The first time I saw this European Goldfinch, he was hanging out peaceably with a bunch of House Finches (see next photo) at Maintenance Field in the Ramble. He continued in the company of the House Finches for a month or so, until the House Finches started singing. After that, I've seen him solo a few times. I think once he started singing, they realized he was some kind of foreigner and chased him off.