The animal welfare movement is making remarkable progress, which is surprising in a way, because (1) it’s quite small and (2) not much is known about which strategies and tactics are the best ways to reduce the suffering of farm animals. As I wrote last fall, it’s unclear whether lobbying governments, pressuring companies, persuading consumers to eat less meat or doing something else entirely will have the most impact.

So, along with more resources, what the animal welfare movement needs are learning organizations. The Humane League is one. A small group devoted to protecting farm animals, with about 25 full-time staff and a budget of only about $1 million in 2015, The Humane League invests in testing its tactics and messages, to find the way to do the most good at the lowest cost.

“We are just obsessed with metrics, making sure we are being as effective as possible,” says David Coman-Hidy, the group’s executive director.

The Humane League has a small unit called Humane League Labs that tests messages around vegan advocacy. It does grass-roots and campus organizing in 11 cities across the US, distributing more than 1 million pro-vegetarian or pro-vegan booklets a year. And it runs hard-hitting corporate campaigns.

The Humane League is one of a relatively small number of groups working on farm animal protection. The Humane Society of the United States (HSUS), which has a broad agenda and often works with big companies to improve their animal welfare practices, is by far the biggest (2014 revenues: $186 million). Then there’s PETA, which is proudly uncompromising, opposing pets, zoos and drug-testing on animals, as well as eating meat.

The Humane League, Mercy for Animals and Animal Equality focus exclusively on farm animals. They were selected as the three “top charities” in the animal welfare movement by Animal Charity Evaluators (which I wrote about here). (Disclosure: I donated to ACE’s top charities in 2015.)

In contrast to nonprofit sectors where advocacy groups compete for funds and attention, the farm animal protection groups play together nicely, at least according to Coman-Hidy. “We all talk to one another, and share campaign strategies and tactics,” he says. All are relatively new–the oldest, Mercy for Animals, dates back to 1999 — and lean. Together, they spend between $5 and $10 million a year.

Yet the movement has won a slew of victories as the groups campaign to end the practice of raising hens in cages. Just in the last year or so, three big food-service companies — Sodexo, Aramark and Compass Group — agreed to switch to cage-free egg supply chains, away from the more abusive cage-egg production systems. So did McDonald’s, Dunkin’ Donuts, General Mills, Nestle and Costco. This month alone has brought similar commitments from ConAgra, Mondelez International, Target and Denny’s which found humor in the news.

Says Coman-Hidy: “Literally millions of hens will no longer face a lifetime of suffering in a cage, thanks to these new policies.” Cage-free eggs are far from a panacea for chickens, but they are a meaningful step in the right direction. (For more, see my story for The Guardian, Why the egg industry is scrambling to set hens free, and this comparison from HSUS.)

Among the animal welfare groups, The Humane League is known for its hard-hitting campaigns. “We’re very aggressive,” says Coman-Hidy. In the recent campaign against Costco, for example,The Humane League produced a video in which hundreds of college students promised never to shop at the warehouse store, leafleted outside Costco stores and organized online petitions. HSUS, for its part, obtained and released undercover footage exposing abused chickens at a Costco supplier and filed a shareholder proposal asking the company to disclose any risks it faces around its animal welfare practices. Supermarkets are likely to be the next targets for the cage-free coalition, so the victory over Costco, a big food retailer, is significant.

But are corporate campaigns the best use of The Humane League’s resources? It might seem so because persuading a few companies to change their practices can influence the lives of millions of animals. It’s harder, presumably, to convince thousands of consumers to change their eating habits. On the other hand, the corporate commitments to buy only cage-free eggs bring only incremental improvements to the lives of chickens, who can still be required to live their entire lives indoors, in crowded conditions. Persuading more people to adopt vegetarian or vegan diets has a deeper and longer-termimpact. It’s also hard to know whether corporate campaigns build a bigger, more powerful animal welfare movement or, alternatively, lead to complacency among consumers who feel satisfied because chickens have been liberated from their cages. These are the kinds of questions that confront the movement.

The Humane League isn’t tied to any single approach. For a time, its campus organizers focused on cage-free campaigns. But, after seeing campaigns around Meatless Mondays working elsewhere, they shifted gears, deploying their organizers to urge K-12 public school systems to go meatless on Mondays. They had success in Philadelphia, Boston and Baltimore.

“We try to be nimble,” Coman-Hidy says. In its review of The Humane League, Animal Charity Evaluators says:

The Humane League has one of the best understandings of success and failure that we have seen among animal advocacy organizations. They actively work to evaluate their own programs’ efficiency, quantitatively when possible, in order to determine what is working best and what they need to do less of or modify

At Humane League Labs. the organization studies such questions as “Which request creates the most diet change, ‘Vegan,’ ‘Vegetarian,’ ‘Eat less meat” or ‘Cut out or cut back on’ animal products?” and “Which vegan meals to omnivores find most appetizing and accessible?” (Hint: It wasn’t the “tofu scramble.”) Coman-Hidy told me that the ethical arguments against eating meat work seem to work best with young people, health arguments for a plant-based diet strike a chord with older people and “the environmental argument is the least powerful, unfortunately.”

Coman-Hidy is quick to note that these aren’t peer-reviewed, scientific studies, but says The Humane League has raised some earmarked money to hire a full-time researcher and statistician. He’d like to be able to test out campaigns in different geographies and then buy supermarket data to see which worked best.

“We’re in the age of Big Data and we owe it to animals to take advantage of it,” he says.

Like most nonprofits, The Humane League also relies on data to do outreach, recruit members and raise money. It’s a big user of Facebook ads, and has found that Spanish-language ads deliver the highest return.

This ad, for example, which is headlined “End animal abuse,” and asks, “Have you seen that millions of people are changing the way they eat?” “Watch the shocking video!” it says. “See how the animals are treated before reaching our plate.” It reached more than 500,000 people and, more importantly, cost just 39 cents for each person who watched the video and then ordered a vegetarian starter kit. Another ad required spending $2 for each person who ordered the vegetarian kit and was, of course, dropped after testing.

The Humane League is in growth mode, Coman-Hidy says. Just 26, he joined the group five years ago, becoming its second full-time employee, working with the founder, Nick Cooney. (An author and activist, Cooney now works for Mercy for Animals and wrote How To Be Great at Doing Good, an excellent book about effective altruism. He’s on The Humane League board.) The Humane League has expanded steadily, using donors or groups of donors to finance new local offices which over time become able to sustain themselves. This year, it expects to spend roughly $2 million, about twice what it spent in 2015. That’s good news for farm animals, and for those who care about their mistreatment.