New Jersey’s only mammal genuinely capable of powered flight — and its most overlooked source of pest control.

Of the nine bat species that call New Jersey home, the one Montclair residents are most likely to encounter is the big brown bat: a hardy, adaptable species that has become the state’s most commonly seen bat, often roosting in attics, soffits, and behind shutters right alongside people.

The little brown bat, once just as familiar, is now far rarer after a devastating disease swept through the region — a shift worth understanding before writing off a bat sighting as unremarkable.

What they look like

Big brown bats are mid-sized, with a body about four to five inches long and a wingspan of roughly 12 to 13 inches — noticeably larger than the little brown bat, with slower, more deliberate wingbeats and an audible chattering in flight that can help tell the two apart. Their fur is glossy brown, their wings and ears dark and hairless, with finely webbed skin stretched between elongated finger bones.

Family life

Bats don’t pair up. Big brown bats are promiscuous, with both males and females mating with multiple partners, and genetic studies have found litters with more than one father as a result.

Mating for most of New Jersey’s bat species happens in autumn, often at “swarming” sites — caves or mines where large numbers congregate specifically to find mates before settling in to hibernate. Females then store sperm through winter and don’t become pregnant until spring, a strategy called delayed fertilisation that lines births up with warmer weather and abundant insects.

Females gather in maternity colonies, often in an attic, barn, or tree cavity, where each typically gives birth to one or two pups in late spring or early summer. Little brown bats, by contrast, almost always have just a single pup a year — a slow reproductive rate that’s a major reason their population has struggled to recover.

Pups are born hairless and helpless but grow quickly, able to fly within three to four weeks. Maternity colonies are all-female; males roost separately through summer, rejoining only as autumn approaches.

Daily habits

Bats are almost entirely nocturnal, emerging around dusk to feed and returning to roost before dawn. They are voracious insect predators: a single little brown bat can eat several thousand insects in one night, and big brown bats favour beetles in particular, including agricultural pests like cucumber beetles — a meaningful, free form of pest control for farms and gardens alike.

Bats locate prey using echolocation, emitting rapid, high-frequency calls and interpreting the returning echoes with extraordinary precision, all while flying at full speed in the dark.

New Jersey’s resident bats hibernate through winter, typically in caves, abandoned mines, or occasionally buildings, living off fat reserves built up in autumn. Three further species — the hoary, red, and silver-haired bats — aren’t residents at all, but migrate through the state each spring and autumn on their way to warmer wintering grounds.

A persistent myth: bats and rabies

Bats carry an outsized reputation for spreading rabies, but the actual risk from a bat that isn’t behaving abnormally is very low — many big brown bats, in particular, carry a natural immunity.

The real risk comes specifically from direct contact. A bat should never be handled bare-handed, and anyone bitten or scratched by a bat, or who wakes up with a bat in the room, should have the bat evaluated and seek medical guidance, since a bite isn’t always felt or visible.

A bat simply flying near a house at dusk, or roosting quietly in an attic, poses no meaningful risk on its own.

Living alongside bats

• Leave a roosting colony alone in summer if possible. A maternity colony in an attic is temporary and provides significant pest control. Exclusion — sealing entry points after bats have left for the season — is far more humane and effective than removal during pup-rearing months.
• Never handle a bat directly. If one needs to be removed from a living space, use a container and stiff paper rather than bare hands, and consult a wildlife control professional for anything beyond a single stray bat.
• Consider a bat box. A well-placed, well-designed bat house gives displaced or dispersing bats an alternative roost away from a home’s interior.
• Support hibernation site protection. Undisturbed caves and mines are essential winter habitat, and disturbance during hibernation can be fatal to already-stressed populations.
• Skip pesticide use. Healthy bat populations reduce the need for chemical insect control, and pesticide-laden prey affects the bats that eat them.

A bat overhead at dusk is one of the more overlooked forms of pest control a neighbourhood has — and, for the declining species among New Jersey’s nine, a sighting worth appreciating rather than fearing.