My thought is that the many chipmunks that I saw prior to the feral cat's presence were pretty much all killed by the momma cat to feed her litter. With the disappearance of the cat, the population of chipmunks were able to recover.
If we do some very basic back-of-envelope calculations, this makes a kind of sense:
- Chipmunks take a year to reach maturity, producing one or two litters each year (Feb-Apr and Jun-Aug).
- June 2009: Widespread death of chipmunks near the barn; assume no surviving females nor young, therefore no young litter from the June-August litters and no juveniles.
- Feb-Apr 2010: Cold average temperatures: assume unlikely that chipmunks in surrounding areas have new litters; death of juveniles likely high.
- Jun-Aug 2010: Production of first viable litters in 2010 in surrounding areas.
- Jun-Aug 2011: Production of second viable litters moving into the area around the barn.
- Feb-Apr 2012: Anomalously warm winter and spring create further litters in the surrounding areas.
- Jun-Aug 2012: Relatively high number of chipmunks in the area surrounding the barn.
- Sep 2012: Mast year for nuts mean that chipmunks are running around, looking for nuts
Still, it's a further reminder of the predatory potential of house cats. One more reason why people should spay and neuter their cats: so feral cats won't have litters and end up killing small mammals and birds.