Trout and Vision in the Ult...

In the images above (click on them to see enlarged view) you will note that the pseudo-color blue (used to indicate strong UV reflectance) is found principally on the yellow-dyed ostrich plume. Some of the UV reflectance from the ostrich also leaks through the white goose feathers and the orange-dyed goose quill. Small points of UV reflectance are also found on the pheasant wing.

It would be the work of a lifetime to record all of the plumage used in fly tying in both visible light and UV, so this article will present some facts, some concepts, some speculation, and some conclusions derived therefrom.

• Fact -- One study indicated that UV plumage reflectance is common among bird species. Of the 312 bird species representing 142 families tested in a study by Eaton and Lanyon [2] it was found that 63% of the feather patches sampled reflected UV light .
• Proviso -- The laboratory study was done with a UV light source much more powerful than sunlight. Therefore, it is likely that many of the feathers showed a UV reflectance greater than in natural light.
• Proviso -- The study showed a 5% reflectance as significant. If we look at the more reasonable threshold of >20% to account for natural light and UV absorption by water, the result is only 13.6%.
• Proviso -- The authors chose a spectral range of mean UV reflectance between 340 and 380 nm. This would indicate that much of the UV was below the UV visual range of trout.
• Conclusion -- Rainbow trout have a maximal UV sensitivity at approximately 360nm; thus, for our purposes, UV reflectance of feathers in a wavelength much below this is of no value.[3]
• Fact -- Songbirds, where both sexes are human visibly identical, use UV patterns for differentiation.
• Conclusion -- Fly tiers no longer use many songbird feathers; so, while this may increase the significance of certain traditional patterns, it will not change our present ties.
• Speculation -- Female ground-nesting birds - pheasant, grouse, etc. - that rely on camouflage to protect their nest are unlikely to have UV-reflective feathers. The males of these species already are more colorful, so further differentiation is unnecessary except for "honesty".
• Fact -- "Honesty" markings on birds are those which describe, to the female of the species, the courting bird's age, vitality, etc. Sometimes these are UV-reflective marks which deteriorate with age - though the male may look young to the human observer, the female can detect, in the UV, its age. (Let us be thankful for Grecian Formula).
• Conclusion - Even if the male of a species may generally have some plumage which is UV-reflective, the fly tier would not know how vibrant that UV marking is without testing each feather.

Personal Test Results
I photographed a series of undyed hackle necks. I use the term "undyed" because the necks are treated to preserve them and it is possible that different chemical treatments will bestow different UV-reflective properties. The results were interesting:

• Medium Ginger neck - highest UV reflectance - the darker hackles reflecting more than the lighter
• White neck - medium overall UV reflectance
• Grizzly neck (three examined) - almost 0% UV reflectance
• Black neck - almost 0% UV reflectance
• Gray, Dark and Medium necks - almost 0% UV reflectance
• Furnace neck - almost 0% UV reflectance
• Ginger Chinchilla Neck - high UV reflectance, good coverage
• Dark Brown neck - almost 0% UV reflectance
• Red Brown (Coachman) neck - low UV reflectance, but uniform

"In birds, the colors ultraviolet, blue, and green originate from feather microstructure rather than pigmentation."[1] Since the necessary microstructure is a cross-layering it is often found in association with iridescent or "shiny" feathers. Thus the old admonition to always buy shiny hackle for dry flies may actually have provided the tier with more UV reflective hackle.
However, the traditional use of "natural" necks has, unknowingly, prevented us from tying dry flies with a significant and dependable element of UV reflectance in the hackle or wings. Dyes, on the other hand, often impart, either through the dye-stuff or the mordant, a much higher UV reflectivity than is present with the undyed feather - as you will note from the series of photographs below. Since this UV reflectivity is produced by pigments rather than the natural microstructure, it can be reproduced fairly dependably.

White Turkey feathers - some striated UV reflectance (see photos below - click to enlarge)

White Quills

White Quills w UV in blue

Dyed Marabou - you will notice that the yellow dyed has the greatest UV reflectance, followed by red, light green, and dark green. The white has a coronal point UV reflectance, as does the gray. The brown and black exhibit no significant UV reflectance.

Dyed Marabou

Dyed Marabou w UV in blue

Next in the series will be "Body Materials"

Interesting factoid -- Most birds have good vision in the near-UV (UVA, 320-400nms). A small falcon, the Kestrel, is perhaps the most startling example of UV vision. The kestrel feeds primarily on field voles, Microtus agrestis, a four-inch rodent with the habit of marking its path through the grass to and from its nest with spots of urine. Since urine is UV reflective (dependent on metal content, concentration, freshness, etc.) the kestrel is able to detect the path of the vole across a dense meadow, determine the freshest path by the higher UV reflectance of the latest urine patches, and track the rodent to its grassy nest.