This AP story notes

This AP story notes that a American helicopter was shot down in Iraq today just north of Baghdad. This is the fourth downed helicopter since January 20th — at least three of which appear to have been shot down. The AP says that 54 helicopters have been lost in Iraq since the invasion and about half of them from hostile fire. Events like this can randomly cluster together. But something else seems to be up — whether it’s a more aggressive posture on the Army’s part, more use of helicopters to avoid IEDs or better weaponry on the part of the insurgents. If you know more about this, I’d be interested in hearing more.