Interspecific Covariation in Courtship Displays, Iridescent Plumage, Solar Orientation, and Their Interactions in Hummingbirds.

Many animals communicate using multiple signals. Historically, most attention was paid to how multiple signals evolve and function in isolation, but recent work has focused on how they 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 in regard 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 covary with behavioral and plumage properties, which themselves negatively covary, 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 covary with signals, we were able to discover the complex multilayered 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.