Figure 1) A Ruffed Grouse (Bonasa umbellus) drumming on a log

Just like how all humans have unique voices, various studies have shown that many vocally active species have vocal individuality as well. Individual variation in vocalizations can signal important information such as age, body size, sex, and social status of the emitter to other members of the same species (Favaro et al. 2015). In birds, evidence for individual acoustic variation in vocal signals exists in a wide range of species such as the Eastern Screech-owl, the African Wood Owl and the Bald eagle (to name a few!) (Cavanagh & Ritchison 1987;  Delport et al. 2002; Eakle et al. 1989). Vocalizations, however, are not the only acoustic means birds use in mate selection (O’neil 2014). Non-vocal acoustic signals created by mechanical means, are rather common but are less well understood (O’neil 2014). One example of a mechanical courtship display is the courtship display of the male Ruffed Grouse (Bonasa umbellus).

Sound Clip 1) Ruffed Grouse Drumming (Garcia et al. 2012a)

As mentioned in my first blog post the Ruffed Grouse produces a low-frequency sound known as “drumming” to attract females and defend their territories. Drumming is accomplished by series of swift downward wing strokes produced by a stationary bird perched on a log or stump (O’neil 2016). Drumming activity peaks in intensity during the last week of April which coincides with the breeding season (O’neil 2016). The ruffed grouse drums throughout the day, with drumming activity peaking approximately 1 hour before sunrise followed by a smaller peak just before sunset (O’neil 2016). Though all “drumming” sound may sound the same to the human ear, they show individual variation just like vocal individuality in other birds (Garcia et al. 2012a)! Garcia et al. 2012a found that the number of pulses and pulse rates in a “drumming” event where highly individually specific and could potentially provide information on individual identity to other Ruffed Grouse, though more work is needed to prove this!

Figure 2) Shows the averaged hourly drumming across 7 weeks during the 2013-2015 drumming season. Visible are two peaks in drumming activity just before sunrise and a smaller peak in activity just before sunset. (O’neil 2016)

Directionality is also a key feature of vocalization in birds that again is less understood in non-vocal acoustics (Garcia et al. 2012). A study by Garcia et al. 2012 examined the directionality of the sound discovering that the acoustic radiation drumming display differs from that of vocalizations in that the sound-pressure levels are just as high behind the bird as they are in front of it. In comparison, vocal acoustics in birds is loudest in front of the bird and quietest behind (Patrcelli et al. 2007)

Figure 3) A polar plot depicting the pattern of acoustic radiation of a Ruffed Grouse. As you can see sound pressure are levels are highest in front and behind the bird (Garcia et al. 2012).

The non-vocal drumming of the Ruffed grouse is also much quieter than the vocal acoustics of other birds (Garcia et al. 2012). The ruffed grouse drums at a sound pressure level of 65-70 dB whereas a typical songbird sings at a peak sound pressure level of 80-94 dB (Garcia et al. 2012). Low frequencies are able to travel farther through the environment, however, are more difficult to localize (Bradbury and Vehrencamp 1998).  This directionality lifely affects its detectability by predators, however, this may make it difficult for females to find the “drumming” male (Garcia et al. 2012).

In summary, the non-vocal “drumming” display of the Ruffed Grouse is similar to other avian vocalizations in the way that it shows individual variation, however, it differs from that of vocalizations because of its low frequency and unique directionality.

References

Cavanagh, P. M., Ritchison, G. 1987: Variation in the bounce and whinny songs of the Eastern Screech-owl. Wilson Bull. 99, 620—627.

Delport, W., Kemp, A. C. & Ferguson, J. W. F. 2002: Vocal identification of individual African Wood Owls Strix woodfordii: a technique to monitor long-term adult turnover and residency. Ibis. 144, 30—39.

Eakle, W. L., Mannan, R. W. & Grubb, T. G. 1989: Identification of individual breeding Bald Eagles by voice analysis. J. Wildl. Manage. 53, 450—455.

Favaro, L., Gamba, M., Alfieri, C., Pessani, D., & McElligott, A. G. (2015). Vocal individuality cues in the African penguin (Spheniscus demersus): a source-filter theory approach. Scientific Reports. 5, 17255

Garcia, M., Charrier, I., Rendall, D., & Iwaniuk, A. N. (2012a). Temporal and Spectral Analyses Reveal Individual Variation in a Non-Vocal Acoustic Display: The Drumming Display of the Ruffed Grouse (Bonasa umbellus, L.). Ethology. 118(3), 292–301.

Garcia, M., Charrier, I., Iwaniuk, A.. (2012). Directionality of the Drumming Display of the Ruffed Grouse. The Condor, 114(3), 500–506.

O’neil, P. (2016). The use and function of the ruffed grouse (bonasa umellus) non-vocal drumming display across the breeding season. Masters thesis, University of Lethbridge, Lethbridge, Alberta.