Pigeons on the Pill

When I was in Paris last, a tour guide in Notre Dame said that the city fed pigeons birth control tablets ground up in feed to keep the pigeon population down.

I’m assuming the pills are made of progesterone, like a lot of versions of the Pill. If this is true then birds give off the same chemicals in pregnancy as humans? If not, why does this human hormone affect the birds?

To control pigeons (and rats, incidentally), they use a chemical called Mestranol. Mestranol is a synthetic analog of the natural estrogen, which helps regulate the ovulation cycle. Estrogen presence in the body during pregnancy (in humans, anyway) spikes to prevent other eggs from being released - high estrogen levels, no eggs, no pigeons.

And yes, this is one of the estrogens that’s present in some birth control pills… and it’s a carcinogen.