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Where the yard's bats go in September and October

September to October

The bats over the lawn on August evenings thin out and vanish through September and October. Some fly south, and the ones that stay drop into a winter sleep lasting until spring. In most of eastern Massachusetts that yard bat is now the big brown bat.

Why this time of year

September and October are when a yard that had bats all summer goes quiet: nursery colonies break up in late summer, insect prey collapses with the cooling, and the bats respond by migrating or by going into hibernation. No source gives a hibernation onset date for Massachusetts, so this is a transition to watch rather than a date to mark.

How to notice it

  1. On a mild evening go out to the same spot in the yard about 20 minutes after sunset and face the lightest part of the sky. Bats read as flickering, jerky silhouettes, nothing like the steady flap of a bird.
  2. Count how many you see in ten minutes and write the number and the date by the door.
  3. Repeat on the next mild evening, and again in mid-October. The count usually falls off, and one night there are none.
  4. Write down the date of the last bat you see. That date, and not a rule in a book, is when your yard's bats went to winter.

Try this with kids: The bat you saw in August might be asleep in a cave a hundred miles away right now, or asleep inside a building down the street. Which would you rather be, the bat that flies south or the bat that sleeps all winter without eating?

a hibernaculum is the cave, mine or building where a bat spends the whole winter asleep

Safety

  • Never enter a cave or a mine to look for bats. Massachusetts closed its five most important bat hibernacula to the public because waking a hibernating bat burns the fat it needs to survive the winter.
  • Do not go into an attic or crawlspace looking for bats, and do not disturb bats in a building in winter.
  • Never pick up a bat with bare hands. If anyone has contact with a bat, or a bat is found in a room with a sleeping person, do not release it: wash the area and call local health officials.
  • This is a yard activity. Everything worth seeing happens in the open sky above you.

Sources

  • mass.govMassWildlife Natural Heritage and Endangered Species Program, and the source for the species framing: nine bat species in Massachusetts; the big brown bat is the only one listed Common, statewide, wintering in buildings, caves and mines; the little brown bat declined by more than 99 percent after white-nose syndrome and is state Endangered, as are the northern long-eared, eastern small-footed and tricolored bats; silver-haired, eastern red and hoary bats are listed as migratory in winter; the five most important hibernacula are closed to public access.
  • mass.govMassWildlife: bat eviction is permitted only in May and from August 1 to mid-October, that late window being after pups can fly but BEFORE the bats enter hibernation. Also that big brown bats often hibernate in homes and attics, that winter disturbance can kill them, and the rabies handling protocol used in safety_notes. READ CAREFULLY: this establishes a regulatory boundary set safely ahead of hibernation. It is NOT a measurement of when hibernation begins.
  • massaudubon.orgMass Audubon: nursery colonies break up in late summer; little brown bats migrate to hibernation caves and mines mainly in western Massachusetts, upstate New York and Vermont, so the hibernacula are not in eastern Massachusetts; big brown bats may migrate to caves or spend the winter in the same buildings they occupy the rest of the year; the eastern red bat migrates along the east coast; white-nose syndrome dramatically reduced the little brown bat, once the most common bat in Massachusetts.
  • extension.psu.eduPenn State Extension, Northeastern bats, corroborating the autumn sequence independently of Massachusetts sources: maternity colonies disband in late summer and early fall; autumn prompts silver-haired, red and hoary bats to begin migration; bats swarm at cave entrances before gathering to hibernate; big brown bats occasionally overwinter in an attic or basement; bats arouse in March and reach summer roosts in April, which sets the length of the absence.
  • mass.govMassWildlife species spotlight: little brown bats increase body weight by over 30 percent before hibernation; winter roosts are abandoned mines and limestone caves; heartbeat drops to about 8 beats per minute; and MassWildlife's own guidance to stay out of caves and mines in winter, which is the basis of the first safety note.

Checked against these sources on September 9, 2026.

In the guide for September, October

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