Suppose a piece of software is distributed under some permissive license, e.g. BSD. Now, I modify this software and want to distribute my derivative work. My questions:
Who is the owner of the derivative? Is it the original author, the new author or there is joint ownership?
You own the copyright to the portion of the code that you modified.
The previous contributors own the copyright to the portions they created*
Joint ownership is a good way of looking at the copyright on the source code for the project. Keep in mind that copyright is separate from the license of the project. In this case, copyright is ownership of code. License is the rules others have to follow in order to use that code.
Is a copyright notice (say under the original copyright) sufficient to establish the new ownership?
Yes, although it is not necessarily required under the Berne Convention.
The caveat to all of the above is that some projects may require you to assign copyright to the project before they'll accept your contribution in the main branch. This note from the US Copyright office discusses how copyright may be transferred to another.
Since it sounds like you're forking the project, there probably isn't any concern about the original project requesting copyright ownership of your modifications. I primarily wanted to point out that copyright can be transferred.
Under the Berne Convention, everyone owns their own work. So you own your changes, and the original authors own theirs. You can't change their ownership.
If you really care, you should add your copyright to the copyright notice for any file you modify, leaving all other copyrights in that file and in all other files intact.
The short answer is that you can only claim ownership/copyright for that what you have created yourself.
The longer answer is that if you contribute to an open source project, then you can claim copyright for the modifications you have made and the copyright on the rest of the code remains with their respective authors.
A source code license does not give you ownership of (the copy of) the code, but it only gives you certain rights on what you can do with the code.
How to indicate that you have contributed to the code depends a bit on the license under which the code is released and the project itself. Sometimes all the contributers to a file are mentioned in the copyright statement in that file, and sometimes there is a file called AUTHORS (or something similar) that lists all the contributers to the entire project.
If you are unsure how to claim your attribution, first check how it is done in the project you are contributing to, then check if there are any requirements imposed by the license and if that still doesn't help you, add your name to the copyright statement in the files you modified.
