Ducks of a feather

I conducted this project as part of Professor Pringle’s EEB321 class at Princeton University.

The local pond is constantly brimming with the jovial, inquisitive, and—around dinner time—inpatient quacks of a group of 20 ducks. The group is a patchwork of Pekin, Indian runner, wild mallard, and crested ducks. As many of the ducks are domestic breeds, they are fed corn daily, but foraging under the water of the pond as well as in surrounding fields remains a key aspect of their lifestyle.

A crested (front) and Pekin (back) duck at the local pond one evening, seemingly in search of an after-dinner snack or resting place.

Although I don’t have an hour-by-hour analysis of how the ducks spend their time outside of foraging, most of them seem to spend a significant amount of time preening their feathers, a process which relies on them spreading oil from a specialized gland over their feathers. The adaptive value of these oiled feather structures is often showcased as the ducks glide through the water and exit the pond with water droplets rolling gracefully over them, as if they are wearing a sort of water-repellant suit. This past summer, when one of the ducks sadly fell ill and ceased its preening behavior, the condition of its feathers quickly deteriorated and the outer layer no longer acted as an effective shield from water and other particles. The vital water repellant function of the oil-coated feathers also seems apparent from these loose feather samples coated in beads of morning dew:

Interestingly, however, in addition to these somewhat regular-looking water repellant feathers, I often come across disorganized, ‘fluff’-like structures that don’t seem to repel water in the same way as their more structured feather counterparts. Although the typical structured feathers do tend to have downier, disorganized-looking regions at their base, the relatively disorganized unit below appears somewhat distinct to me (i.e. not taken from the base of a regular feather), although more observations are needed to confirm this. The snaking tendrils of this fluffy structure give me an impression of knotted tinsel, but maybe I’m just overly eager for the holidays.

I was curious to examine and compare the micro-level organization of these fluffier structures with the more organized feathers to gain more insight into the respective physiological and functional roles they play to suit ducks so uniquely for life in the water. Using my Foldscope, I produced the images below of an example typical structured feather from a Pekin duck:

Although these images certainly would benefit from clearer focus, they nonetheless depict interesting microstructures around the central shaft of the feather. In the left image, the central shaft appears to be comprised of an interesting sort of crisscrossed structural network. In addition, what I assume are barb structures extend outward from the shaft at a relatively consistent angle on each side. The consistency of this angle seems to contribute to the observed macro-scale regularity of the feather structure. In contrast, the central structure in the image on the right is slightly larger and shows what I believe is another portion of the shaft structure. This part of the shaft seems bilaterally symmetric, consistent with the macroscale bilateral symmetry of the feather. The composition of the shaft is a bit more difficult to decipher here (as the image wasn’t able to distinguish a clear pattern like the crisscross pattern in the left image), but barbs once again appear to extend out at consistent angles, although they appear slightly less consistently angled than in the left image. Overall, I am curious about the differences between the left and right images of the putative shaft structure: assuming that they indeed depict different parts of the same shaft structure, how and why do these differences arise along the shaft? What benefits might these progressive changes in structure potentially confer to the feather unit and feather assemblage as a whole, if any? I’m also curious about the relative amounts of oil on the surface of the respective regions of the feather, which I can’t directly judge from the image, but which I would predict to be higher further from the shaft base, which is downier and closer to the skin.

I obtained the following two Foldscope images from the disorganized ‘fluff’ structure, presumably also from a Pekin duck:

Once again, these images are plagued by a regrettable lack of focus and poor lighting. However, at the very least, they capture the disordered nature of the tiny strands comprising this fluffy structure. The sizes of these strands seem variable compared to the relatively constant sizes of the barbs extending from the more ordered structure above, and these strands also appear to lack the same rigid linear shape, likely illustrating a different structural composition. I’m curious if their individual protein structures differ at perhaps the tertiary and quaternary levels, or if these differences arise at some other structural levels. Furthermore, I wonder about the benefits and growth locations of these downy structures compared to the more ordered feather structures; I would predict that these would grow in more basal layers closer to the skin for the primary purpose of temperature regulation, while the more ordered structures would primarily serve as environmental barriers, their regularity creating a water repellant shield with broad coverage once coated in oil. Once again, I am also curious about the amount of oil on these components of the fluff structure, which I would predict to be minimal or low compared to the oil content on the components of the feather structure.

Overall, these images reveal some intriguing differences in the microscale structural properties of ordered feathers and disordered ‘fluff’ on ducks, demonstrating the complexity underlying these signature avian adaptations. After all, ducks of a feather swim together, in large part thanks to these fascinating microscopic properties.

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