Impressionistically, all birds that fly do so forward. But is there a bird that can fly backward? The answer is yes, and it's the hummingbird, the world's smallest bird.

Belonging to the family Trochilidae, hummingbirds comprise around 340 species.

These diminutive birds are known for their vibrant plumage, rapid wingbeats creating a jewel-like blur in flight, and long, slender bills that can be straight or curved. They are characterized by their small size, narrow wings, and delicate feet. They are named after the humming sound produced by their wings during flight, resembling that of bees.

Hummingbirds can be found foraging and nesting in various terrestrial habitats. Their preferred habitat includes areas with abundant nectar-producing flowers, such as arid shrublands, desert oases, coastal lowlands, tropical rainforests, boreal forests of the New World, and alpine tundra.

They inhabit a wide range of altitudes, from as high as 4,876 meters in the Andes Mountains to below sea level in inland deserts. Due to bird feeding and the establishment of plantations, hummingbirds are sometimes found in urban areas and suburban areas with natural food sources.

The small size of hummingbirds makes their skeletons difficult to preserve as fossils, leaving their evolutionary history somewhat mysterious. Most hummingbirds are native to Central and South America, where fossils dating back one million years have been discovered, suggesting an origin in the Pleistocene epoch.

However, scientists have discovered the oldest known hummingbird fossil in southern Germany, dating back over 30 million years to the Oligocene epoch. This indicates that hummingbird ancestors appeared far earlier, during the Eocene epoch.

Research indicates that the most recent common ancestor of all modern hummingbirds lived in certain regions of South America approximately 22.4 million years ago. This suggests a relatively recent diversification of these birds, especially considering the many species still extant today, revealing rapid diversification upon reaching South America.

Given that modern hummingbirds are only found in the Americas, how did they migrate from Eurasia to South America? Considering their high energy demands, researchers suggest they may have traveled via the Bering Land Bridge from Eurasia to North America, then continued southward to South America.

Once in South America, these nectar-feeding birds settled and diversified rapidly, splitting into nine main clades or groups containing hundreds of species. Approximately 120,000 years ago, these birds migrated back to North America, and around 5 million years ago (at least five times) invaded the Caribbean.

New species seem to have rapidly proliferated within these biogeographic regions (e.g., North America vs. South America). However, over their 22.4 million years of evolution, the rate of new hummingbird species appearing in the Andes Mountains was remarkably similar to the rate of new species appearing in non-mountainous habitats, indicating the crucial role of mountainous regions in the diversification of species.

These findings are consistent with some studies on the impact of the Andes Mountains on biodiversity.

Genetic analyses suggest that hummingbird diversity continues to rise and expand today, with the rate of new species formation surpassing the rate of extinction.