Some sort of Brassicaceae species. The Brassicaceae includes: Radish(Raphanus raphanistrum), Winter Cress (Barbarea vulgaris), Turkish rocket(Bunias orientalis), White mustard(Sinapis alba), Pepper Cress(Lepidium sativum), Honesty(Lunaria annua), Dame's rocket(Hesperis matronalis), Wild Rocket(Diplotaxis tenuifolia), Wasabi Arugula(Diplotaxis erucoides), Arugula(Eruca vesicaria), Suneson Camelina(Camelina sativa), Garlic mustard(Alliaria petiolata) - many others. There are also of course
- the Brassica species as well.
None of what I listed seems to resemble what you have exactly.
I am having a hard time thinking of any Brassicaceae species that include toxic plants - some of the types that I mentioned are best eaten in small quantities, possibly oxalic acid issues - nothing that will kill you though. Whatever you have should
be safe for the chickens to eat. Suppose you could try saving seed considering it is already flowering - see what younger plants look like, could be a rare species as well. Could work in a permaculture
setting considering it is probably edible and reseeds itself.
Good luck on finding the exact genus / species though.