It has been reported on the news in the last few months that in different neighborhoods of NYC there has been a significant decline in the number of stray cats. No bodies have been found so it's not a case of poisoning, but possibly catnapping? People have been urged not to let their pet felines out at night or during the day. In my neighborhood, we have noticed a decline in the stray population and have no idea why. I doubt that rescue groups are rounding them up for adoption purposes. It is possible that the spay/neuter programs run by some rescue groups has resulted in less number of kittens being born and hence a decline in the population.
I hope nothing horrible is happening to the cats. You do point out an interesting fact about the spay and neuter program. In places where it is mandatory they have seen a decline (which is good).