Electronic Supplementary Material:
Reflectance spectrometry (following Avilés et al. 2008): Briefly, reflectance data on the
cere of scops owl nestlings was recorded with an Ocean Optics equipment [S2000
spectrometer connected to a deuterium-halogen light (D2-W, mini) by a coaxial
reflectance probe (QR-400-7-UV-vis) and the OOIBase32TM operating software (Ocean
Optics, Inc. Dunedin, FL, USA)]. Reflectance was measured with the probe placed at a
constant distance and reaching the nestling at 45º. Measurements were relative and
referred to a standard white (WS-2) and to the dark, which we calibrated before the
measurement of each nestling. All measurements were taken within a portable hide with
opaque wall set in the surrounding of the nests. Nestlings were returned to their nest
before fifteen minutes, and subsequent visits to these nests confirmed us that our
manipulation was non-harmful. Measurements were repeated three times and mean
values per nestling were calculated and used in the analyses.
Calculation of colour variables (following Montgomerie 2006): We summarized cere
reflectance data by calculating: (i) total brightness as the summed reflectance in the
interval 300-700nm; (ii) UV chroma as the summed reflectance in the interval 300-400
nm divided by total brightness; and (iii) λUV peak as the wavelength at which the
maximal value of reflectance is reached in the UV (i.e. 300-400 nm).
Description of Linear Mixed Model on body mass gain: To test for the effect of
experimental reduction of ultraviolet reflectance on parental feeding allocation we
performed a Linear Mixed Model (MIXED SAS procedure) with body mass of nestlings
at different dates as within effect, and experimental treatment (UV reduced versus
control) as between effect. Because we were interested in analysing whether the
treatment mediated nestling growth we tested the interaction time x treatment on body
measures. Initial body mass was entered as covariate in the model to control for a
possible role of competition ability on our results. Nest ID was introduced as a random
factor in this analysis to account for non-independence of measures collected at the
same nest.
Effect of the UV-blocker and the control treatments on cere reflectance: We qualified
changes in cere reflectance before and after application of the UV-blocker and the
control treatments in seven and six nestlings, respectively (figure 1b). The UV-block
treatment did not significantly affect summed reflectance in the human visible (400-700
nm) wavelengths ((mean ± s.d.) 6276.64 ± 2326.76 before versus 6137.94 ± 1917.75
after; Paired t-test: t6= 0.31, P = 0.76; figure 2b), while markedly reduced UV chroma
(0.33 ± 0.08 before versus 0.071 ± 0.05 after; Paired t-test: t6= 6.30, P = 0.0007; figure
2b) and displaced to longer wavelengths the λUV peak (348.62 ± 15.87 before versus
409.90 ± 12.15 after; Paired t-test: t6= 10.79, P = 0.00009; figure 2b). The control
treatment did not significantly affect either summed reflectance in the human visible
wavelengths ((mean means ± s.d.) 7173.64 ± 1074.95 before versus 6365.71 ± 696.88
after; Paired t-test: t5= 1.61, P = 0.16), UV chroma (0.35 ± 0.05 before versus 0.38 ±
0.04 after; Paired t-test: t5= 1.09, P = 0.32), or the location of the UV peak (346.16 ±
10.81 before versus 348.77 ± 4.04 after; Paired t-test: t6= 0.63, P = 0.55).
The shape of a reflectance spectrum often does not directly equate to how the
signal is processed by the receiver, which makes desirable the use of models of birds
vision that account for trait coloration, the contrast and luminal environments in which
this visual display is perceived (i.e., the nest), and the perceptual capacities of the
intended receiver of the display (i.e., the parents) (Endler 1990; Kelber et al. 2003).
Unfortunately, we lacked a suitable avian model for the receiver at this time since
spectral sensitivity data for the cones and rods in the scops owl have not been obtained
yet. Also, we missed information on cone type ratios and cone/rod ratio for the scops
owl or any closely related species which precluded model parameterization. Thus,
although the experiment induced a change in parent behaviour related with a
manipulation of shape reflectance, it cannot be known precisely how much of the
variation in the UV peak is perceived by parents.
Full results of the Linear Mixed Model on body mass gain:
Effect
treatment
time
F/Z, P
F1,52 = 8.51, P = 0.005
F1,52 = 195.32, P< 0.0001
treatment*time
F1,52 = 6.68, P = 0.012
initial body mass
F1,52 = 0.24, P = 0.62
nest
Z=1.30, P = 0.97
Literature cited
Endler, J. A. 1990 On the measurement and classification of colour in studies of animal
colour patterns. Biol J Lin Soc 41, 315-352.
Kelber, A., Vorobyev, M., and Osorio, D. 2003 Animal colour vision - behavioural tests
and physiological concepts. Biol Rev 78, 81-118.