Monday, September 24, 2012

Bird-Brained Rheologists

I ran across a couple of reports 1 and 2 (see pages 74 and 75) that both state that mud daubers and swallows are able to build their mud-based nest by taking advantage of thixotropy. From reference 1:

"Mud dauber wasps build little cells of mud, stuck to walls. When they add new pellets to the cells, they add a bit of water from their crop, and they buzz. The vibrations liquefy the mud, letting it spread into the earlier, drier pellets. Then old and new pellets vibrate together, achieve the same consistency, and are stable when vibration stops.

That brings us back to barn swallows. They collect mud pellets from puddles and gradually add several rows of pellets to form the nest cup. But they apparently don’t just whack each new pellet into place. Instead, when they add pellets to the growing base, they use a dabbing or dabbling motion. This jiggles the old (drier) and new (wetter) pellets until the water content is similar and their consistency is equalized. As soon as the dabbling stops, the junction of new and old pellets becomes stable. Wouldn’t it be fun to find out if young adult barn swallows know to do this automatically or if they have to learn the hard way (if their first nest-building attempts collapse)!"

I'm having a difficult time finding a primary literature reference documenting this behavior and more importantly to my interests, the rheology of the mud. (Without having rheology curves, we don't know if this is thixotropy or shear-thinning or both, but thixotropy is such a cool term, let's just go with it.) While I don't doubt that something like this is occurring, I have a few questions:
  • It has been my observation that swallow nests are not built in a single day. This would then mean that the uppermost layer of the mud in the partially built nest would be quite dry the next morning. How is this large disparity in the moisture content addressed?
  • What is the nature of the mud at the very base of the nest - the part that is in contact with the house/cliff/barn...? This mud must not only adhere to the external structure, but also to the rest of the nest. (It is the original "tie-layer" or compatibilizer, isn't it?) How is meeting this disparity in requirements accomplished?

I would love to hear from any serious ornithologist or entomologist about this, particularly if they can provide any primary research about the nature of the mud(s) used in nest building.

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