Why should i cook at 450 degrees?Is it for less steam?I have cooked chicken at this temp it's good but never understood why.Now i cooked a 5lb. bird @450 for 1hr 30 min. How do i figure out a bird 8lbs it's time.Or just 18 min a lb?Thanks in advance..

no more than 40 mins

I usually use 400 degrees and it's about 30 to 35 minutes. Also depends on if there is a bone and if they're really large.

It depends on their weight and whether or not they're at room temperature when they go in the oven. I'd start checking them after 20 minutes.