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[BlacktailSlayer]

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[Dioni A]

It's much less than scientific but my observation is that they start to change food sources after a sustainaed week of freezing night time temperatures. My best recollection is that Aspen's will change color slightly before it. None of that initiates the migration in my area though. And I would argue that not all areas even ones that are in close proximity have migrations that are driven by the same motivator. Some units are driven by snow while others the deer move solely for breeding. I'd love to hear what @robby denning has to say