I tend to count from the oldest chick except when it comes to banding dates when I use the age of the youngest chick because we really need to make sure the chicks are old enough to be banded.
So, the West Winnipeg chicks are on Day Eight - oldest two chicks are eight days, third is seven days old.
They can thermoregulate at 10+ days so the rain will not be as big a danger. At 10 days they can move on their own, tend to move into the shade/under cover when the weather changes - and the West Winnipeg site has a roof they can get under to avoid both rain, sun and winds from most directions.