Given a definition of a nest as where pokémon of a certain species reliably, frequently spawn in multiples (that is, seeing 2 or even more of the same species on your screen at the same time), I've never yet encountered a nest. I've seen people encounter nests on YouTube, but I've never been at one, myself.
(Not counting pidgies, ratattas, and spearows. Because. Obviously. Infestations!)
I've heard that while nests formally migrate, frequent spawn areas (where pokemon of a certain species reliably but not too frequently spawn one at a time) don't. However, a nearby frequent spawn area in a nearby park has switched from bellsprouts to tentacools. I'd always before been able to pick up 2-3 bellsprouts on a walk there; now, no bellsprouts at all, but 3 tentacools.
"nests" and "frequent spawn areas"
(Not counting pidgies, ratattas, and spearows. Because. Obviously. Infestations!)
I've heard that while nests formally migrate, frequent spawn areas (where pokemon of a certain species reliably but not too frequently spawn one at a time) don't. However, a nearby frequent spawn area in a nearby park has switched from bellsprouts to tentacools. I'd always before been able to pick up 2-3 bellsprouts on a walk there; now, no bellsprouts at all, but 3 tentacools.
Hmmmmm.