If you lose one of the hens, before you send the other to a race put another hen in
with her cock bird in one half of the nest then let the hen to be raced into the nest section for a few seconds, only enough
time for her to see the other hen with her mate
This system can be also used successfully with cock working in reverse
I have based this on 20 birds racing but it can be used with any combination and number of birds