I have been very intrigued to watch these young blackcaps in the garden this week. They were eating woody nightshade berries which are poisonous.
Immediately after eating the berries they were flying down to a patch of bare soil and picking up small bits of earth. At first I wondered if they were looking for insects but it was clear they were swallowing the soil.
To prove what they are up to here are a couple of brief video clips, the first with a tree sparrow joining in.
Soil eating is known as geophagy, literally earth swallowing, and is observed in many types of animal. It is common behaviour in many herbivorous mammals as a way of providing essential minerals otherwise missing from their diet. I found an interesting article by Jared Diamond reviewing reasons why birds (in his case parrots) might eat soil. They include 1) to assuage hunger, 2) to help grind up swallowed food, 3) to buffer acidic or alkaline foods, 4) to counteract viruses or bacteria, 5) to provide essential minerals, and 6) to detoxify plant poisons.
Of these I wondered if the last is the explanation in these blackcaps. Woody nightshade berries (Solanum dulcamara) are toxic and the other birds seem to avoid them. Then I found this extract in a book called Birds and Berries by Barbara and David Snow.
So it seems that blackcaps deliberately seek out the woody nightshade berries. I think the probable explanation for what I have seen is that the blackcaps are eating the poisonous fruit and then eating the soil as an antidote. The question is why don't they just eat the blackcurrants, gooseberries and raspberries like the other birds do? They must get something out of it. Any comments will be welcome.