The income from deer seems to fluctuate randomly for me. I have about 20 deer and sometimes I get 15+ per minute while at other times I get less than ten for several minutes.
And it doesn't help that I have no info on how many deer there actually are in my city. The FOOD indicator only tells me the total income from deer, but not the number of deer. If anyone knows how to see the number of deer, please let me know.