American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Interspecific covariation in courtship displays, iridescent plumage, solar orientation, and their interactions in hummingbirds”

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Richard K. Simpson and Kevin J. McGraw (Oct 2019)

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The complex evolution of hummingbird visual courtship signals produced varied and unique signal interaction properties

Hummingbirds evolve to be either acrobatic and flashy or colorful and consistent during courtship

A female broad-tailed hummingbird sitting on her nest; females are the targets of the elaborate male courtship displays studied here.<br />(Credit: Richard K. Simpson)
A female broad-tailed hummingbird sitting on her nest; females are the targets of the elaborate male courtship displays studied here.
(Credit: Richard K. Simpson)

Animals exhibit a staggering array of exaggerated ornaments, such as peacock tails or deer antlers, and complex behavioral displays, like the elaborate dances of birds of paradise. Ornaments and behaviors are often used for courtship, and many animals use both ornaments and behaviors simultaneously, which prompts the questions of 1) why there is such a diversity of signaling traits, and 2) why animals use multiple signaling traits together. Previous work addressing these two questions has focused on understanding the information conveyed through animal signals, and how signals are transmitted and detected through the environment. However, Drs. Simpson and McGraw set out to tackle these questions from a different angle. Previously they found that male hummingbirds can alter the presentation of their iridescent throat feathers during courtship dances and that this signal interaction changes how males appear to females. For example, when a male is dancing for a female, he can manipulate how he orients his feathers relative to the female and sun to create a flashy, strobe-like color appearance. To tackle questions about signal evolution and diversity, Simpson and McGraw tested how signal interactions vary among multiple hummingbird species, and how signal interactions may be co-evolving with other signaling traits. They quantified the courtship dances, iridescent feathers, and signal interactions (male color appearance during courtship) of six hummingbird species in the American Southwest and found that signal interactions do vary among species, with some having very flashy color appearances during courtship and others having very consistently colored but brighter color appearances. Further, they found that species with more complex dances have flashier color appearances, while species with larger and more colorful feathers have brighter, more consistent color appearances, illustrating how these different signaling traits can co-evolve. This work demonstrates the need to incorporate signal interactions into future research on multiple signals so that biologists can better and more deeply understand the evolution and diversity of animal signals.

A male broad-tailed hummingbird perched on his territory and showing off his iridescent throat plumage.<br />(Credit: Richard K. Simpson)
A male broad-tailed hummingbird perched on his territory and showing off his iridescent throat plumage.
(Credit: Richard K. Simpson)


Many animals communicate using multiple signals. Historically, most attention was paid to how these traits evolve and function in isolation, but recent work has focused on how signals may interact with one another and produce unique signal interaction properties. These interaction properties vary within species, but little is known about how they vary among species, especially with regards to how the expression of particular signals may drive different signal interaction mechanisms. We studied the evolutionary relationships between iridescent plumage, courtship (shuttle) displays, solar environment, and male color appearance during a display (i.e. the signal interaction property) among six species of North American “bee” hummingbirds. We found that color appearances co-vary with behavioral and plumage properties, which themselves negatively co-vary, such that species with more exaggerated displays appeared flashier during courtship, while species with more exaggerated plumage appeared brighter/more colorful with minimal color-changes. By understanding how signal interaction properties co-vary with signals, we were able to discover the complex, multi-layered evolutionary relationships underlying these traits and uncover new potential drivers of signal evolution. Our results highlight how studying the interaction properties between animal signals provides a richer understanding of how those traits evolved and diversified.