Any luck? After years of iOS development, I still run into unknown build and linker errors which could be due to various different reasons.
One of the most common problems is not linking the library to the project target. Make sure you add "Sparrow" in "Target Dependencies", and add "libSparrow.a" in "Link Binary with Libraries". You also need to make sure to link all the libraries that Sparrow depends on.
Alternatively, you can link "Sparrow.framework" instead of the Sparrow project, but I won't discuss that in this post.
If this doesn't fix your problem, please post the error log.
You can avoid the source tree route by using "Sparrow.framework" or adding the "Sparrow.xcodeproj" root directory as a sub-directory to your actual project.