I found the below National Geographic article about a case in Scotland where they found Red Deer killing young seabird chicks and eating mostly the legs, heads, and bones.
The Red Deer were believed to be supplimenting their increased calcium requirements during the time of year when bucks were growing thier antlers and there was a calcium difficiency in the local vegetation they were eating.
Whitetail deer in North America have also been known to stomp on fish in streams and eat them. There is case after case showing deer will seek out alternative food sources to suppliment certain minerals or nutrients missing in their regular diet.
When I’m planning my food plots and mineral sites, I always try to figure out what nutritional gaps I can fill in the area.