EN LAS MADRIGUERAS DE LOS CASTORES

So recently I was contacted by a beaver watcher in Navarra Spain who asked some questions about lodges and food caches.  I told her my thought that beavers were unlikely to use food caches in that climate. It just so happened that around this time a European beaver buddy shared a notice for this presentation on beavers. So of course I sent it her way.

To my surprise, she very excitedly attended and really enjoyed it, sending me the handouts afterwards. One of them was a list of articles on castor fiber which started with Duncan Haley’s population paper. I thought he might like to know this so I sent it his way. He was very pleased and sent along some of his observations about the issues I thought you’d be interested in. (Pause a moment to check out the temperature recorded by the nightcam.)

It’s my experience that Eurasian beavers prefer to dig burrows when they can (it seems N. American beavers are more prone to building lodges; and it seems only them that build ‘island’ lodges out in the water – all Eurasian lodges I have ever seen are bankside.

So if a deep enough bankside of soft enough material for digging is available, with deep enough water beside it (about a metre or more), that is where they will burrow. Over the years I have gotten so I can (and have) predicted where the burrow would be in a site occupied by beavers, by looking at an overview of the terrain available.

The roof of the chamber at the end of the burrow is shallow, and air can get through by diffusion. It is so shallow that it often collapses by itself (and will almost always do so if e.g. a tractor drives over the top). If that happens then it is roofed over with sticks and mud. Sometimes this is called a ‘burrow lodge’ – the lodge part (of sticks and mud) being connected to the water by the burrow.

Where it’s not practical to dig a burrow – where the soil is shallow or stony, or the terrain so flat that there is no rise high enough to burrow in and build a chamber above water, then Eurasian beavers will readily build a lodge. They also usually move their main burrow/lodge site quite frequently, moving it around within the territory and exploiting heavily the resources nearest the lodge, before moving and refocusing (while the boundaries of the territory seem to be stable). So there are usually several burrow or lodges, or frequently  a mix of types, inside a territory.

It is said in the literature that the whole family lives in one lodge most of the year, but that only the breeding female and the young use it after she gives birth. At that time the others use old or secondary burrows/lodges.

Beaver food caches look like a big pile of sticks in the water (see attached photos), and are always found immediately outside the main lodge/burrow entrance. In cold climates like Norway, it is one good way of finding active burrows. Research needs to be done on what triggers beavers to build food caches. Anecdotes say that northern beavers reintroduced in the south prepare for a northern winter the first year, but then adapt. Where currents keep water open, beavers are active in winter even when it is very cold, down to at least -27C in my experience. In climates like Navarre, most beavers will not need to make winter food caches. It seems that most in such places do not build them. I would be interested to know if any of them do, and if it is related to altitude. In the 2008 photo, the lodge is on the right (the large sticks, often stripped of bark). The cache is the mass of twigs in the water on left. Most of the mass is underwater, like an iceberg (and so available even after the surface freezes).

The ‘North Lodge’ photos show a large beaver lodge just in shot to bottom left, and a very large food cache when it is at its biggest in early December; a beaver clearing a path in the snow to allow it to add material to the lodge; and a beaver pulling a branch up onto the lodge in the foreground while another feeds at the food cache.

Duncan Haley, Ph.D. Norwegian Institute for Nature Research | NINA

Don’t you feel better now? My Spanish pen pal  was very grateful for the information and now is fully committed to learning more about beavers. So I feel my work here went very well.

By the way, this was the other handout at her conference. It’s the introduction to the epic poem by Henry Wadsworth Longfellow “Hiawatha“. Reading the Spanish version makes me think he might just have preferred it.