Mammals—including us—use facial muscles to communicate, by, say, smiling or frowning. Reptiles and birds don’t do that: they don’t have the right muscles for it. If you think a bird looks grumpy, or angry, or has any similar human-type facial expression, you’re projecting your human perceptions onto an animal that really doesn’t work like that. (Now, whether the bird actually is grumpy is a different matter; I’m just saying that you can’t tell if it is by looking at its face.)
So the appearance that all these junco chicks have of possessing some serious attitude is merely an entertaining illusion.