In the 30+ years I’ve had Great Blue Herons in my backyard, I’ve never spotted one doing this, so I Googled it. From NPS.gov: On hot days, a great blue heron can be spotted with drooped wings and an open beak. The bird will be fluttering their throat muscles to increase evaporation, an action called “gular fluttering,” an avian version of panting. The drooped wings allow air flow to help remove excess heat.
Stella was tired from the Great Blue Heron visit.