To the OP. Like the others said, try spot metering and try to get closer or zoom in more. As also mentioned, flash is something to play with too. On-board might be a little weak for the job, but if you have something more powerful, you can have some fun.
I probably have a hundred shots invested in this hummingbird, its a wonder the poor thing is not blind. I think they are actually getting used to the flash, it really doesn't scare them anymore.
Here's one of my better ones, original size and final crop. This was daytime, but stopped down quite a bit. Heavy tree cover behind. I've got a few more pixels to work with on the d7100, but you could still get a decent shot not cropping so tight. I'm finding the hummingbirds harder to flash than other birds. I guess they are iridescent. If the angle is just right, they glow like a road sign and look really bad. Still learning.