When birds puff out feathers

It is truly entertaining to watch birds and their antics, in particular when birds fluff their feathers, and you wonder why in this hot weather.
     I eventually learn that birds do that to keep cool.  As birds naturally have a higher body temperature, are always active and have a high metabolic rate, they generate even more body heat so they need to keep cool more.  When they puff out their feathers or spread their wings, they allow air to circulate and cool their hot skin.  They may also hold their wings up to lower the body temperature.
     I was once told that it's the bird's way to de-stress when it fluffs its feathers.

     

    However, when this Asian Koel fluffed its neck it was because it was startled when a branch cracked up next to it and dropped off.  Yes, birds can be shocked too.