Alerts Provide an Early Warning System

white and black bird on brown tree branch

Biological Strategy

Black-capped chickadee

Sherry Ritter

Image: Skyler Ewing / Pexels / Free non-commercial use

Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Cooperate Within an Ecosystem

An ecosystem is a community of organisms (plants, animals, and microbes) interacting with one another and the nonliving components of their environment (such as air, water, and mineral soil). This interaction can be passive or active, and can cooperatively enhance the functioning of the ecosystem as a whole. For cooperation to contribute to maintaining communities within an ecosystem, it must be beneficial to at least some members of the community. Cooperation consists of symbiotic relationships, such as mutualism (in which two or more species in an ecosystem benefit) and commensalism (in which one species benefits and the effect on others is neutral). An example of a commensal relationship is that between bromeliad plants and trees: bromeliads live on trees without harming them. Bromeliads have mutualistic relationships with other species, including insects, frogs, and worms. The plants capture water in their base, forming a pond that these organisms join. The nutrients that these organisms excrete in their droppings nourish the bromeliad.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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Predator-specific alerts allow multiple species to survive by providing an early warning system.

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Introduction

The sound of birds singing in a woodland can seem like a confusing jumble of trills, whistles, and chirps. We don’t tend to think about the potential function of all this noise that other animals produce, but research on animal communication has slowly built up a startling picture of just how much communication is actually going on. Scientists have found that birds and mammals broadcast warnings about nearby predators, for example, and understand each other’s messages. They can even share information on what kind and size of predator is around, and what it’s doing.

The Strategy

Black-capped chickadees are tiny black and white birds who live in forests. They’re best known for their “chickadee dee dee” calls. These and some other calls keep birds in contact with each other or let each other know where to find food. Less noticed by humans is a very quiet, high-pitched “seet” call. They use this call to warn others that there’s a hawk or owl flying nearby.

Chickadees also have another call when they see a perched hawk or owl. It’s a louder and longer version of its usual “chickadee dee dee” call. This altered call draws in other birds to “mob” the predator. The more dangerous the predator is to the chickadees, the more times they say “dee”. For example, the tiny northern pygmy-owl, who eats a lot of chickadees, gets a lot of “dee” calls. The bigger great horned owl is less of a threat and so the birds make fewer “dees”. The harsh “chickadee dee dee” means the chickadee sees a hawk or owl and needs other nearby birds to mob the predator until it flies away.

The “seet” call has a different purpose. Because it’s so quiet and high-pitched, it’s hard for a predatory bird to hear. That means it’s a safer call for a chickadee to make compared to the deeper, louder “chickadee dee dee.” The “seet” call warns other chickadees to hide when a hawk or owl is flying nearby. Like an early warning system, the alert can spread through a forest from one bird to another faster than the hawk can fly. By the time the predator arrives, the birds are safely in hiding.

Other species, including nuthatches, jays, squirrels, and chipmunks can also listen to the chickadees’ warnings. Birds in other regions also react to the “seet” call even if chickadees don’t live in the area. Scientists have played a recording of the chickadee’s “seet” call all over the world, and other birds understood it and took cover.

The early warning system goes beyond directly saving birds and mammals from predators. This cooperation allows them to spend less time watching for predators and more time getting the food they need to survive.

Hawk being mobbed by chickadees

The Potential

Further research could decode more sound signals that have near-universal meanings, reopening lines of communication between humankind and other species. Technologies which tap into these patterns of communication could help make humankind a better participant in the conversation, and thus a better neighbor to those species. Speakers could be placed on skyscrapers near flyways to prevent avian collisions with difficult-to-see architectural glass, or around airports to reduce collisions with airplanes.

Gardeners and farmers could also use knowledge about communication among birds to repel birds or other organisms that eat crops, or to attract organisms that feed on species that have other detrimental effects on agriculture.

Image: Frank D. Lospalluto / CC BY NC ND - Creative Commons Attribution + Noncommercial + NoDerivatives

Northern Pygmy-Owl with branches for scale.

Image: Rick Leche / CC BY NC ND - Creative Commons Attribution + Noncommercial + NoDerivatives

Black-capped chickadee

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Last Updated November 3, 2020

References

“In addition to being one of the most subtle and sophisticated signaling systems yet discovered, this system is unusual in that it combines aspects of both referential and risk-based antipredator vocalization systems. To denote the presence of a rapidly moving predator (e.g., raptor in flight), chickadees produce a “seet” alarm call. When they encounter a stationary predator (e.g., perched raptor), they use the “chick-a-dee” mobbing call. These two vocalizations appear to be functionally referential to the type of predator encounter (i.e., each denotes a specific type of encounter). In addition, we have shown that subtle variation in the “chick-a- dee” mobbing call reflects the size of a specific predator, a characteristic of a risk-based system. Thus, chickadees convey information about predators at two different levels: A coarse level of encoding (“seet” or “chick- a-dee”) signifies the type of predator encounter, and a fine level of encoding (variants of “chick-a-dee”) signifies the degree of danger presented by that specific predator encounter.” (Templeton et al. 2005:1937)

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