This site offers a large amount of information on bird skulls and anatomy, displaying species from all over the world.
No bird has been killed to get its skull or skeleton for this collection. All wild birds died of natural causes or accidental, and in a few cases due to legal hunting by others, or died in captivity. The Experimental Zoology Group of Wageningen University has been authorized by the Dutch authorities to own protected European birds after their death. The permit nr is: FEF 27/06/2/98/0051. The Wageningen University also has a CITES exemption, nr NL004.
Who wants to keep (part of) a found bird should inform oneself about the local laws on bird protection.