Avian bills are fascinating structures, having evolved a myriad of forms and functions, as depicted in the montage on the right from: https://en.wikipedia.org/wiki/Beak
The authors make the major point that hornbills use their bills like toucans do (as radiators of body heat, showing evidence of greater relative blood flow recruitment at warmer temperatures), but that their calculations suggest that the per area rate of heat exchange in the hornbills is less than the toucans.
This is a nice addition to the recent work done by a number of us in the field (see references below, e.g. Symonds and Tattersall, 2010), emphasizing that bills are not the dead, static structures that many assume they are.
One main point the authors note (they base their comparison mainly vs. toucans from our 2009 paper) is that the bills of hornbills are not as effective as dissipators of heat compared to the toucan and quite likely for multiple reasons: 1) the hornbills’ bill is thicker, so is a heavier barrier to heat exchange, likely because the hornbills use their bills as digging implements and must be more robust, and 2) toucans “switch on” their blood flow at lower temperatures than hornbills, and the lower the air temperature, the greater the driving force for heat loss, and 3) the hornbill bill is relatively smaller than the toucans. The reason for the hornbill vs. toucan comparison is because the two lineages have long been viewed as old world vs. new world convergences (although that is really an oversimplification).
One point I’d like to emphasize that is often missed in the popular press. These large billed birds are not unique in their bills’ thermal radiator function, since many other birds exhibit similar capacities to modify blood flow to their bills (geese, ducks, sparrows to name a few). They are, however, striking in how large their bills are, which is why they garner such attention.
Here is a photograph of a hornbill (what mother could not love that face?):
Some technobabble about how the measurements are made:
Like myself, these researchers used thermal imaging to obtain their results (see their paper for images and thermal videos), which means that heat exchange is an estimation of events and is done under sampling assumptions. I’ve been working on calculations to make things easier for myself and colleagues (see below for R package), and came across a couple of things. Firstly, some of the assumptions van de Ven et al (2016) used are different than the ones we made (Tattersall et al 2009). We assumed higher wind speeds (5 m/s) in our modelling, which I think are a little on the high side of reality. van de Ven et al (2016) on the other hand, assumed a wind speed of zero and therefore incorporated only free convection estimates. Gates (2003 – Biophysical Ecology) advises that forced convection is the more realistic scenario for heat exchange in animals since wind speeds are rarely exactly zero, and suggests values between 0.5 and 1 m/s. This means that van de Ven’s estimates of convective heat will be underestimates, while our toucan data are over-estimates. Anyhow, the good news is that van de Den realized this when making comparisons to our previous work and made comparisons using free convective heat exchange (while politely not criticizing us for our over-zealous wind speeds). The take home message though is that their published values for both species are on the low side, while our published values are more for what a flying bird might experience.
For anyone interested in using their thermal images for similar analyses, I have create a package in R called “Thermimage” to help in the calculations.
See https://cran.r-project.org/web/packages/Thermimage/index.html for the package itself and https://github.com/gtatters/Thermimage/blob/master/heatcalc.R for some sample scripts.
Gates, DM 2003. Biophysical Ecology. Dover Publications, Mineola, NY, 611 pp.
Symonds M.R.E., Tattersall G.J. 2010 Geographical variation in bill size across bird species provides evidence for Allen’s rule. American Naturalist 176, 188-197. (doi:10.1086/653666).
Tattersall G.J., Andrade D.V., Abe A.S. 2009 Heat exchange from the toucan bill reveals a controllable vascular thermal radiator. Science 325, 468-470. (doi:10.1126/science.1175553).
van de Ven T.M.F.N., Martin R.O., Vink T.J.F., McKechnie A.E., Cunningham S.J. 2016 Regulation of Heat Exchange across the Hornbill Beak: Functional Similarities with Toucans? Plos One 11, e0154768. (doi:10.1371/journal.pone.0154768).
Greenberg R., Danner R.M. 2012 The Influence of the California marine layer on bill size in a generalist songbird. Evolution 66, 3825-3835. (doi:Doi 10.1111/J.1558-5646.2012.01726.X).
Greenberg R., Danner R., Olsen B., Luther D. 2012 High summer temperature explains bill size variation in salt marsh sparrows. Ecography 35, 146-152. (doi:Doi 10.1111/J.1600-0587.2011.07002.X).
Greenberg R., Cadena V., Danner R.M., Tattersall G.J. 2012 Heat loss may explain bill size differences between birds occupying different habitats. Plos One 7. (doi:ARTN e40933 DOI 10.1371/journal.pone.0040933).
Hughes A.L. 2014 Evolution of bill size in relation to body size in toucans and hornbills (Aves: Piciformes and Bucerotiformes). Zoologia-Curitiba 31, 256-263.
McNab B.K. 2001 Energetics of toucans, a barbet, and a hornbill: Implications for avian frugivory. Auk 118, 916-933.
Seki Y., Bodde S.G., Meyers M.A. 2010 Toucan and hornbill beaks: A comparative study. Acta Biomater 6, 331-343.