I too have a bird feeder just off the deck in the yard and my cats sit and watch the birds all day through the sliding glass door (they are also inside kitties). Occasionally a bird will fly right into the door as if they are planning to come right on in the kitchen (and believe me it's not because the door is super clean). Sometimes they appear dazed but so far they have all been able to fly away. Maybe yours flew into the glass and hit it a little too hard.