When you walk down a hall, the familiar sounds of your own footsteps provides a simple observation: locomotion generates sound. This everyday experience is general: essentially all animal motions produce sound; and conversely, most animal sounds originate from muscle action and motion (Clark 2016). In the Clark lab, we study this relationship between sound and behavior, particularly of animal flight, and particularly of hummingbirds: a fun mix of bioacoustics and biomechanics. We ask research questions such as:

  • What physical acoustic and aerodynamic mechanisms produce flight sounds?
  • How are flight sounds related to the aerodynamics of flight?
  • How do incidental sounds of locomotion attain communication function?
  • What information do flight sounds contain (such as of flight performance) that may be of interest to receivers?
  • How do behavioral displays evolve?

As such, our research spans bioacoustics, ethology, behavioral ecology, evolution, functional morphology, and flight biomechanics. Research in the Clark lab roughly falls into three overlapping areas:

Animal Aeroacoustics-

We investigate the physical mechanisms by which air flowing over a feather or wing generates sound. Mechanisms include aeroelastic flutter, in which a feather oscillates at a stable frequency set by both its resonance frequency and the airflow.

We have in the lab:

  • An Acoustic wind tunnel (open jet, so that microphones can be out of airflow)
  • An Acoustic camera (localizes sound sources) (see videos in Gallery)
  • High speed cameras

Species studied:

  • Hummingbirds
  • Smithornis Broadbills in Africa (suboscine passerines) (Clark in press)
  • Feathers of various other species (Clark and Prum 2015)

We also study how flight sounds evolve. Birds such as hummingbirds, shorebirds, flycatchers, manakins, or cotingas can already vocalize, so why do they repeatedly evolve non-vocal communication sounds? All animal motions generate sound, intrinsically, so one answer is simple: courtship displays automatically include sound as a byproduct of motion, thus these sounds are readily available for selection to grab hold of and turn into communication sounds.

Birds have independently evolved flutter sounds into communication signals produced during displays dozen of times (Clark and Prum 2015). Perhaps the most acoustically diverse clade are the 38 species of 'bee' hummingbird, in which sexual selection has caused the tail morphology, and associated sound, to diversify. Nearly every species produces unique flutter-induced sounds with the tail-feathers during courtship displays (Clark 2011; Feo and Clark 2010).

Courtship and Locomotion-

We also study courtship display performance in its own right. Males of taxa such as hoverflies or hummingbirds attain high velocities, accelerations, or wingbeat frequencies in their aerial displays. We posit such behaviors become exaggerated through directional female preference for a dynamic male feature, similar to how female preferences for static features are thought to give rise to exaggerated morphology such as the elongated tails of birds or fish (Clark 2009; Clark 2012).

Research questions include:

  • Why do some hummingbirds nearly double their wingbeat frequency during displays?
  • What limits display performances?
  • How do males differ in their ability to perform displays?

Courtship displays can feature extreme behavioral performances not easily observed in other experimental contexts, and thus make interesting case studies of extreme performance per se. Tim Higham (Biology, UCR) and I are investigating how hummingbirds are able to hit extremely high wingbeat frequencies during courtship displays.

Evolution of Behavior-

Hummingbird courtship displays are also fascinating from the perspective of behavior. Over the past decade I have conducted fieldwork in ~13 countries in North Central and South America, most recently including Cuba and Argentina, trying to obtain display descriptions from every species in the Bee Hummingbird (Mellisugini) clade. Thus far this project has been descriptive:

  • Described new displays
  • Described new hybrid combinations
  • Split one hummingbird species into two

But we now have enough data that the bigger picture is now emerging. In the next few years we will use our data to examine:

  • How tail sounds coevolve with behavior
  • How behavioral components ('elements') of displays evolve
  • How nonvocal sounds coevolve with vocalizations
  • The role of Song Learning

Hybrid hummingbird-finder (and lab tech) David Rankin and I are investigating the phenotype of rare hummingbird hybrids such as Anna's x Allen's, Anna's x Costa's, and Black-chinned x Allen's that we have found in Riverside. David has found 5 such hybrids on and around the UCR campus in the past year alone. David gets a raise when he finds and records the display of a Costa's x Allen's.

Allen's x Rufous hybrid zone: There is an undescribed hybrid zone between Allen's and Rufous Hummingbirds. The hybrid phenotypes are quite interesting, since the displays that the two parent species perform are rather different. In collaboration with Alan Brelsford (UCR) and Kevin Burns (SDSU).

Other Projects

We have large outdoor aviaries and permits to work with hummingbirds. On campus we have Anna's, Costa's and Allen's hummingbirds year-round, plus Black-chinns from April-Sept. I'm happy to collaborate with anyone wanting to take advantage of these resources.

Ongoing projects include:

  • Pollination biology (in collaboration with Erin Rankin (Entomology, UCR) and Amy Litt (Botany, UCR)

[ References ]

  • Clark, C. J. 2009. Courtship dives of Anna's Hummingbird offer insights into flight performance limits. Proceedings of the Royal Society of London B 276:3047-3052.
  • —. 2011. Wing, tail, and vocal contributions to the complex signals of a courting Calliope Hummingbird. Current Zoology 57:187-196.
  • —. 2012. The role of power versus energy in courtship: what is the 'energetic cost' of a courtship display? Animal Behaviour 84:269-277.
  • —. 2014. Harmonic hopping, and both punctuated and gradual evolution of acoustic characters in Selasphorus hummingbird tail feathers. Plos One 9:e93829.
  • —. 2016. Locomotion-Induced Sounds and Sonations: Mechanisms, Communication Function, and Relationship with Behavior in R. A. Suthers, and T. Fitch, eds. Vertebrate Sound Production and Acoustic Communication. New York, Springer Handbook of Auditory Research.
  • Clark, C. J., D. O. Elias, M. B. Girard, and R. O. Prum. 2013a. Structural resonance and mode of flutter of hummingbird tail feathers. Journal of Experimental Biology 216:3404-3413.
  • Clark, C. J., D. O. Elias, and R. O. Prum. 2011a. Aeroelastic flutter produces hummingbird feather songs. Science 333:1430-1433.
  • Clark, C. J., D. O. Elias, and R. O. Prum. 2013b. Hummingbird feather sounds are produced by aeroelastic flutter, not vortex-induced vibration. Journal of Experimental Biology 216:3395-3403.
  • Clark, C. J., and T. J. Feo. 2010. Why do Calypte hummingbirds "sing" with both their tail and their syrinx? An apparent example of sexual sensory bias. American Naturalist 175:27-37.
  • Clark, C. J., T. J. Feo, and K. B. Bryan. 2012. Courtship displays and sonations of a male Broad-tailed × Black-chinned Hummingbird hybrid. Condor 114:329-340.
  • Clark, C. J., T. J. Feo, and I. Escalante. 2011b. Courtship displays and natural history of the Scintillant (Selasphorus scintilla) and Volcano (S. flammula) hummingbirds. Wilson Journal of Ornithology 123:218-228.
  • Clark, C. J., T. J. Feo, and W. van Dongen. 2013c. Sounds and courtship displays of the Peruvian Sheartail, Chilean Woodstar, Oasis Hummingbird, and a hybrid male Peruvian Sheartail × Chilean Woodstar. Condor 115:560-577.
  • Feo, T. J., and C. J. Clark. 2010. The displays and sonations of the Black-chinned Hummingbird (Trochilidae: Archilochus alexandri). Auk 127:787-796.
  • Feo, T. J., J. M. Musser, J. Berv, and C. J. Clark. 2015. Divergence in morphology, calls, song, mechanical sounds, and genetics supports species status for the Inaguan hummingbird (Trochilidae: Calliphlox “evelynae” lyrura). Auk: Ornithological Advances 132:248-264.