Hummingbirds are a unique and fascinating group of birds. With their ability to hover mid-air, fly backwards, and flap their wings up to 80 times per second, they stand out from other avian species. But what other types of birds are hummingbirds most closely related to? Here is an in-depth look at the evolution and taxonomy of hummingbirds to uncover their closest living relatives.
Overview of Hummingbirds
Hummingbirds belong to the family Trochilidae, which includes over 330 described species. They are found exclusively in the Americas, with the highest diversity occurring in South America. Hummingbirds are well known for their small size, with the smallest species like the Bee Hummingbird reaching only around 2 inches in total length. Despite their tiny stature, hummingbirds have remarkably high metabolisms and must consume nectar frequently to meet their energy needs. Their long bills and tongues allow them to retrieve nectar from flowers, while their wings can rotate in almost all directions during flight. While hummingbirds drink nectar, they also consume small insects for protein.
Taxonomy and Phylogeny of Hummingbirds
In terms of scientific classification, hummingbirds belong to the following groups:
- Kingdom: Animalia
- Phylum: Chordata
- Class: Aves
- Order: Apodiformes
- Family: Trochilidae
Within the order Apodiformes, hummingbirds belong to the suborder Trochili. Historically, hummingbirds were classified along with swifts in the order Apodiformes, which also contains the treeswifts. This was based on anatomical similarities, particularly the unique wing shape shared by swifts and hummingbirds that allows for adept flight control and hovering abilities. However, more modern molecular analyses have revealed that hummingbirds diverged from a common ancestor with swifts more than 35 million years ago. As a result, many taxonomists now separate them into their own order, Trochiliformes.
Analysis of Genetic Relationships
By analyzing genetic data, especially DNA sequences, scientists can construct phylogenetic trees that estimate evolutionary relationships and the timing of divergence between different groups. Several phylogenetic studies have aimed to clarify the closest living relatives of hummingbirds within the order Apodiformes:
- A 10-gene analysis by Prum et al. (2015) suggested that hummingbirds are sister taxa to the clade containing swifts and treeswifts. This indicates hummingbirds split off earlier from a common ancestor and are not embedded within the swift family.
- Sibley and Ahlquist (1990) used DNA-DNA hybridization data and concluded the sister group to hummingbirds were the Eurasian tree-swifts in the family Hemiprocnidae.
- A whole-genome phylogenomic study by Claramunt et al. (2020) also supported the hypothesis that hummingbirds are the sister lineage to Hemiprocnidae based on genetic markers across thousands of loci.
While some uncertainty remains, most evidence points to the Eurasian tree-swifts as the closest evolutionary relatives of hummingbirds based on genetic data. However, hummingbirds diverged from these groups tens of millions of years ago, so significant genetic and morphological changes have occurred since that split. Next, we will look at the specific bird families under consideration as potential sister groups.
Swift Families Closely Related to Hummingbirds
Within Apodiformes, there are three main families that may be the closest relatives to hummingbirds:
Tree-swifts (Hemiprocnidae)
This small family of swifts contains just 5 species restricted to tropical Asia, such as the crested treeswift and whiskered treeswift. They have distinctive body plumage and nest on cliffs or trees rather than inside cavities like many swifts. Molecular evidence suggests they are the sister taxon to hummingbirds.
Swifts (Apodidae)
The true swifts contain over 100 species and are found worldwide. Well-known species include the chimney swift, common swift, white-throated swift, and white-collared swift. They have very short legs adapted for clinging to vertical surfaces and spend most of their lives airborne. Swifts appear closely related to hummingbirds but branched off earlier evolutionarily.
Todies (Todidae)
This is a family of just 5 species of tiny, colorful birds endemic to the Caribbean. Unlike swifts, they perch on branches and have longer legs. They were formerly classified close to kingfishers but molecular data now places them in Apodiformes as a sister group to swifts, although less closely related to hummingbirds.
Conclusion
In summary, most evidence indicates that Eurasian tree-swifts are the closest living relatives and sister taxon to hummingbirds, having diverged from a common ancestor approximately 35-40 million years ago. However, significant anatomical and physiological adaptations have occurred since that split, resulting in the wildly differing lifestyles between treeswifts and hummingbirds today. While swifts appear more closely related to hummingbirds than todies based on genetic data, the exact details of these evolutionary relationships are still being uncovered as more data becomes available. But we can say conclusively that among living species, hummingbirds appear most closely related to other members of the order Apodiformes, particularly certain families of technicolor, aerial-adapted swifts.
