'Totally Unreal' - Hawk Seen In HCT Otter Den Baffles Experts

HARWICH – Let's say you are a private detective, working an otter den case at the Robert F. Smith Cold Brook Preserve. Your job is to keep an eye on comings and goings in what may be North America's first human-made den. Luckily, you have an ace up your sleeve: a motion sensor camera that records a short video clip when a critter checks out the digs.

Perhaps you sit in the Cold Brook parking lot, sipping coffee, checking the camera feed and admiring the results of the recent Harwich Conservation Trust (HCT) eco-restoration project that transformed the Preserve into a wildlife oasis.

You see the usual suspects popping into the den pretty regularly, including a rabbit and a muskrat. An otter scoped out the den on Oct. 6 but hasn't been seen since.

The stakeout is settling into a dull routine, but then something shocking happens. On Nov. 11, a Cooper's hawk trips the camera in the den. It seems impossible, but feathers don't lie. You dial up HCT headquarters, and suddenly the investigation takes on new life.

"I am not sure where to start regarding the Cooper's hawk entering the otter den other than to say the occurrence is totally unreal," wrote HCT Trustee Herb Raffaele in an email. "If it were not for the photo, every raptor specialist in the world would likely treat the report of a Cooper's hawk behaving in this way as nonsense."

Raffaele, who researched and designed the HCT otter den, is also a noted ecologist, author of books on wildlife conservation and the birds of the Caribbean, and former chief of international conservation for the U.S. Fish and Wildlife Service.

To understand why the bird's behavior seems so unusual, let's take a look at the den's design. The subterranean chamber, made of cinder blocks, is connected to the outside world by two buried pipes, each with an eight-inch diameter. One of the pipes runs 20 feet from the den into a pond at the Cold Brook Preserve to provide water access for otters. This pipe is straight.

The other pipe connects the den to the upland. It's about 18 feet long and runs in a zigzag pattern, surfacing in the upland at about a 20-degree angle. This is likely the route that the Cooper's hawk took to get into the den — a long, dark journey into the unknown.

It's a stunning turn of events that may have no precedent.

"A friend of mine who is a world expert on accipiters (this group of hawks) said the closest case she knows of was a falconer who trained Cooper's hawks to follow wood rats into their small dens, but these were trained birds — a big difference," wrote Raffaele. "I looked through the ‘Handbook of Birds of the World’ along with a number of other references and there is no mention of Cooper's hawks ever entering a burrow — not to mention an 18-foot one."

‘Spelunking Down A Dark Tunnel’

Mark Faherty, noted bird expert and science coordinator at Mass Audubon's Wellfleet Bay Wildlife Sanctuary, said he was "flummoxed and perplexed" by the bird's expedition. 

"There's nothing about Cooper's hawk behavior that makes this make sense to me," he added. "This hawk was just straight up spelunking down a dark tunnel, with no idea what was in there."

Faherty said Cooper's hawks are known for their ability to fly quickly through dense wooded areas on a quest for prey, which is mainly smaller birds. They also eat some smaller mammals, like chipmunks and squirrels.

He thinks that the hawk was likely seeking a snack in the pipe, as opposed to following some sort of general curiosity. 

"There's one thing on a hawk's mind, and it's the next meal," said Faherty. "It wasn't house hunting, it was going after prey."

While the camera provides proof that the hawk was in the den, there are still many unknowns. The short video shows the Cooper's hawk striding into the pipe that heads toward the pond. Connor O’Brien, HCT's director of land stewardship, said that since the pond is tidally affected, the hawk may have been able to exit via the pond pipe during lower tide times. This line of speculation would mean the hawk made a nearly 40-foot underground journey.

The theory gains some credibility due to the fact that the hawk only triggered the motion sensor camera once, on its way into the pond pipe. Had it turned around, it's possible that it would have again triggered the camera as it re-entered the den.

It also does not appear that the hawk somehow got stuck in one of the pipes. About half an hour after the hawk passed through the den, a rabbit entered via the upland pipe. Five hours later, a muskrat is seen entering the den from the pond pipe. If a stuck hawk was blocking the pipe, it seems fair to assume that one of those animals would not have entered the den.

So, what we have here is a first-class hawk mystery. In an attempt to glean more insight, HCT is planning to install an additional camera outside the den, near the upland pipe exit. Meanwhile, birding experts are chirping about the hawk's amazing adventure.

"It makes me claustrophobic just thinking about what this bird was doing," said Faherty. "This brave pioneer of a Cooper's hawk, just heading down the tunnel."