an animal estrus detection apparatus adapted to be secured to the rump portion of a first animal , and actuated by a second animal mounting the first and thereby applying a predetermined amount of pressure thereon a depressible pin . the pin &# 39 ; s motion is transferred by a linkage mechanism to a release mechanism , which then disengages an outer cover causing it to fall away and deploy an estrus indicating streamer . the release mechanism may include either a pneumatic or an escapement type release timer which is powered by said linkage and which prevents said linkage from powering said release mechanism until a predetermined period of time has elapsed . the release mechanism may also include a counting mechanism which actuates the release arm only after a predetermined number of successive mountings of said first animal . a second , hydraulic timer , actuated when the outer cover is released , measures the time elapsed since the outer cover &# 39 ; s release .