try showing him the reward first as an incentive, then phase that out as he gets better at exploring.