Might be Magpies. (picture not mine)

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Whos panting? the Pekins? That would be my guess since they tend to get over heated easier. Good idea not to leave them outside they are way to young for that unless there is a heat source. I am sure your little ones appreciate having the Pekins for the extra warmth in side.
How did they do with out heat over night? If they were all piled on top of each other they may have been too cool.