Why my website get many "/favicon.icon" request?

I know that the browser will automatically request "/favicon.icon".

But many requests did not visit my page, it only visit "/favicon.icon".

They don't hava Referer and User-Agent.

The nginx log like this:

47.88.135.135 - - [02/Sep/2019:03:31:22 -0400] "HEAD /favicon.ico HTTP/1.1" 404 0 "-" "-"
47.74.138.142 - - [02/Sep/2019:03:31:24 -0400] "HEAD /favicon.ico HTTP/1.1" 404 0 "-" "-"
47.254.113.21 - - [02/Sep/2019:03:31:24 -0400] "HEAD /favicon.ico HTTP/1.1" 404 0 "-" "-"
64.71.142.65 - - [02/Sep/2019:03:31:25 -0400] "HEAD /favicon.ico HTTP/1.1" 404 0 "-" "-"

Then I tried to set the "favicon.icon" file, but I still receive a lot of "/favicon.icon" requests. And the nginx log like :

161.117.143.163 - - [02/Sep/2019:06:25:26 -0400] "HEAD /favicon.ico HTTP/1.1" 200 0 "-" "-"
64.71.142.65 - - [02/Sep/2019:06:25:26 -0400] "HEAD /favicon.ico HTTP/1.1" 200 0 "-" "-"
47.91.195.140 - - [02/Sep/2019:06:25:27 -0400] "HEAD /favicon.ico HTTP/1.1" 200 0 "-" "-"
80.231.126.202 - - [02/Sep/2019:06:25:27 -0400] "HEAD /favicon.ico HTTP/1.1" 200 0 "-" "-"
47.244.73.59 - - [02/Sep/2019:06:25:28 -0400] "HEAD /favicon.ico HTTP/1.1" 200 0 "-" "-"