The culprit:
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I think he sees his reflection in the window and is trying to protect his territory. I don't think he hits the window hard enough to hurt himself, but I'm still concerned. He does all the time - when the (black) curtains are open or closed. I don't know what to do to make him stop. Sometimes his girlfriend hangs out too, but she usually waits in the bushes by the front door.

Action shot:
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Full photo set

I was the shadow of the waxwing slain
by the false azure in the windowpane;
I was the smudge of ashen fluff -- and I
Lived on, flew on, in the reflected sky . -- John Shade, American poet

Oooh, a bird! I had visions of something along the lines of Crackpot Religions

(Cute!!)

If you want the cardinal to stop, you'll need to do something to make the window less reflective, at least temporarily. Birds will do this with car mirrors, too. Putting a plastic grocery bag over the mirror stops it. So maybe...waxed paper taped to the window from the outside? (I'm trying to brainstorm ways that will still allow you to get light) Or maybe masking tape? I know that tape and decals help with birds unintentionally striking windows.

Hm. looking at the pictures again, you're high up, aren't you? Go with the masking tape first. Poor birdie. So het up with birdie hormones that his little brain (not a strong point to begin with) is on overload.

You think he'd swerve to miss the other bird, but maybe he's trying to fight a perceived rival.

you take great pics, btw

My parents have this problem every couple of years. First, the window starts to develop a cloudy haze from the bird's oils. Then the haze turns reddish, as the bird repeatedly slams its head into the window. It gets nasty. We usually have to shoot the bird, unfortunately.

Maybe some kind of netting?

I get this with my patio doors. Every now and again there'll be a ghost bird on the glass. I'm not sure what to do about the cardinal. Maybe a plastic owl somewhere near the window?

There's a cardinal trying to break into our home.

Lousy Vatican.

A cat in the window will also deter the bird.

Eponysterical.

Since we're on the third floor of the building, I don't know what we could do to the outside of the window to keep him away. When we get close to the window, he sees the motion and goes away. But we can't be by the window all the time to protect him. Maybe we can get some sort of motion device - like those electronic cat toys that move the feather on a stick.

I also have to credit my camera for the photos. I wasn't that close for the first picture, but with 10 megapixels, I can blow up the picture. :)

Here in the urb-burbs* Spring is a time for extreme nature events. We've had two road kills in front of the house, a raccoon and a possum (neither of which species I've ever seen before around here) and now some buzzards or vultures (vultures, I'm pretty sure), have moved some possum parts into the yard and are feasting on it. Squick.

*urb-burbs - a zip code that contains both city and county, and receives some services from the city and some from the county.

Maybe an electronic perpetual motion type device put in the window?

I rather fear birds, especially non-pet birds in houses. I won't go into it, but it's a bad omen. Better he's trying to get in than trying to get out!

I found this list of tips to keep your windows safe from birds. Some of them are rather drastic (whitewashing your windows?), but there's a couple that seem to be worth trying.

Oh yeah, I should mention - they're also trying to sell you some product call CollidEscape.

call = called

I'm done now.

Maybe I have a solution. The picture window has two sections, one on either end, that open out. I can fashon a few (bird shaped?) plastic dangles and suspend them from a fishing line between each opening window. I can do this from inside my apartment instead of finding a ladder to reach the third floor on the outside of the building. The site that unsuprising linked to said if the diversion clacks against the window, it will help draw the bird's attention. And the movement of the dangles in the wind could scare him away.

My family cottage has picture windows all around, floor to ceiling. One year, a window on the forest side, an owl crashed through and we found it behind the couch, in spring — must have been autumn when the cottage was closed up when it happened.

We hung this wooden mobile of a hawk [it was articulated, pull a nylon string and the wings moved as in flight] in front of that window. It may not be moving, but it'll twist in any draft.

No troubles since.

I think those cut outs would work. Hawk, owl, put 'em all on the window.

Poor culprit [that cardinal is so sweet], knows where the food is, in da house, are you feeding them and have you forgotten to lay out a spread of suet¿

Did you know that bears, once they break into cottages for food, know exactly to head for the fridge. Many reports of fridges being toppled over, dragged across the floor, because they've learned where the food is stored. Never mind the tinned stuff. They want that fresh storage.///

Could cardinals be any different¿
nah.

Dinner./ Give, please, or I'll bust that window and you'll be shrieking 'BIRDS'.///