There is a window every year when sealing a bat colony out of a building is the worst thing you can do. Understanding that window is most of what you need to know about bat work.
What maternity season is
Female bats gather in warm roosts to give birth and raise pups. The pups cannot fly for several weeks. During that period the adults leave nightly to feed and return to nurse.
Exact dates vary by species and latitude, but broadly the window runs from late spring into late summer. In warmer southern states it starts earlier and runs longer.
Why sealing during it goes badly
A one-way door lets adults out and keeps them from returning. If flightless pups are inside, you have just separated them from their mothers permanently.
The pups die in the wall or attic. You get an odour problem, a fly and beetle problem, and adult bats chewing and clawing at the seal trying to get back in, frequently finding a new way into the living space instead.
This is the single most common failure in bat work, and it is almost always the result of someone wanting the job done immediately.
What can be done during the window
Plenty. The inspection, the quote, the full exclusion plan and all sealing of points that are not in active use can proceed. Only the primary exit gets left until the pups can fly.
Single bats found in living space can be removed at any time. That is a different job from colony exclusion.
Guano cleanup in accessible areas can often start, depending on roost location.
How exclusion works once the window closes
One-way valves are fitted over the active exits. Bats leave to feed and cannot get back in. After several nights of confirmed zero activity, the valves come off and the points are sealed permanently.
Every other gap on the building is sealed first, because bats are efficient at finding the next opening. Anything larger than roughly half an inch is worth closing.
Guano and why cleanup is separate
Accumulated guano can support a fungus whose spores cause respiratory illness when disturbed. Cleanup uses containment, filtration and protective equipment rather than a shop vacuum and a bin bag.
Contaminated insulation is usually removed rather than treated, because the material holds odour and pheromone that continues to attract bats to the site.