slow area migrations

has anyone noticed that over a few days or weeks you might get an abnormal number of one type of mon that appears to phase out over time. These don't seem to be related to nest migrations. a couple of weeks ago the main drag in my town had heaps of skarmory caught like a dozen or so haven't seen any before or since at the moment it is abra/kadabra it only started in the last 3 days and already appears to be fading.