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In NJ, hibernating bats will usually retire to caves or mines in the fall before truly hibernating during the winter months. Around the end of August thousands of little brown bats retire to the Hibernia mine in Rockaway Township, Morris County, New Jersey. At that time they will fly out at sundown to feed during the night and their mass exodus from the mine is the spectacle we will try to witness.

The Hibernia mine is under the protection and management of the bat conservation office of the NJ Department of Fish, Game and Wildlife.

Mike Valent, the bat specialist from the NJ D of FGW, is leading a trip to the mine to see the bat exodus on Sep 14 (Thu) at 7:30 pm.


Route 80 to Hibernia exit (Exit 37, County Rd 513).
North on 513 off the exit ramp for about 3 miles.
Look for a small side road called Lower Hibernia Road, on the right.
The parking lot is immediately next to the road.
People will congregate in the Pkg Lot at 7:30.

There will be a short tutorial introduction and then it is off to the mine. See you there!


Sun Sep 17 14:58:29 EDT 2000

bats leaving mine
The entrance to the mine is protected by a gate and a concrete wall. The bats can be seen streaming out of a hole left in the wall. This picture was taken on a return trip to the mine on Sep 24. Photo by J. Mocenigo.

Despite the original directions being a bit off, the weather was good as Mike Valent lead a group of about 30 people to the mine.

In the fall, bats leaving the mine have more than feeding on their mind. It's also mating season. The bats will swarm at the entrance of the cave looking for a mate before dispersing into the surrounding country.

I recorded some isolated bats on the way to the cave, but pretty soon the air was filled with flying shadows and only a continuous "rain", then "storm" of bat calls could be heard (through the detector of course). This storm has it's "gusts" as well as calmer moments. We didn't see an actual dense swarm but there were moments when it seemed hundreds of bats were swirling around, some flying quite low overhead. The entrance to the mine is protected by a massive iron gate to keep humans out, and we were assembled a respectful 20 feet away. With the help of a powerful flash light the bats could be seen flying in and out like bullets, passing through the openings of the gate at speeds that, for a human, would be quite imprudent. The light didn't seem to bother them too much -- maybe it allowed them to go even faster!

Sample recordings in compressed mp3 format.[1]

Three seconds from the echolocation pulse train of a bat passing overhead
a005 image and sound

Six seconds from a bat starting to circle overhead
a075 image and sound

Four seconds "gust" of overlapping bat echolocation calls.
a155 image and sound

Ten seconds cut from the thick of the "storm". The storm is relentless, I had half an hour of this on tape before I finally left!
a300 image and sound

Reported by V. Goffin.

[1] If your browser doesn't already play mp3 compressed files, you may want to consider configuring it with WinAmp or freeamp.