My 16 girls (and one roo) were hatched on May 1st this year. Each afternoon when I get home form work I've let them out for an hour or so of supervised free-ranging. They wait by the inner door to the coop, I open the outer door first, stand in the "hallway" and open the inner door. There then comes a tsunami of chickens making a break for the outside. The coop empties in under 2 seconds. We just had our first snow a couple of weeks ago; THAT day was different.
The girls were their usual excited selves envisioning an afternoon of dust-bathing, scratch and treats. I opened the inner door and the wave rolled out as usual......UNTIL.... the first half of the flock RAPIDLY decided that snow on chicken feet was not for them and immediately made a 180 and charged back INTO the coop with the same vigor as they had previously exited.. The half who were at the end of the out-going wave met the head on the in-coming wave mid doorway. The resultant clash of forces formed a wave of wings, feathers, feet, and frenzy that crested at a height of several feet, and resulted in overturned farm implements, water buckets, and grain tubs. Several scattered into the trees, but despite some loss of feathers most managed to regain an upright position within the coop. After dark, and with some coaxing, the remaining revelers were returned to the roost. Since that time, the speed rate of the outgoing wave had diminished greatly. The term "snowbird" was apparently not coined to describe chickens.