I've seen evening grosbeaks, crossbills and pine siskins at fire pits,
mineral springs, and road cuts eating ashes and/or soil. I've always
hypothesized that they need minerals to augment their (in many cases) 100%
seed diet, assuming other birds get their minerals from their animal
(insect) prey, or, for other birds that eat only plants, from nectar,
leaves and/or fruit. These cardueline finches also drink more than other
birds, I think, perhaps again because seeds don't furnish sufficient water.
There is probably stuff about this in the literature, but I can't think of
a reference.

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Dennis Paulson phone: (206) 756-3798
Slater Museum of Natural History fax: (206) 756-3352
University of Puget Sound email: dpaulson at ups.edu
Tacoma, WA 98416