The first hummingbird species to presumably nest and roost in colonies has been discovered in the high Andes of Ecuador.
As ornithologists and most birders know, hummingbirds are tiny but mighty – mighty as in mighty aggressive. These small birds have rightly earned their warlike reputation because they viciously defend their territories from intruders. In fact, most bird watchers have never seen aggregations of hummingbirds that were not actively fighting over food or mates, but astonishingly, one species has overcome their bellicose nature to nest peacefully in colonies.
This species, known as the Chimborazo hillstar or Ecuadorian hillstar, Oreotrochilus chimborazo, is a hummingbird that lives high in the Andes Mountains of Ecuador.
Initially, the study’s lead author, Gustavo Cañas-Valle, a Flora, Ornithology and Mammals Guide and Tour Leader with Naturetrek Wildlife Holidays, couldn’t believe his eyes when he realized what he was seeing: a group of Chimborazo hillstar hummingbirds in his native Ecuador were nesting and roosting in a colony.
“It was mind-boggling,” Mr Cañas-Valle said. “I knew hummingbirds were usually very aggressive to each other, so finding them nesting in the same location was amazing.”
As Mr Cañas-Valle continued watching them, he “realized that males and non-reproductive females were also roosting in the same space as reproductive females, and that goes way beyond normal biological phenomenon. Most colonies of birds tend to have reproductive females, but rarely non-reproductive females.”
So of course, Mr Cañas-Valle simply had to share the news of his amazing discovery with one of his professors. He reported his observations to conservation geneticist Juan Bouzat, a professor in the Department of Biological Sciences at Bowling Green State University. Amongst other professional activities, Professor Bouzat is an active member of an undergraduate program that focuses on mentoring underrepresented minorities and women in science and technology careers.
But upon hearing the news, Professor Bouzat was skeptical.
“Hummingbirds are not a species like penguins where you see hundreds of them together,” Professor Bouzat, the study’s co-author who also studies penguins, pointed out in a statement. “These are hummingbirds that live in the High Andes, above 10,000 feet, in a very, very harsh environment above the tree line.”
The Ecuadorian hillstar hummingbird lives on the dry Western slopes of the Chimborazo volcano in the High Andes of Ecuador. These hummingbirds live at elevations ranging from 3,745 to 4,882 m (12,286.7 – 16,017 ft) above sea level. Mr Cañas-Valle identified 23 adults and four chicks nesting and roosting within a single cave on the Chimborazo volcano. This particular cave was located over 3,600m above sea level. The habitat in this area is cold, windy and treeless with sparsely vegetated volcanic soil that receives between 145 and 500 mm (5.7 – 19.7 inches) average rainfall per year.
The Chimborazo hillstar is the only hummingbird species present in this harsh Puna-like habitat. It feeds on nectar produced by alpine flowers, mainly by the Chuquiragua, Chuquiraga jussieui, a flowering plant in the daisy family (Asteraceae), that dominates the landscape in this area.
In this challenging environment, nesting and roosting substrates appear to be limited to a few rocky volcanic outcrops, vertical substrates within ravines, natural caves, and man-made structures, including under-road culverts and houses.
“The first intuition was to say there might not be enough space – the landscape doesn’t offer them suitable locations to nest in safe, protected places,” Professor Bouzat explained.
But how could the team test whether Chimborazo hillstar hummingbirds nest together due to a lack of sites away from predators and the tough environmental conditions of the High Andes?
“[W]e surveyed the landscape for potential sites that would be appropriate for solitary nesting,” Professor Bouzat explained (Figure 1).
During the dry season in June to July 2021, Mr Cañas-Valle and Professor Bouzat spent 26 days looking for nests. They found 82% of these nests were clustered in 7 nesting aggregations, whereas 13 had one solitary nest each. Nesting aggregations comprised a total of 61 nests ranging from groups of 3 to 16 nests, with an average of 8.7 per locality. Roosting hummingbirds were never seen at solitary active nests, and peculiarly, nesting aggregations were consistently associated with roosting groups of non-reproductive individuals – unique amongst birds.
Additionally, Mr Cañas-Valle and Professor Bouzat found unoccupied nesting sites that probably were suitable for solitary hummingbird nests, including ravines with concrete culverts that allow for water runoff to pass under roads. This finding allowed them to test their hypothesis about the role that the local environment may play in this hummingbird’s social groupings.
“Some culverts showed signs of past use but were not being recurrently used across years as it happens with most birds, which was very strange,” Mr Cañas-Valle said. “If our hypothesis that the hummingbirds are getting together because of reduced availability of nesting sites was correct, then every site available for solitary nesting should be used. But that didn’t happen – many culverts were not used.”
This finding suggested that Chimborazo Hillstar hummingbirds probably evolved group living for other reasons. As seen in other social species, it is reasonable to assume that these hummingbirds gain some sort of advantage by being part of a group.
“In evolutionary biology, reproductive success is the currency for adaptive evolution. Colonies typically evolve over a long time and become socially cohesive, in some cases leading to complex cooperation among individuals,” Professor Bouzat explained. “Somehow, they get a benefit – increasing their reproductive success – from being associated with the social group.”
But are these birds truly colonial? From an evolutionary perspective, large groups living together in groups typically arose for the mutual benefit of all the members of the group, relying on behaviors such as working together to find food or detect predators (i.e.; ref).
“There is a theory of information exchange as one of the potential explanations for coloniality,” Mr Cañas-Valle said. “This is only speculation right now, but hummingbirds may get easier access to mates and food resources by an exchange of information between members of a colony. Those are the two most important things the hummingbirds gain, with also gaining a safe place to sleep that is slightly away from the harsh winds of the Chimborazo volcano region.”
Nevertheless, it took years of meticulous observations to convince Professor Bouzat these birds may be living in colonies.
“It took me probably two years for Juan to say, ‘Well, Gustavo, you convinced me. We can call this gathering of nests a colony from now on’,” Mr Cañas-Valle said. “I was thinking, ‘Finally’.”
“That was a priceless moment.”
Source:
Gustavo X Cañas-Valle and Juan L Bouzat (2024). Ecological drivers of coloniality in Oreotrochilus chimborazo chimborazo (Chimborazo Hillstar), a hummingbird endemic to the High Andes of Ecuador, Ornithology, ukae063 | doi:10.1093/ornithology/ukae063
The authors did not reply to multiple emailed requests for comment and information. Materials and photographs courtesy of Bowling Green State University.
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