[continued from previous message]   
      
   orientation).  The hen will leave the nest about once a day to drink and   
   eat until around day 18 of incubation when she somehow senses the chicks   
   in the eggs and goes into a sort of trance.  She stops turning the eggs,   
   and does not get off the nest.  She remains in this trance for up to 72   
   hours after the first chick hatches on day 21.  This gives all the   
   chicks a chance to hatch and be ready to move off the nest.   
      
   Commercial layers have been selected to not go broody.  Crosses indicate   
   that the behavior is disrupted in at least two ways.  You can cross two   
   nonbroody lines and produce broody birds, so the defects are independent   
   and can complement each other.  You have to be homozygous for each   
   defect.  Both defects seem to block the start of incubation, so the   
   birds keep laying eggs.  The egg counting mechanism is messed up or the   
   transition to incubation is somehow blocked.  I used to have an inbred   
   Leghorn line that was nonbroody and laid a lot of eggs, but they   
   retained the nesting behavior and would try to gather eggs underneath   
   the hen.  So they still had that part of the behavior, but for some   
   reason they never started incubation and kept laying eggs.  Some   
   researchers have gotten the birds to brood chicks so they haven't lost   
   that ability they just never get to the point of having chicks hatch   
   under them to stimulate chick brooding behavior.   
      
   You have physical stimulation that triggers hormonal changes that result   
   in altered behavior.  The hormonal levels seem to just change a bit, and   
   it isn't an on and off switch.  This seems to be enough to get the brain   
   to send the next batch of signals.   
      
   Ron Okimoto   
      
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