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An excellent article on bird survival tactics in winter can be found at

https://www.allaboutbirds.org/how-do-birds-survive-the-winter/?utm_source=Cornell%20Lab%20eNews&utm_campaign=72de864285-Cornell-Lab-eNews-2019-02-11&utm_medium=email&utm_term=0_47588b5758-72de864285-317763661

It is written by Bernd Heinrich, author of many articles and books about bird behavior. He deals with winter survival in his book “Winter World: The Ingenuity of Animal Survival.”

Thanks to Paulo Boute and Douglas Carver who posted this information on the email network BirdChat.