Context: My informant is a Ukrainian immigrant who grew up in the Soviet Union, lived in East Germany, and currently resides in California. This piece of folklore comes from personal experience. She will be referred to as “L”.
Main piece: Whistling inside the house is extremely bad luck. We will lose money. Also if birds get into the house, that is an omen that someone will soon die.
Background: My informant grew up extremely superstitious. She is also an immigrant, and finds that continuing these superstitions is a way of preserving culture in her new home.
Notes: I think this fear comes from this idea of bad things invading the sanctity of the home. It’s no secret that birds can carry disease, I’m sure there are some connections to birds and death (ravens as a symbol of death?). Whistling could perhaps be a sign that one isn’t paying attention to their wealth, meaning it could be stolen away.