Hummingbirds found nesting in colonies have completely defied expectations

A team of scientists has recently found an unexpected social twist in the behavior of certain Andean hummingbirds.

Contrary to their usual reputation for aggressive territoriality, these small, high-energy birds are sometimes nesting close together in a communal fashion. The finding has sparked fascination among ornithologists worldwide.

Rethinking hummingbird nesting

Typically, hummingbirds are highly aggressive and territorial. But the researchers found that in the harsh environment of the Andes Mountains, a particular species – the Chimborazo Hillstar – appears to forgo territorial aggression and cluster its nests instead. 

“It was mind-boggling,” said Gustavo Cañas-Valle, an ornithologist and Bowling Green State University (BGSU) alumnus. “I knew hummingbirds were usually very aggressive to each other, so finding them nesting in the same location was amazing.”

The surprising finding left Cañas-Valle’s colleague and co-author Juan Bouzat, a BGSU evolutionary biologist, initially skeptical. 

“Hummingbirds are not a species like penguins where you see hundreds of them together,” Bouzat said. “These are hummingbirds that live in the High Andes, above 10,000 feet, in a very, very harsh environment above the tree line.” 

Yet after years of research, the experts have produced data that demonstrate this unusual social nesting.

Constraints of hummingbird nesting sites

In seeking to understand this communal nesting, the researchers hypothesized that perhaps the Chimborazo Hillstars were simply forced into shared spaces due to limited safe nesting areas in the rugged high-altitude environment. 

“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,” Bouzat explained.

To test this hypothesis, Cañas-Valle meticulously surveyed the region, including the possibility that birds might nest alone in ravines or in human-made culverts. If scarcity of safe spots were the only factor, those solitary sites should be fully occupied. But that was not the case. 

While some birds did nest alone in culverts, many were left unused. Meanwhile, over 80% of the observed active nests were grouped in colonies with roosting birds.

Evolutionary reasons for communal nesting

The results point to factors beyond simple habitat shortages. According to the scientists, colonies usually evolve over a long period of time and become socially cohesive, in some cases leading to complex cooperation among individuals. 

This suggests that close nesting may enhance reproductive success, the key measure of success in adaptive evolution.

“There is a theory of information exchange as one of the potential explanations for coloniality,” 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, while also gaining a safe place to sleep that is slightly away from the harsh winds of the Chimborazo volcano region.”

Getting these findings recognized was not straightforward. Before the paper was accepted for publication, Cañas-Valle had to convince Bouzat, his advisor, that this “colony” of hummingbirds was real. 

“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,’” Cañas-Valle said. “That was a priceless moment.”

A window into evolutionary adaptations

Beyond explaining the immediate puzzle – why hummingbirds would nest together in the Andes – this study may illuminate how harsh environmental pressures can drive unconventional adaptations. 

Rather than typical one-on-one territorial aggression, these hummingbirds appear to have adopted a semi-cooperative strategy, enabling them to endure cold, windy conditions above 10,000 feet.

By showing how extreme habitats can shape social behaviors that seem out of character for a species, the BGSU team’s research also opens the door for broader discussions about communal nesting and the ways in which different species can adapt to harsh climates.

For now, the Chimborazo Hillstar stands as a striking example of evolutionary flexibility, reminding ecologists that even among famously territorial creatures, cooperation can occasionally win out in the face of nature’s most daunting challenges.

The study is published in the journal Ornithology.

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