On journeys through the woods, I often take note of any dead pigeons I see (a bit morbid but there are a lot this year in particular, I reckon the buzzards are back). Something I noticed is a common colour between the stones; they all seem to be orange stones. Probably to do with local rocks available; there is a stream on site and quite a lot of orange flint available, but I wonder if they select based on colour. Has anyone else noticed something like this? They’re woodpigeons in the UK if that makes a particular difference.
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I wonder if maybe they select some orange stones of a type that stains (or something else they eat is orange) and over time, the other stones become stained. Kind of like white laundry becoming pink if you wash it with colors.
There’s a commercial pigeon grit fed to domestic pigeons that is a rusty orange color from added iron. I always assumed that was as a supplement to prevent anemia, but maybe they found pigeons prefer that color?
Not sure, just a couple of guesses.
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Fascinating question. I found this as a possible answer: Why birds eat colourful grit: colour preferences revealed by the colour of gizzard stones
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Different coloured stones are different minerals, different hardness. Like choosing what works for a millstone when grinding flour? Not just the aesthetics of colour, but how effective the grit is in their crop.
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I also found a tangential article, “Gizzard Stones from Adult White-Tailed Ptarmigan (Lagopus leucurus) in Colorado” which in the abstract stated some interesting findings about the mineral contents of their gizzard stones.
Quartz, the Colorado Lagopus leucurus population’s “favorite” type of stone, is a Moh’s hardness of 7 (pretty high), so I assume some birds choose from available stones with the greatest resistance to scratching. I wouldn’t want to have a stone so scratched it cracks in half in my digestive tract! They choose to go round with bigger ones I assume to avoid getting their throats sliced open. Also, like humans, they need different types of “teeth” to digest their food (molars and large millstones for grinding, sharp and small ones for cutting).
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