daydreamer.
I spoke with a "chicken friend" today and he experienced similar problems with smaller breed chicks ordered during cold weather. Or I should say, mixed orders during cold weather. He had 1/25 die when ordering during warm weather and another buddy has made several orders over the past 2 years with very minimal losses but they only order during late spring or summer.
I think the smaller chicks cant take the stress and abuse the larger chicks put on them during shipping in cold weather. The small breed chicks pile up under the hottest spot below the light and the larger bodied chicks spread out more so they obviously need it warmer than the big chicks.
When the replacement order comes in, I will be ready with a separate area for the small chicks to keep the big ones from bullying them and pecking. I can bump the temp up a bit for them since they seem to need a warmer spot too. If I can get them past the first 4 days or so, they should be strong enough to join the rest of the pack.
Wish I had known about the mortality potential of shipping mixed chicks in cold weather. Its been in the low 40's here with rain and high humidity and below freezing in IA where they are shipped from so its probably a nasty journey for baby chicks.