UC Davis has a list of toxic plants to livestock, table 3 has notes on toxicity levels. These are for horse, cattle, and sheep.  https://alfalfasymposium.ucdavis.edu/-files/pdf/livestockpoisoningplantsnov2010.pdf

A retired OSU poultry professor has said as long chickens have feed available the probability is low that they will eat toxic plants.If you are concerned, you may need to fence off a portion of your yard for the chickens to use and remove all the toxic plants of concern from that part of the yard. It would be rare that chickens eat toxic plants of any kind unless they are quite hungry. 

Oregon State University  information on Creeping Buttercup. This  is a resource for cattle and sheep producers and includes information on managing the plant. http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/16924/pnw399.pdf 

Another good resource is University of California Veterinary Medicine program:
https://socalnestbox.com/foods-toxic-to-chickens/

https://ccah.sf.ucdavis.edu/about/ucd-toxic-plant-garden

I reached out to UC Davis and the NestBox Team with your question. Here was the response: 


We asked Dr. Olukayode Jegede, a toxicologist that we work with at UC Davis. This was his response:

Yes, both plants are toxic to birds (including poultry). The creeping German buttercup is toxic to all grazing animals, including poultry birds. There are many sources pointing to this, and poultry birds usually avoid these plants especially when they have other plants on the range/backyard that they can feed on.
I saw this simple blog with a list of poisonous plants to birds: https://everythingbirdsonline.com/toxic-plants-for-birds/

We currently do not have a list of plants that are toxic one our website, but we do have a list of toxic foods that might be of interest to you since it includes some common edible plants: Foods Toxic to Chickens - SoCal Nestbox. We plan to write both an article on toxic ornamental plants and an article on toxic invasive and native wild plants so this information is more easily accessible in the future.

If you suspect your chickens have ingested either of these plants, please contact your local veterinarian immediately or contact one of the two animal poison control centers (https://www.petpoisonhelpline.com/ or https://www.aspca.org/pet-care/animal-poison-control).
If you decide to remove these plants, be cautious as even fragments of these plants remaining in the soil can continue to grow. You may need to remove them repeatedly over multiple seasons if they continue to regrow. Be sure to get the root pieces out intact.
Best regards,
Joseph GendreauSoCal NestBox Team

You can also reach out to Dr. Olukayode Jegede (<personal data hidden>) if you have more questions.

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