ISRAEL JOURNAL OF
VETERINARY MEDICINE
SEEING THE INVISIBLE
S. Santos
Based on the hypothesis that diurnal avian species which appear as sexually monochromatic to
the trichromatic human eye might be dichromatic if the avian visual range (320-700 nm) and
capabilities (tetrachromacy) are considered, this study was designed to reveal concealed plumage
sexual dichromatism using reflectance spectrometry.Reflectance spectrometry is an objective
method for colour assessment that has the advantage of including parts of the spectrum to which
humans are blind (UV 320-400 nm) and is independent of the observer's visual
capabilities.Feathers and plumage are non-uniform rough surfaces. Therefore, their physical
characteristics have to be considered when choosing a colour assessment technique. Angle
dependency (influence of illumination and observation angles on the reflectance spectrum) is
expected in both structurally coloured and pigmented feathers. In Chapter 2, angle dependency of
spectrometric results was shown to be significant, both for iridescent (as visually detectable) and
non-iridescent feathers feathers. The results of this preliminary study emphasized the need for a
plumage colour assessment technique superior to the commonly used single angle reflectance
spectrometry. In the studies that followed, multiple angle spectrometry was used to explore
sexual dichromatism in five different bird species previously classified as sexually
"monochromatic".In Chapter 3, we have explored sex differences on a group of blue-fronted
Amazon parrots (Amazona aestiva). Based on these findings a model that enabled 100% correct
sex prediction based on one particular body region measured with different angle geometries was
proposed.In Chapter 4, the plumage of the long-tailed finch (Poephila acuticauda) was chosen as
a study object since this species had been previously classified by single reflectance spectrometry
as monochromatic by others (Langmore and Bennett 1990). Sexual dichromatism of this species
was revealed by multiple angle spectrometry. Moreover, a model for sex discrimination was
proposed based on the use of multiple angle geometries for a particular body region (grey
crown).In Chapter 5, a comparison between sexual dichromatism results obtained with both
individual feathers and plumage was made using the most commonly used angles for plumage
colour assessment. Both plumage and individual feathers revealed sexual dichromatism, but
occasionally at different illumination and observation angles. Whichever method was used, the
reflectance spectrum changed significantly when the angles of illumination and observation were
changed, which resulted in sexual dichromatism being visible at one angle and not at another
angle.In Chapters 6 and 7, respectively, the sexual dichromatism of the Java finch (Padda
oryzivora) and European magpie (Pica pica) was explored and verified by multiple angle
spectrometry. Additionally, in the European magpie, the proposed sex determination technique
was confirmed with a new set of birds.Finally in Chapter 8, intraspecific co-evolution of the UV
visual system and UV plumage sexual dichromatism was hypothesized for the studied species,
based on the simultaneous presence of a spectral peak, sexual dichromatism in the UV range and
the presence of a mutation for the UV sensitive opsin. From this study we can conclude that
certain plumage colour characteristics can be missed by the use of single angle, rather than
multiple angle spectrometry. Single angle spectrometry may occasionally lead to erroneous
conclusions both in regards to morphological characteristics of birds, and in related fields of
biology, such as taxonomy, behaviour or evolution.So far our initial hypothesis that all diurnal
birds are sexually dichromatic, even when not visible to the relatively colour blind human eye,
has not been rejected.