OOW your thoughts on BR ring true, but in a War scenario any Attacker who hasn't been chained usually has plenty of population space. KDs are usually forced to invest a lot of FBs into Avians (usually someone with more offence to chain instead) because of this perk. In a War scenario I would rather have +BR than +Income, as a hitter... After the War, Income is nice, but then you are pumped again soon enough, and the BR has lost its use but Income doesn't mean a lot either. The reason that Avian passes Human is actually the Human runes penalty, the income and BR evens out on my calc, Human has a bit more BE (science) but they have an acre penalty, effectively. Whereas Avian does not...
While this is not included in the table, Avian has a cheaper Elite too.