Storm + Winooski River Paddle Revisited

Bonus post: Just wanted to post a quick video I put together from that epic thunderstorm last night. For a better synopsis of conditions leading up to and during the storm, check out The Weather Rapport.

What: Last Wednesday and Thursday I paddled the Winooski River from Bolton Dam down to Salmon Hole in Burlington (about 30 miles or so). There are a couple of great paddling guides online (1) (2) that give a sense of what to expect from the conditions on the river, but do little to describe the sheer joy of getting lost in the meandering freedom of the Winooski's bends and folds. Only occasionally did the old span bridge or rumble of a semi's engine break remind me of the thin border of silver maples separating me from the faster pace of asphalt and clocks.

Ecological notes: I was particularly fascinated with the enormity and odd grace of the committee of turkey vultures that seemed to defy my cultural expectations of what a vulture should be (on Friday I'll post more about collective nouns, like committee, for animals). Wednesday evening I came to a fork in the river where it split around an island. I saw a turkey vulture circling low on the south stretch of the island and so I set down that direction. As I slowly approached, another turkey vulture landed on a gravel spit about a quarter mile down from me, followed shortly by three more. I tied up my boat and crept slowly along the shore line to get a better view. As I rounded the bend I was treated with a spectacular view of about 25 turkey vultures either sunning themselves on the rocks or perched up in the trees preening themselves.

In that late afternoon heat I wanted nothing more than to spread my own arms out and sun myself on the rocks with them. I couldn't help but wonder if that was the ultimate motivation for the turkey vultures' behavior. When I got back, Callan and I came up with a pretty short list of reason why we thought the vultures (and other birds) might do sun themselves. Here's what we came up with:

1. Kill bacteria by drying them out (they're detritivores, an animal that eats dead stuff, which is probably a source of all kinds of nasty bugs)
2. Kill other parasites, like taking a dust bath
3. Produce Vitamin D
4. Warm themselves up before going to bed
5. Warm themselves up to increase metabolic rate, digest food faster
6. Visual display of power to other turkey vultures
7. Dry off

I've also seen cormorants do this (see photo above), pelicans, and green herons. Some quick research shows that many birds do this, but their posture is different from the vultures and cormorants shown above. Indeed only a few others, like anhingas, storks, and other herons, do this standing up with wings spread. The ring-billed gull colony I encountered the following afternoon had about 300 individuals (plus a small handful of mergansers) all standing facing the sun, but with wings closely tucked in).

Some patterns in the list of birds that sun themselves standing up with wings out emerge: many are aquatic, they tend to be darker, they're carnivores, they're very large birds, they live longer than most birds, and most are in the stork order, Ciconiiformes. After writing this I looked up the order of pelicans (Pelicaniformes) to see if it used to be included in Ciconiiformes, and sure enough Pelicaniformes is often proposed as part of Ciconiiformes. What's even cooler and more surprising is that new world vultures (as opposed to buzzards and their old world kin) are close cousins of the storks. So if this behavior of standing up and sunbathing is monophyletic (occuring only in one evolutionary branch of birds), then it might be reasonable to assume it originated in a single ancestor of new world vultures, herons, storks, cormorants, and pelicans. The behavior might then serve a unique purpose among these birds.

I didn't find anything conclusive, but it looks like birds will sunbath for all the reasons that Callan and I came up with. A 1957 paper on the topic indicates that birds might also sunbathe socially; if one starts sunbathing then others will join her/him. I'm not sure what the primary purpose of sunbathing is for these birds, but I'll bet spending lots of time in/near stagnant water (for cormorants, herons) and eating carcasses (vultures) makes these birds susceptible to parasites so having a way to kill them would be good (maybe the black of a vulture heats their feathers up to a higher more lethal temperature).

Additionally, vitamin D deficiency can pose skeletal problems, making large body size a critical reason to sunbathe. Ciconiiformes and vultures don't migrate far, so in the winter they're not getting as much vitamin D as say migratory song birds might. Vitamin D deficiency prevents an animal from absorbing calcium from food. Prolonged deficiency (as with people in northern latitudes) can result in weakened bones that bend and curve under the body's weight (like the stereotypical bow-legged Eskimo). Sunning would be an effective way of producing vitamin D.

Another source says that warming up feathers "resets" their curvature, bringing them back to their most efficient shape, an obvious advantage for a turkey vulture, which spends most of its time soaring looking for food. But I couldn't verify this with any reputable sources. 

Where: Winooski River, about 2 miles upstream from Richmond.