On the swarming behavior of Dratinis (or other rare Pokemon) in the wild

I realize that Pokemon Go quite possibly hasn't been out long enough to answer my question, but I'm curious if anyone else has seen a similar occurrence:

I discovered pretty quickly after downloading the app that a small pond along my route to work seemed to be absolutely crawling with Dratinis - something like six showing up within the radar. Catching one, I figured I'd check again the next day to see if they were still there.

Same thing the next day, a couple showed up on the radar, caught another. Sweet, this must be a Dratini nest, I think to myself.

Skip forward a few days to today. I've been past the same spot at about the same time in the morning, and the numbers of Dratini encountered have gone to zero the past two days. Which leads me to wonder...

Is there perhaps a "swarming" behavior for rare Pokemon in Pokemon Go? As in, rare Pokemon appear at a location in large numbers for only a limited amount of time?

Solution 1:

There are places in Pokemon Go called "nests" where many Pokemon of the same, often rare, type spawn. This is why there are (or were, depending on when you're reading this) many Electrabuzz in Central Park and many Bulbasaurs in a park near me.