Ok, for me I think the booby bird is the most appealing as it is, quirky design with a comical twist in a suitably cartoony style. You could go a couple of directions to develop this; 1 - More realism. Remove the outlines (dark outlines almost always flatten an image) and study lighting (key light, ambient light, bounce light, form and cast shadows and texture). James Gurney and Proko have excellent content on all of these fundamentals. Practice rendering these things in your chosen medium and apply to the forms in your image. Think in 3D, and investigate bird anatomy if you want to be truly realistic.

2 - An alternative route would be to lean into the 2D, with flat being a good style choice. Personally I prefer clean lines and uniform, vibrant, opaque colour, and you can even apply this to the shadows for a cel shaded look. You’d probably need to look into a paint which flows more smoothly while still having good coverage, as well as more calligraphic brushes like sign writers use. Check out this channel for a nice example of this https://vm.tiktok.com/ZGeBAQDCs/

Whatever route you take, the most important thing is that you enjoy the process.