Well no place that still has freezing temps has a problem, obviously, because it’s to cold for maggots to hatch right now.
I’m in Oregon and I can attest that there are no houseflies here, although there will be by April.
Perhaps you just have had enough early warming to allow an early hatching and if the winter was mild than a larger percentage than usual of the egss survived winter.
Birds don’t migrate based on temperature, so an early hatch would have fewer predators and make for an unseemly annoyance.
I don’t read books predicting end times (except for the original). Even the angels don’t know, so I’m pretty sure human authors don’t.
Besides which, it’s not like the end times can be delayed or averted, now can they? So what exactly are believers supposed to do differently once armed with this secret arcane information???