Hello my chickadees!
I get asked this question a lot, and just did again, so I thought I'd share the latest:
Q: "Can I keep hens in the backyard in Cambridge, Somerville, or Brighton? If yes how many hens are allowed? What are the requirements (permit....etc) ?"
Somerville is the most permissive of the three - in that they are the only one that actually has an active permitting process that allows residents to keep chickens - and the details are here. Click on the "rules" tab, and note that I've updated my space recommendations since I wrote the Chickens section of this ordinance and now strongly recommend 12-15 square ft of secure "outdoor" run space per bird, and 2-3 sq ft of "indoor" coop space per bird. For more coop construction info see my previous blog post here.
Boston passed an urban ag ordinance in Dec of 2013 that I also helped write, but unfortunately it didn't end up being as useful for chickens as we'd hoped, mostly because each town within the City of Boston has their own prohibitions on livestock. See this page for info on Article 89 and note that while it is technically possible to get a permit for backyard chickens in the city of Boston the process is prohibitively challenging for most. That said, many people with City limits have chickens, and as long as you play nicely with your neighbors (i.e. be super respectful of space that isn't yours, noise, waste management, aesthetics, etc) the City usually isn't going to come hunting down your renegade chickens. BUT, the reality is that you don't have any recourse if something happens and someone complains, so I strongly recommend at least trying for the permit, such that even if you don't go all the way through the process you have designed your coop setup to abide by as many of the requirements as possible (setbacks from property lines and neighboring buildings, size recommendations, flock size, etc).
As for Cambridge, they developed an Urban Ag Task Force a few years ago to start looking into adding some Urban Ag-focused zoning, and passed a zoning amendment allowing the keeping of honeybees in Dec of 2017, but it doesn't look like they've had much growth since then. So, officially, keeping chickens is prohibited in Cambridge. That said, many people in Cambridge also have chickens, and treat it similarly to Boston - be sure you have enough space and play well with neighbors and you likely won't have an issue BUT you have no protections if something comes up. When in doubt, abide by the recommendations in the Boston chicken permit for setbacks and such to be most certain that you're being as non-invasive as possible and you'll raise your chances of being able to keep your girls if you do get "discovered".
I hope that helps. Let me know if you have any other questions, and good luck!