25 August 2017

Why do birds look fat in winter and thin in the summer?  Have they lost weight?

No.  They’re thin because they’re trying to stay cool.

Underneath their smooth outer feathers, birds wear down coats all year long.  The down is warmer when it is fluffed out to hold heated air near the skin. Our puffy down coats use the same principle. Both we and the birds look fat on cold winter days.

When it’s hot, birds cannot take off their down coats so they force hot air out of the down feathers by compressing their outer feathers and squeezing down the fluff.  This makes them look thin. It is counter intuitive to us because tight clothing makes us feel hot so we wear loose clothing in the heat.

The cardinal on the left, above, is not the thinnest one I’ve ever seen.  Cris Hamilton took his picture in May when the temperature was pleasant.  He’d look considerably thinner this month.

It’s just another way that birds cope with heat.

 

p.s. We think of down as white but northern cardinals have black down and black skin as seen in this photo of a bald cardinal by Matt Webb. Notice that the downy base of the feathers is black.

Click here to see a northern cardinal’s body feather, called a semi-plume, black at the root and red at the tip.

(photos by Cris Hamilton and Matt Webb)