Feather Your Own Nest

Draws on the very real bird behaviour of lining a nest with soft material, feathers, down, fur, to make it comfortable and ready before eggs are laid. The English idiom applies that same image to people who quietly make themselves comfortable, typically at someone else's expense, while appearing to serve some other purpose. It's a genuinely old phrase, appearing in the 1553 play Respublica and turning up in several other works of the period, well established enough to be considered a cliché by the 18th century.