COLOMBO — The rich diversity of bulbuls in South Asia can be traced as far as the Sundaland region of Southeast Asia, in the islands that today make up Indonesia and Malaysia, a new study shows.

Bulbuls, the Pycnonotidae family of fruit-eating songbirds, number more than 150 species throughout Asia and Africa, with 24 in South Asia, half of them endemic to the region. Genetic analysis now shows that these South Asian bulbuls trace their origins to Sundaland, coming in several waves of colonization that crossed through mainland Southeast Asia to get to the Indian subcontinent.

“When these bulbuls’ genetic identities are placed on the global bulbul evolution tree, they fit into different spots suggesting that these species occupied South Asia at different time periods,” said Ashish Jha, lead author of the new study and a researcher at the Laboratory for the Conservation of Endangered Species in Hyderabad, India.

Jha and his colleagues compared the genetics of the South Asian bulbuls mainly with their Asian cousins to recreate a tree of evolution and map other factors to study their phylogeny, or how they developed over time. Data for some South Asian endemic bulbuls were missing from the existing global phylogeny of bulbuls, so the team used the available genetic data and supplemented it with genetic data of endemic species from India and Sri Lanka.

The researchers put a special focus on the roots of the region’s endemic bulbul species, which evolved in response to specific climatic and environmental conditions.

“Once the entire Indian peninsula was covered under moist and humid forest, but around 12 million years ago during post-Miocene era, it began to turn dry,” Jha told Mongabay. “This rapid aridification restricted rainforests in the region mostly to high elevations, resulting in the members of bulbuls that got isolated in these spots to evolve as separate species.”

Molecular dating shows the oldest of the South Asian bulbuls are the striated bulbul (Pycnonotus striatus) and mountain bulbul (Ixos mcclellandii) of the Himalayas, and the yellow-throated bulbul (P. xantholaemus), and the white-browed bulbul (P. luteolus) of peninsular India, which diverged during the late Miocene (11.63 million to 5.33 million years ago). The endemic bulbuls of India’s Western Ghats diverged from their sister species during the Pliocene, around 5.33 million to 2.58 million years ago.

The oldest endemic species in the peninsular region are the yellow-throated bulbul of India’s Deccan region and Sri Lanka’s yellow-eared bulbul (P. penicillatus). “Both these species don’t have a closely related sister taxa, so they are unique in the tree of evolution,” Jha said.

The African connection

The study also established that South Asian bulbuls belonging to the Pycnonotus genus colonized Africa during the Pliocene, moving through the Arabian Peninsula and the Middle East, while those from the Hypsipetes genus “island-hopped” their way to Madagascar and the Mascarene Islands during the early Pleistocene, about 2.58 million to 12,000 years ago.

Today, there are two Hypsipetes species in South Asia: the black bulbul (Hypsipetes leucocephalus) in the Himalayas, and the square-tailed bulbul (H. ganeesa) found in the Western Ghats and Sri Lanka.

“Many would assume that the Himalayan species is sister species with the Western Ghats and Sri Lankan square-tailed bulbul, but it is in fact related to the Philippine bulbul [H. philippinus], whereas the square-tailed bulbul was found to be closely related to the Madagascar bulbul [H. madagascariensis],” Jha told Mongabay.

“These variations within the bulbul family make them a very good model to understand local as well as regional diversity while studying long-term impacts associated with climate and geological changes over time to species,” said study co-author Sampath Seneviratne from the University of Colombo.

For example, in Sri Lanka, the yellow-eared bulbul is probably the most common bird in the central highlands, occurring even in home gardens and urban areas. But at elevations lower than 1,200 meters (4,000 feet), they simply disappear. Likewise, the square-tailed bulbul is very common in rainforests and sometimes found in home gardens, but is absent in areas without rainforest. By contrast, the most widespread bird in Sri Lanka is the red-vented bulbul (P. cafer), which nests on apartment balconies and is comfortable in urban surroundings.

Many recent science publications have looked into how the rapid aridification and other environmental factors that played out millions of years ago have contributed to the colonization and evolution of South Asia’s biodiversity today.

“It is interesting to study how different groups of animals respond to historical climatic and environmental changes,” Seneviratne said, “as this helps to understand what will happen in future, if similar changes occur.”

 

Citation:

Jha, A., Seneviratne, S., Prayag, H. S., & Vasudevan, K. (2021). Phylogeny identifies multiple colonisation events and Miocene aridification as drivers of South Asian bulbul (Passeriformes: Pycnonotidae) diversification. Organisms Diversity & Evolution, 21(4), 783-794. doi:10.1007/s13127-021-00506-y

 

Banner image of a square-tailed bulbul (Hypsipetes ganeesa) found in India’s Western Ghats and Sri Lanka, which has sister species in Africa’s Madagascar islands. Image courtesy of Erich Joseph.