When my chicks gobbled up my baby rhubarb I did some search on this. It's the oxalic acid that could potentially be toxic at a very high dose. With a normal dose the main problem is that it blocks calcium absorption. It's also contained in chards, beet leaves and spinach and orach, and of course oxalis, all of which my chickens loved.There is a bit of a debate about whether chickens should be eating rubbarb. From what I've read it is in fact toxic but to reach a level of toxicity to be life threatening they need to eat an awful lot. I haven't managed to find out exactly what an awful lot is. I know some chicken keepers use it to combat internal parasites. Other keepers report no ill effects apart from a runny bum if the chickens have eaten a lot. In the past, some days they've eaten a bit and moved on to other things. Two in particular have been doing some very watery poops and are eating more than the rest. I'm going to err on the side of caution.
Apparently in eastern country they make sure to eat yogurt if they eat rhubarb? Maybe it's an old wife's tale.
At the time Gaston's team was not laying so I worried more about my rhubarb than about them. It did them no harm that I could see.