You’ve raised a number of interesting issues here, thank you!
I have a doubt about clay ingestion. I observed in Andes,that Local People use a special coal made of special tree bark assiciated with a special clay. This is used when they have to go in high places above 4.000 metres. This mix is used to potencialize the benefits of Coca Leaves. So, Calcium and Magnesium elevates the PH. Alkalinize.
As it turns out clay ingestion, charcoal ingestion, and lime (or other alkaline materials) ingestion are potentially quite different. Here’s a brief review in case it’s of use:
Clay can do any number of things in a vertebrate gut, releasing minerals and adsorbing charged particles are the best known, and since they take place virtually simultaneously, it’s hard to get one without the other, regardless of ‘why’ the bird or mammal chose to eat the clay in the first place. But clay also causes cellular and acellular changes in the lining of the gut, and these changes can have a profound influence on digestive efficiency, on the ability of the gut to sustain chemical insults from dietary components, as well as other functions, mostly unstudied.
Charcoal which is, broadly speaking, cooked rather than burned wood, has an incredibly high surface area and has an affinity to most small compounds regardless of their electrical charge. That’s why it is so often used in emergency treatments of humans who have ingested dangerous chemicals - it functions like a chemical sponge, soaking up all sorts of things in various parts of the gut.
Alkaline materials such as lime can, as you say, make certain plant constituents available to vertebrates - and yes, that’s why coca leaf and betel nut and other plants are consumed with lime. In fact, it’s the same with corn which in parts of Central and South America is treated with lime to become masa harina - the lime adds calcium of course, but its alkalinity also makes the available niacin go through the roof, and therefore the corn becomes far more nutritious. Few clays we’ve tested so far have the high pH required to provide this kind of role for the parrots.
The extinct Scarlet Macaws in North Bolívia, based in ex-trapper information, used to eat some “salts” in Bat´s feces deposits in tree holes. The trapper used to fix a trap in tree hole, and wait the birds come to “salitrar”, and then he trap a Macaw by neck. All Scarlet Macaws in this area were trapped in 80´s by this way. This is a very curious fact.
Of course macaws and other parrots do enter cavities which aren’t their nests, however in my experience, that’s been about drinking fresh water. We’ve also found soldier fly larvae in the crops of chicks in Peru, and those kinds of larve are often to be found in the bottoms of cavities (including in parrot nests) because they love the moist piles of bird and bat poop typically found there. It was never clear if the adults were grabbing these larvae in their own nests, in other cavities, or if the chicks (which where big enough to do so) were possibly picking them up off the cavity floor. (If the last is true, it’s a bizarre and amusing sort of recycling!)
That said, there have been some interesting and weird behaviors seen at cavities, notably a group of Pionites leucogaster which frequented a cavity near Puerto Maldonado (on Lake Sandoval if memory serves). It seemed at the time that there was more going on there than just drinking - too habitual and too many birds, etc. I wonder if this cavity had a huge bat roost in it and therefore a similar sort of salitrar. We ran a picture of these guys on the back of a PsittaScene with a caption noting that it was never clear what was going on here (http://www.parrots.org/pdfs/our_publications/psittascene/2003/03Feb54.pdf)
A family of bats (or families depending on the size of the cavity) can produce quite a lot of excrement over time, and the insects living in the bottom of the cavity can process that all very quickly. It would be wonderfully bizarre for macaws to consume either fresh or composted bat guano as a means to acquire sodium, however, stranger things have happened. My guess is that, if they are in fact consuming such delectable delights, there’s more going on here - pursuing insects or protein perhaps - and further study would be of great interest.
Eating feces has a name incidentally - coprophagy (or allocoprophaghy if it’s someone elses feces , it’s something our close relatives the gorillas do with some frequency (http://cat.inist.fr/?aModele=afficheN&cpsidt=14573986), and of course it’s very widespread in small herbivorous animals like rabbits and guinea pigs if I’m not mistaken. Some even produce two kinds of feces, one to eat and one to not eat (night feces or cecotropes). To me, it would seem that eating bat feces, however processed, would be asking for serious disease and parasite problems, but as this primate paper suggests, there could be some upsides to that as well.
OK, this is beyond gross, but deeply intriguing!
All best wishes,