Well, this is a surprise! I never realised that Liam was a songwriter as well as a great vocalist. I guess it's in the bloodline.
"Songbird" is simple, honest, catchy and effective. A more inexperienced or insecure songwriter might have been tempted to try and beef it up with extra sections, and to increase its length. At this point a comparison with the Beatles is helpful (though normally I hate it when people compare Oasis with the Beatles). John and Paul never bothered padding out a song to make it conform. If they had a good idea for a song that only lasted two minutes, then they left it as a two-minute song.
Liam may not have the song writing skills of John and Paul, but he has their good sense to keep "Songbird" simple and to the point. But this simplicity can be deceptive. The melody and rhythm actually have a number of subtleties that contribute to making them work so well. Perhaps this is due to Liam's long singing experience, and working with a band arranging and performing songs.
The lyrics are also attractive, and they certainly go together nicely with the music. They are open to many interpretations: e.g. talking to his better half, to a new girlfriend, to his emotional sense, a woman friend, an imaginary friend, an ex-girlfriend, a one-night stand, etc. The more I think about the lyrics, the more depth they seem to reveal.
The arrangements on this song are good as well, particularly what's going on in the instrumental sections.
Overall this is a refreshingly simple and quality song from Oasis.