Ed Yong in Not Exactly Rocket Science:
In 2004, the chicken became the first bird to have its genome fully sequenced. Its DNA revealed something odd—or rather, an odd lack of something. It was missing a gene called T1R2, which we and other mammals need to taste sweet foods. Chickens, it seemed, can’t taste sweets.
They aren’t alone. Maude Baldwin from Harvard University and Yasuka Toda from the University of Tokyo looked at the genomes of 10 different birds, from falcons to finches and ducks to doves. None of them had T1R2. Alligators do, and they’re some of the closest living relatives of birds. So at some point, as birds evolved from small dinosaurs, they lost their sweet tooth.
What about hummingbirds?
Hummingbirds feed largely on nectar, the sweet liquid that flowers produce. They love the stuff and the sweeter the better; they’ll actually reject flowers whose nectar isn’t sweet enough. They lack the T1R2 gene, but they can clearly taste sugar.
Baldwin and Toda have now discovered their workaround: they repurposed two other taste genes that are normally responsible for detecting savoury tastes. On a hummingbird’s tongue, these savoury sensors are sugar sensors too.