maybe the eldest chicks
I see already the blue on the end of the wings:
"As the new feather, also called a pin or blood feather, emerges from the skin’s feather follicle, it looks like a spike, quill or much like the feather shaft itself. This new feather has a blood supply flowing through it and is encased by a keratin (type of protein) coating or feather sheath. The base, where the feather is developing and being nourished by blood, is a dark blue color."
(https://vcahospitals.com/know-your-pet/ ... s-in-birds)