House vs. bird

Yesterday afternoon, I was surprised sitting in my office by a bird slamming into the window. This happens from time to time. But yesterday, this sparrow was hurtling full speed, and when he hit the window, he stunned himself and slid down to the porch. I ran out and the little guy lay motionless except for breathing about a thousand times a minute. This was a dangerous situation as our cats like to hang out in just that spot. I stood guard for several minutes, hoping the little guy would come around, but he just lay there. I nearly gave up on him, and went inside to retrieve a piece of cardboard to move him with; when I returned, he still lay there but as I approached, he revived and flew away. Me cheering!

About an hour later, another bird slammed into the window again! Recovered nicely, and flew off.

I have two hypotheses.

First, something about the window glare fools the birds. The windows are under the porch roof, not on the perimiter of the house, and are face North. So it's unclear to me what a bird could mistake them for.

My second theory occurred to me only during a bike ride later that evening. The wind was blowing strongly out of the South. Could it be that a bird has to work really hard flying south against the wind, and that when it drops down below the roofline in the lee of the house, it suddenly finds itself moving twice as fast as it expects?