This Northern Parula was gleaning insects from the potted pepper plant that sits right outside the kitchen window. Parulas are normally found in tree canopies away from human houses, but after the breeding season they wander into other habitats.
The bird was ramming its sharp beak under leaves...
...and into flowers. Like some other migrant warblers, this species supplements its diet with nectar during the non-breeding season. Not content with sipping, the visitor yanked a few flowers off the pepper plant.
David George Haskell 
I have one of these that is flying repeatedly into my dining room window from my lilac bush – is it doing this to get flies, etc., that might be flying against windows? I was worried there was a reflection it was going up against or something because of the constant attention to the windows and slid the screen, but it is still happening. Thank you. :)
Hmm. Does it appear to be feeding (grabbing at insects, etc), or just lunging? If it is just flying up, then it is likely attacking its own reflection. The only way to stop that is to put up some barrier temporarily to block the reflection/access.
It flies to the window and then “up” the window, or across the window or down the window, depending on where. Then it flies back to a branch, then does it again. Since I’ve had the problem before, I slid the screen up (because usually that breaks things up and reduces/eliminates the reflection) and it then flew and landed and grabbed hold of the screen sometimes.
Strange behavior. Sounds like it is after its reflection. Have you tried blocking off the window temporarily with something more substantial?
I will try that tomorrow. With aggressive male cardinals the screen has usually done the trick.