Heron With Injury

The Great Blue Heron pictured above was photographed at the Cowichan Estuary in a ditch beside the road, not the prettiest background.  It was one of the more shy herons I have seen and allowed 2 photos before it flew away.  It wasn’t until I uploaded the photos to my computer that I noticed that there was an injury at top of wing, cropped photo below.  Not good quality but you can see the injury.

I wonder how it happened?