Ever wonder why birds look fat in winter and thin in the summer? They’re trying to stay cool. Underneath their smooth outer feathers, birds wear down coats all year long. The down keeps them especially warm when they fluff it out to hold more heat next to the skin. This fluffing makes them look fat on cold winter days. When it’s hot, they can’t take off their down coats so they force hot air out of the down by compressing their outer feathers.

Interestingly, cardinal down is actually black under all that red.

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Painting derived from a photo reference by Don Barrick. Unrestricted license granted for use of the image.