Birds that eat insects leave Pennsylvania for the winter but the omnivores, like this house sparrow, stay behind. Food won’t be a problem but it’s going to get cold so the house sparrows get ready in advance.
A study by Lowther and Cink in 1992 found that house sparrows (Passer domesticus) prepare for winter by molting into heavier plumage. Plumage weight increased 70% between August and September alone. Summer weight is 0.9 grams; winter weight is 1.5 grams.
In September the house sparrows put on their winter coats.
(photo from Wikimedia Commons; click on the caption to see the original.
This article was inspired by page 153 of Ornithology by Frank B. Gill, 3rd edition.) )