Burrowing in a winter wonderland (Image: Bernd Heinrich)

Whether it’s making snow angels or building a snow man, most of us love playing in a winter wonderland – and it turns out some birds do too.

It usually starts with one bird that burrows in the snow, creating tunnels. Another joins in, then a few more, until the entire group is doing it, leaving behind a maze of burrows and furrows.

Bernd Heinrich, professor emeritus at the University of Vermont, studied the peculiar behaviour in Western Maine. He observed a flock of about 150 redpolls (Carduelis flammea) making at least 252 cavities and short tunnels between November 2012 and February 2013, and has just published his findings (Northeastern Naturalist, doi.org/xvj).


But exactly why they do it – or what triggers the behaviour – is still a mystery.

Birds dig it

“I have wondered a lot about what the trigger is, and I have not seen anything obvious,” says Heinrich. Although, he says, there seems to be a social aspect to it. “Where one does it then a lot of others do too.”

He didn’t see any evidence that redpolls are seeking food in the snow and there wasn’t any vegetation near the tunnels. The birds were also unlikely to be bathing, as they were very clean, and they didn’t appear to be taking shelter – so were they just having fun?

“Play is defined as behaviour with no immediate function, so in that sense, yes, it is ‘just’ play,” Heinrich says.

But he suggests snow tunnelling may be adaptive.

“Snow tunnelling may have a function way up north in the high Arctic – their home: the shelter from cold,” he adds.

Spending the night in snow tunnels may help them survive the freezing nights in the Arctic tundra, even though it would also make them vulnerable to burrowing predators, such as short tailed shrew, and could potentially leave them in an ice tomb.

Beaks under blankets

In the Arctic, Redpolls can be quite certain when they duck down under the fluffy snow blanket that it will still be fluffy the next morning because it rarely thaws.

But the snow regularly thaws in Maine, and the icy crust that develops as temperatures drop overnight would trap the birds. “The best adaptation becomes the best death trap, if conditions change,” Heinrich says.

The fact that redpolls in Maine don’t go under the snow at night suggests they have some inhibition to chance it.

“They may recognise the forest here is not the Arctic tundra, and maybe also they have some temperature or hunger threshold before they would risk all by spending the night in the tunnel,” says Heinrich.

Reference: Northeastern Naturalist, DOI: 10.1656/045.021.0404