My #1 passion (at this point in my scientific career) and probably what I want to dedicate the rest of my life to, is feather coloration and how birds use it- whether to be for sexual selection, to determine dominance, to mimic other birds. Well really, mostly just sexual selection, and also couple the feathers with courtship rituals and I just about die and go to heaven. Anyway! Although I am doing work with vocalizations and behavior in my graduate degree, in my undergrad I had the opportunity to have my own feather research project. Since it will be Thursday by the time this will be posted, and Throwback Thursday is a thing, this post is dedicated to my past research. It will also serve as a gateway for future feather posts.
Feathers can tell us a lot about the bird. In many species, males and female have different feather colors.
In all bird species, plumage can indicate the age of the bird to some degree. In some species, the age is blatantly obvious because birds of different ages will have different colored plumage. In other species, not so much, there’s always skulling to be sure, but we’re talking about feathers here! And I have never actually learned how to age based on the skull ossification so….:(
One of the things I think is coolest about feather coloration is that in some cases, it can indicate the health of the bird, and therefore indicate quality as a potential mate! This is because important bio-molecules called carotenoids. Carotenoids are a pigment molecule that is produced in plants, specifically in the chloroplasts. They are responsible for the reds, the yellows, and the oranges we see. Carotenoids cannot be synthesized by birds, so in order for a bird to have all that beautiful red, orange, and yellow plumage, they either need to eat plants or eat insects which have been eating plants with carotenoids.
But before birds can get all those gorgeous feathers, they must allocate carotenoids for the greater good- their immune system. Carotenoids are used in the immune system to aid in activating the immune system to fight off illness. Specifically, they stimulate effector T-cell function, enhance macrophage and cytotoxic T-cells, and stimulate T and B lymphocyte multiplication. They also help get rid of free radicals that the birds produce from metabolic function. Basically, carotenoids rally the troops whenever disease and parasites rear their ugly head in birds. If a bird is getting enough carotenoids in it’s diet, it can allocate the excess to coloring their feathers up.
In many species of birds, carotenoid coloration in the feathers can be used to determine how healthy a bird is. This can be done by measuring the brightness, saturation, and hue of a feather (you could also extract and measure the amount of carotenoid in the feather, but that’s chemist territory). Usually, the higher and saturated hue of a feather, the greater the carotenoid content. I should also mention that in order to make comparisons between the feather traits and health of a bird, or feather traits and amount of carotenoids in the feathers, tests of immune health or pigment extraction occur. Showing off how healthy you are is important for males. If a male bird is healthy enough to use carotenoids to color up his feathers he’ll be a more attractive mate; he may have better genes that help keep him healthier, he may be an optimal forager with a territory full of food, or the feathers look appealing to the lady bird’s eye. Super cool stuff right?
My research project was assessing variability in feather coloration by gender and indices of immune function in Common Yellowthroats captured in North Eastern Pennsylvania. That’s quite a mouthful of scientific jargon, isn’t it? Let’s break it down.
Assessing variability=Looking for differences
Feather coloration= Brightness, Saturation, and Hue
Gender= Male and Female
Indices of Immune Function= Amount of white blood cells in a blood sample
Common Yellowthroats captured in North Eastern Pennsylvania= My advisor had all these feathers/blood cell counts on hand from the past and his study site is in Pennsylvania and I wanted to do feather work so…
The objective of this study was to test the hypothesis that carotenoid pigmentation in feathers is influenced by age, gender and immune function in Common Yellowthroats. We did not have bib feathers, which are known to correlate with immune function. However we did have tail feathers. The rectrice feathers feathers of the Common Yellowthroat are colored by carotenoids (one called lutein) and melanins (brown/black/grey pigments the birds produce themselves). Luckily for me, the feathers had already been pulled from the birds and some birds had a blood slide made for them with the numbers and types of white blood cells counted. Eighty hours of feather scanning with a spectrometer, a run through Spectra Suite and Colour Analysis Program, and some data crunching in SPSS we had our results! And they were not what we were expecting. I should also note that we ended up including keel and fat score in our results because they had some interesting relationships with feather coloration.
Contrary to our original hypothesis, our measures of immune function did not appear to affect any of the feather coloration variables. We found that male rectrix feathers were brighter than female rectrices and that female rectrix feathers were more saturated than males. One hypothesis is that females have darker tail feathers to minimize predator attraction while incubating and males utilize brighter tail feathers as a secondary ornament (in addition to mask and bib attributes) to display health and/or individual quality. The results relating fat score to brightness (positive) are in line with the hypothesis that brightness may signal individual quality. Birds in better energetic condition grew brighter feathers. Further, the fact that males were brighter than females suggests that brightness in rectrix feathers may also be involved in female choice. The negative association between keel score and brightness was unexpected. There is some evidence that accumulation of carotenoid pigments at levels needed for pigmentation of thousands of contour feathers in goldfinches exerts long-term stress and contributes to skeletal muscle breakdown , which we may be seeing evidence of in Common Yellowthroats (probs not though, I had a sample size of 35 birds and one had a keel score of one [which is bad] which probably caused the brightness/keel score relationship significance).
While I was initially disappointed with my results, mostly because I wanted my first project to have super cool super exciting super awesome feather world changing results, I accepted them in the end. Not every scientific endeavor will shake the world with might. Some just get your foot in the door for what you’re interested in, which is what happened in my case. And with that, I will end this #ThrowBackThursday post. I believe I’ve babbled enough about what I love, I need to save my babble for future posts. Until next time!
If you’re super interested in carotenoid coloration after reading this, here are some of the papers I read when I was first learning about it and later used for my research project:
Faris, M. H. Determination and Quantitation of Carotenoids in Setophaga riticulla Deathers. MA thesis. University of Scranton, 2011. Print.
Guzy, M.J. and G. Ritchison. 1999. Common Yellowthroat (Geothlypis trichas), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online
Hatch MI, R.J. Smith, J.C. Ownes (2009). “Arrival timing and hematological parameters in Gray Catbirds (Dumetella carolinensis)”. Journal of Ornithology 151.3: 545-52. Print.
Hill, G. E. (1990). “Female house finches prefer colourful males: sexual selection for a condition-dependent trait.” Animal Behaviour 40(3): 563-572.
Hill, G. E. and W. R. Brawner, (1998). “Melanin–based plumage coloration in the house finch is unaffected by coccidial infection.” Proceedings of the Royal Society of London. Series B: Biological Sciences 265.1401: 1105-1109.
Huggins, K.A., K.J. Navara, M.T. Mendonça, and G.E. Hill. (2010). “Detrimental Effects of Carotenoid Pigments: The Dark Side of Bright Coloration.” Naturwissenschaften 97.7: 637-44. Print.
McGraw, K. J. (2006). “Mechanics of carotenoid-based coloration”. Bird Coloration: Mechanisms and Measurements. G. E. Hill and K. J. McGraw.eds. Cambridge, Harvard University Press: 177-242.
Montgomerie, R. (2006). “Analyzing colors”. Bird Coloration: Mechanisms and Measurements. G. E. Hill and K. J. McGraw.eds. Cambridge, Harvard University Press: 1
77-242 Montgomerie R, (2008). CLR: Colour Analysis Programs v1.02 ed. Queen’s University, Kingston, ON, Canada. Ocean Optics. (2007) Spectrasuite Spectrometer Operating software.
Pyle, P. Identification Guide to North American Birds. Bolinas (Calif.): Slate Creek, 1997. Print.
Saks, L., K. McGraw, and P. Horak (2003). “How feather colour reflects its carotenoid content.” Functional Ecology 17.4: 555-561
Smith, R. J. and F. R. Moore (2005). “Arrival timing and seasonal reproductive performance in a long-distance migratory landbird.” Behavioral Ecology and Sociobiology 57: 231-239.
Smith, R.J. and M.I. Hatch (2008). “A Comparison of shrub dominated and forested habitat use by spring migrating landbirds in Northeastern Pennsylvania” The Condor 110.4: 682-93. Print.
Tarof, S.A., P. O’Dunn, and L.A. Whittingham (2005). “Dual functions of a melanin-based ornament in the common yellowthroat”. Proceedings of the Royal Society B: Biological Sciences 272.1568: 1121-127.Print.