Bat Migration and Hibernation

When we think of migration flights by some species, birds are probably what we are thinking about. But bats also migrate. During these migrations, they will often rest in odd places - window screens, exterior walls of homes and buildings - and will frighten humans.  

Silver-haired bat found in Newark - Photo: K.Perotta via NJDEP 

They are often alone and may remain for a day or two. They are not harmful or threatening to humans and should be left alone. What you should do is take a photo and send it to  MacKenzie.Hall@dep.nj.gov with information about the location and date. The Endangered and Nongame Species Program (ENSP) is able to catch some of these migrating bats in mist nets and then use nanotags to track them for a few weeks to learn about their patterns. Your photos and information can supplement that data. Even if you don't know what species you have found, take several photos from different angles and send them for identification.

Hoary bat found in Avalon - Photo: M.Wright via NJDEP

New Jersey has had an active bat conservation program for many years that has a focus on protecting winter dens of non-migratory species that gather together for hibernation. These winter colonies often number in the hundreds or thousands. Bats are especially vulnerable to disturbance and vandalism during hibernation. Even unintentional human activity can arouse the bats and cause them to use up their energy reserves too quickly. 

The Hibernia Mine in Rockaway Township, Morris County is our most famous "bat cave" or hibernaculum. The site had been used for mining iron ore from the early 1720s until operations ended around 1913. Bats were first observed in the abandoned mine in the 1930s, and by the 1980s more than 20,000 bats were wintering there each year. But vandalism and trespassing were big problems. When federally endangered Indiana bats were found to be using the mine in 1992, New Jersey acquired the property soon thereafter via the Green Acres Program and it is now part of the Wildcat Ridge Wildlife Management Area.

The hibernating colony climbed to around 30,000 bats in the early 2000s, with Little Brown bats being most numerous but bats of all cave-hibernating species seeking refuge there. Sadly, White-nose Syndrome hit Hibernia in January 2009 and more than 90% of the bats died.

For more on NJ bats: state.nj.us/dep/fgw/ensp/bat.htm