TAIPEI (Taiwan News) — Plenty of animals opt for city life as humans do, but despite all the benefits cities offer, concrete jungles can be as dangerous as their natural counterparts.
In a research paper published in 2019, Hung Chih-ming (洪志銘), an associate research fellow at Academia Sinica's Biodiversity Research Center, and his team reported the first case of barn swallow nest predation by the Taiwan whistling thrush. The research demonstrates how interspecies relationships can develop within complex urban environments.
For barn swallows, there are myriad advantages to living in cities, including more nesting sites, fewer predators, warmer temperatures, and protection from extreme weather. After centuries of adapting, barn swallows are now highly successful in cities, and their populations are thriving; many would struggle to survive now without artificial structures.
Barn swallows mainly build their nests in houses. (Getty Images photo)
"Swallows have a close relationship with humans, especially barn swallows, which now nest almost exclusively in buildings instead of natural habitats. This has been going on for thousands of years — since humans began building houses," Hung explained.
On June 2, 2018, a Taipei resident witnessed a Taiwan whistling thrush preying on barn swallow eggs and chicks on Heshun Street in Nangang District. The bird was seen pulling one chick from the nest, tossing it to the floor, and swallowing it.
Despite being closely related, the Taiwan whistling thrush has never previously been recorded preying on other birds as the blue whistling thrush occasionally does.
The barn swallows tried to fend off the threat but failed. Within one week, the Taiwan whistling thrush had wiped out all the chicks and eggs in five swallow nests along the street.
Barn swallows (left) are smaller than Taiwan whistling thrushes. (Getty Images, Wikimedia Commons photos)
The number of Taiwan whistling thrushes in Taipei has been growing in the past few years, especially in Nangang. According to Hung, the actual reason for the increase remains unknown, but it could be because the city has more green space than it used to or because its residents have become friendlier toward wildlife.
Regardless, the case shows that latecomers to the city can erode the advantages enjoyed by early settlers. If the whistling thrushes begin to prey regularly on barn swallow chicks, the latter's numbers could tank.
Ironically, a rare case of barn swallows attacking young whistling thrushes was reported in Taipei's Wenshan District — so the true victim of this relationship remains an open question.
While the dynamics between species are intriguing, Hung is also interested in how animals adapt to their environments. The study of nests, he said, is an excellent tool for getting to the bottom of those mechanisms.
Nests vary by shape and location. They are dug into tree trunks, built on the ground, and hung between branches, among other possibilities.
For instance, most birds that build cupped or domed nests are passerines, such as warblers and whistlers. Only after platform or cavity nests evolved from scrape nests did birds start diversifying their nesting sites, such as in trees and cliffs, while the use of bushes, bamboo, and other non-tree types of vegetation only occurred after birds began building cupped nests.
Bird nests vary in shape, placement. (Wikimedia Commons)
When birds settle into cities, man-made construction also shapes their choice of nesting sites. A great example is the swallows that build their nests on the beams of verandahs, which may be safer than cliffs. Some invasive bird species, like starlings, even build theirs on traffic lights, enduring very hot temperatures, Hung added.
These successful avian settlers then have a better chance of proliferating as cities grow.
Surrounded by mountains, Taipei offers a host of excellent bird-watching spots. Light-vented bulbuls, oriental turtle doves, and Malayan night herons are among the birds commonly found here. Two particularly good spots for checking out the birds are Daan Park and the Taipei Botanical Gardens.
Hung's lab focuses on bird evolution. (Taiwan News photo)