You'd be looking at a ton of sources to come to that conclusion. You can find any of them separately. The easiest hole in it is that litter size is solely based on age and health of the mother, and has nothing to do with population, aside from whatever factor exists allowing low populations to create healthier mothers. There is no correlation between population and litter size except for where there is an overpopulation, which can lead to an unhealthy sampling, or a population so small to be rife with inbreeding, also effecting the health of the mother. But the basis of the idea that coyote population density causes a genetic mechanism that generates larger litters is demonstrably false.