In which circumstances is the identity of a miner known? I started playing with the Blockchain real-time API and noticed that when new blocks were created, they often (systematically?) included a foundBy field. For instance, block with height 446731 (index 1442483) has the following foundBy information:

{
    "ip": "0.0.0.0",
    "link": "http://bitfury.com/",
    "description": "BitFury", 
    "time": 1483612060
}


I have never mined bitcoins; do all miners have to disclose their identity? Obviously, the identity of the miner of block with height 0 is unknown. Is the foundBy field a new feature of recently-mined blocks?
No, it's not necessary to disclose the identity. As Pieter commented below, "foundBy is a feature of blockchain.info, a privately operated website, which gives a guess about the creator of a block. It is by no means something that's part of the protocol". 

If you look at the receiving address of the block's coinbase, you'll see that BitFury sends all their block rewards to that address, so that's probably how blockchain.info arrived at their conclusion.