Birds are known to use vocalizations to warn their fellow birds of nearby danger. But what if the birds hearing the warning are from a different continent and have never heard that call before? A new study suggests that sentinel warning calls, which are used by some birds to alert others of predators, may be universally understood by birds across different regions.
Sentinel warning calls are vocal signals that indicate the presence of a threat, such as a hawk or a snake. Some birds, such as chickadees, tits, and titmice, are known as sentinels because their alarm calls are widely recognized by other bird species in their mixed-species flocks. These flocks benefit from having a diverse set of eyes and ears to detect predators, and from being able to understand the information that the sentinels provide.
How do birds respond to unfamiliar sentinel calls?
Researchers from the University of Illinois Urbana-Champaign and collaborators from Serbia and China wanted to test if bird communities across three different continents could understand the warning calls of a sentinel they had never encountered before: the dusky-throated antshrike. Antshrikes are birds widely distributed across Central and South America that often act as sentinels in their mixed-species flocks.
The researchers played back the warning calls of the antshrike, along with the warning calls of a local sentinel and controls, to flocks of wintering birds in North America, Europe, and Asia, and measured their behavioral responses. They expected that the birds would respond more strongly to the familiar local sentinel than to the unfamiliar antshrike.
However, they were surprised to find that the birds across all three continents responded equally as strongly to the antshrike’s warning calls as they did to the local sentinel’s warning calls. The birds showed signs of increased vigilance, such as looking around, flying away, or hiding, when they heard either of the sentinel calls, compared to the control sounds.
What does this mean for bird communication?
The researchers say that this finding suggests that there is something about sentinel warning calls that makes them so universally recognizable by birds. They speculate that sentinel calls may have some common acoustic features, such as high frequency, short duration, or repetition, that convey a sense of urgency and danger. Alternatively, sentinel calls may have evolved to mimic the sounds of predators, such as raptors, that are shared by birds across different regions.
The researchers also note that this study has implications for understanding how birds cope with environmental changes, such as habitat loss, climate change, or invasive species. If birds can quickly learn to recognize the warning calls of new or unfamiliar sentinels, they may be able to adapt to novel threats and form new alliances with other species.