The secretary bird (Sagittarius serpentarius) is a large bird of prey that is endemic to sub-Saharan Africa. It is well-known for its distinctive appearance, with long legs, a long neck, and distinctive crest feathers on the back of its head that look like old-fashioned pens stuck behind a secretary’s ear. The secretary bird is a diurnal raptor that specializes in hunting snakes, lizards, and other small animals on the ground. While it is not closely related to other raptors like hawks, eagles, and falcons, the evolutionary relationships between the secretary bird and other bird species has been the subject of much research and debate among ornithologists and evolutionary biologists.
Taxonomy and Classification
The secretary bird is the only living member of the family Sagittariidae in the order Accipitriformes. Traditionally, this family has been classified with other raptors in the order Falconiformes based on some similar morphological characteristics related to hunting. However, DNA analysis has shown that the Sagittariidae lineage is actually more closely related to another group of birds called cranes, which belong to the order Gruiformes.
The closest living relatives to the secretary bird based on genetic analysis are two species of cranes: the crowned cranes of Africa and the demoiselle cranes of Eurasia and Africa. While secretary birds and cranes have very different appearances and behaviors related to their ecological niches, DNA evidence indicates they share a common ancestor from around 80 million years ago.
Morphological Similarities to Cranes
Despite the predatory lifestyle and vulture-like appearance of the secretary bird, it shares some subtle anatomical features with cranes that indicate their evolutionary relationship:
- Long hindlimbs adapted for walking and kicking
- Feathers on the head that can be raised into a crest
- Nostrils that are perforated rather than just slit-like openings
- An elongated hind toe
These physical characteristics point to a common descent from earlier ground-dwelling birds that preceded the rapid radiation of modern avian orders.
Genetic Analysis
The emergence of DNA sequencing technology has allowed scientists to go beyond anatomical studies and delve into the evolutionary history of bird groups through analysis of their genetic relationships. By comparing DNA across species, the evolutionary splits between lineages can be estimated.
Several phylogenetic studies of bird genetics have consistently found that the secretary bird lineage split from other raptors over 80 million years ago, while it diverged from the crane lineage around 50 million years ago. This points to cranes as the closest cousins of secretary birds.
One extensive DNA study compiled genetic data from 169 bird species. In the phylogenetic tree generated from this data, the secretary bird was nested within the crane clade, as a sister group to the crowned cranes. This strongly supports cranes as the living sister taxon to the secretary bird.
Ecological Differences Between Secretary Birds and Cranes
While secretary birds and cranes are each other’s closest living relatives, they have a number of striking behavioral and ecological differences as a result of their long evolutionary separation:
Habitat and Geography
- Secretary birds are found exclusively in sub-Saharan Africa.
- Cranes have a much wider distribution across Africa, Eurasia, and Australia.
- Secretary birds inhabit grasslands and savannas.
- Cranes occupy more diverse habitats including lakes, marshes, plains, and tundra.
Food and Foraging
- Secretary birds are carnivorous, feeding mainly on rodents, lizards, snakes, and insects.
- Cranes are omnivorous but mostly herbivorous, feeding on seeds, roots, fruit, and aquatic vegetation.
- Secretary birds hunt by walking through grass and kicking prey.
- Cranes use their beaks to probe the ground or filter feed in water.
Physical Attributes
- Secretary birds have eagle-like bodies with long legs adapted for walking.
- Cranes have slender bird bodies balanced on long legs adapted for wading.
- Secretary birds have wings specialized for short-distance flight.
- Cranes have large wings suited for migratory flight.
These differences in lifestyle and ecology between the two bird groups evolved over tens of millions of years since their divergence from a common ancestor.
Conclusion
While secretary birds and cranes occupy very different ecological niches currently, genetic and morphological evidence conclusively shows that they share a relatively recent common ancestry. Cranes are the closest living avian relatives to the distinctive secretary bird. Evolutionary relationships between bird groups like these continue to be illuminated by advances in molecular biology and DNA sequencing. Going forward, further genomic analysis will provide more insight into the evolutionary branches of the bird family tree.