Sometimes I observe this phenomenon when birds land on water and other times I don’t. I believe it depends on the landing style of the bird but I’m trying to figure out how and why it happens.
1/3200, f/6.3, ISO 640, Canon 7D Mark II, Canon EF 500mm f/4L IS II USM + EF 1.4 III Extender, not baited, set up or called in
I photographed this juvenile Double-crested Cormorant five days ago as it was landing on a local pond. I think photos of birds landing on water can be quite dramatic, in part because of the water splash they create, so I have to pay particular attention to that splash or “plume” when I’m cropping the image to get the composition I like best.
But with cormorants and a few other water birds I’ve noticed something intriguing about the water disturbance they make. That disturbance often extends far out in front of the bird where I wouldn’t expect it to be, especially because I’m quite sure it isn’t water thrown forward by the feet. Both when I see it with my naked eye and when I photograph it that water seems to come almost straight up from the surface of the water far in front of the feet.
If so, and I’m quite confident it is, how can that be? And what makes it happen?
It doesn’t happen with many species, only some. American White Pelicans for example don’t cause water to rise far in front of their feet when landing on water. Instead they appear to keep their webbed feet nearly flat on the surface and simply “water ski” on the water until their slowing speed won’t support their weight and they sink into the water, much like the floats of a landing seaplane.
I have several photo sequences of pelicans landing on water and that’s what happens every time and in every photo in the sequence.
But in my experience cormorants don’t keep their webbed feet nearly flat – instead they’re at a steeper angle to the water (the angle of one foot can be seen here in the same photo as the first one) so they appear to use their feet more as brakes than as water skis and as a result they sink into the water more quickly. And that might make the casual observer assume that the water in front of the bird is thrown forward by the angled feet but I don’t think so. I’m convinced it comes almost straight up out of the water far in front of the feet. If so, how and why does that happen?
Well, I have a theory. Keep in mind this old biology teacher only took one college physics course and that was 49 years ago so my limited knowledge of the subject is rusty at best.
Unlike highly compressible gases, liquids like water cannot be compressed (see first note below). The cormorant’s feet at this steep angle and speed are pushing water molecules forward and those molecules are pushing on other water molecules in front of them. Since water cannot be compressed it must almost be like pushing a near-solid through the water and those pushed molecules in front of the bird have to go somewhere – if they didn’t they would stop the bird like it had hit a brick wall. So where do they go? They take the path of least resistance and go almost straight up with a little forward motion and that’s what we’re seeing here far in front of the cormorant’s feet (if they went down or sideways they’d just hit more dense water).
Well, there you have it – my theory for what it’s worth. I’d love to know for sure, one way or another, so any thoughts on the subject would be appreciated.
That’s assuming of course that you’ve had the stamina and fortitude to wade all the way through this nerdy post. Apologies to those who may not be intrigued by stuff like this but it’s been gnawing at me on and off for years now so I finally decided to just throw it out there and see what might come of it.
And yes, I’m prepared to be wrong but I had to start the ball rolling. If I am it won’t be the first time and after all, that’s what theories are all about.
Ron
Notes:
- Actually water can be compressed but the degree of compression is so negligible, even under extreme pressures, that water is typically considered to be non-compressible. There’s more here from USGS on the compression of water if you have the interest.
- Readers have probably heard it said that if you fall from great distances onto water it’s almost like landing on pavement or cement. It’s true and the reason is that water is essentially non-compressible – it doesn’t “give”. Even a belly flop from a low diving board will knock your socks off. But of course if you dive in a classic diving posture that doesn’t happen because you cleave the water as you enter it.
- My friend Jim DeWitt made in interesting point in a comment on this post on Facebook when he said “Like a lot of diving birds, the cormorant’s center of mass is well ahead of its legs and feet, at the back of the bird, as it were. That makes it impossible for a cormorant to “Water ski,” to use your apt phrase.” That’s because the leg attachment of many diving species is further back on their body than it is in other birds so if they tried to “water ski” with their feet almost flat on the water they’d topple over foreword. American White Pelicans don’t dive but cormorants do.
Ron,
First off I wanted to say thanks for sharing your passion and artwork with us; as someone just getting into bird photography it’s fascinating to hear more of what goes on behind the scenes to set up your shots.
Regarding the hydrodynamics of landing birds, I believe what you’re describing is a bow wave. These are formed by all kinds of objects interacting with a fluid: boats, swimmers, planes, spacecraft. I think part of what makes it hard to tell what’s happening is that we’re seeing it side on. From above I’d bet you’d see the same variety of arcs we expect to see on boats. Rockets and jets are making similar patterns, we just can’t see them with the naked eye (although some strange stuff starts to happen at extremely high speeds).
The most compression happens in the direction of movement, and the geometry of the object interfacing with the fluid is going to change how far out that wave is pushed and the shape it takes. Modern boats have a lot of design work put into minimizing bow waves as much as possible because they exacerbate drag, but I’d imagine some species of bird could benefit from extra drag to make shorter landings.
The cormarant is creating what’s called a stand off bow shock: a wave well in front of the object creating it. In reentry vehicles like space capsules this helps with heat dispersion, I wonder if the cormarant is using it to create a pocket of reduced turbulence in front of it in case it changes its mind and wants to take off right away?
It’s also likely that some birds are able to change this dynamic by altering the angle and position of their feet as they hit the water.
Rockets can actually sometimes do this when re-entering the atmosphere: they’ll fire the engines to create a buffer of exhaust gasses before the bow wave. Pretty crazy to think that the exhaust from a rocket engine is cooler than the air, which is being compressed so much it’s turning to plasma.
Thank you, Pangolin. I should have thought of using that term. So, if it’s a bow wave is it produced by a process similar to what I described?
I learned a lot from the post and comments. 😀
Me too, Arwen.
As a young thing my mother and the youngest of my brothers fought over who did my physics homework. I let them. Essentially colour me clueless.
Love the shots though. And decades after the event wish my well known curiosity HAD extended to physics. And am grateful to all those who were able to provide suggestions for me to ponder.
I’m with you, EC. I missed out on a lot because I was (and still am) largely intimidated by physics.
Hi Ron,
I think Bernie Creswick has the correct idea. I hope it is okay to include a link:
https://skygardener.zenfolio.com/p494133750/h2d9ccadd#ha07f6d22
This is a seven shot sequence of a Bufflehead landing. Note how it sits back with its feet at an angle. I think it is clear, particularly in the last shot that the splash is coming up from the feet.
John, in the case of the Bufflehead in the link you provided I think there’s no question that the water is being pushed forward by the feet and maybe that’s what’s going on here too. Now I’m not so convinced my “theory” holds water. Thanks Bernie and John for shedding additional light on the subject.
And yes, it’s always ok to provide a relevant link.
That does appear to be what’s happening with the cormorant………. 🙂
I am less adept at physics than you, so I have enjoyed the comments to your blog, and will remain mute to your query.
Great shots though, and thanks for sharing!
Great day today, however a tad cold – below 0 – not too bad now, sun isn’t right too much glare. Not complaining really!
Below 0 sounds like it’s more than just a tad cold! Thanks, Dick.
I love watching birds land on water. I know they know what they are doing but boy it sometimes looks so awkward ❗️
Enjoying some much needed rain here in Northern California 😁
Have a great day.
Me too, Diana. Enjoy your rain. We even have some of it here, not nearly enough though…
Your explanation sounds good to me. It seems like the foot is forming a sort of wedge in the water like divers do, which makes the entry splash. I only had a year of college physics and a couple of biomechanics classes and they were over 30 years ago, so I’m no expert at all. You’ve definitely piqued my interest though.
Physics set aside, it is a great shot! Gorgeous warm light on the front of the bird and very interesting wing positions.
Fluid dynamics isn’t my strong point either, Marty. I’m glad you mentioned liking the shot despite all of the other stuff I’ve presented to get in the way. I like it too.
Great shots Ron. Love the Pelican. Hope you had a nice Thanksgiving.
Everett Sanborn, Prescott AZ
Thank you, Everett.
Interesting observations and photos Ron. I think you explained it well, your theory posed makes sense to me. Those birds that can “surf/skate” on the surface are not displacing as much water as those that penetrate the water more. If you drop a small stone in calm water you get ripples. Drop a larger stone and you get larger ripples, and eventually you can get waves where some of the water being displaced will break free of the surface (tension) and appear like they could be a splash, thrown radially out from the point of impact of the stone.
When you land on a float plane is usually smooth as silk, I think that is a good analog to the American White Pelican landing style. On top of the surface, and not as much water displaced.
Thank you, Ed. As many times as you’ve landed in a float plane in Alaska you should know!
Jim DeWitt’s comment makes sense – appears part of the foot, or at least one toe, is a bit out of the water – would have to have an underwater camera to document……… guessing that’s not your thing! 😉 Interesting observation! 🙂 I also notice that the tail appears to be in the water – help slow things down perhaps? Post Thanksgiving fry our brains day! 🙂
“Post Thanksgiving fry our brains day!”
That made me laugh, Judy. Jim often makes insightful observations about birds and about science.
Ron:I do not seem to be getting any of Jim DeWitt’s comments on my copy of your blog. I get it via Feedly and I live in Oshawa,Ontario Canada. Any idea on this?
Jim Holliday.
Jim, Jim DeWitt rarely if ever comments directly on my blog. I post links to my blog on Facebook and he makes his comments there.
Ron, I had to check the meaning of nerd. The online dictionary I use said ia nerd is, “A single-minded expert in a particular technical field.” You are not single-minded, from what I can tell. You seem quite expert in bird biology, and likely biology generally. You’re also very good with a camera. And you seem very human, with interests in relationships with others, for example, former students and other people with an interest in nature. And you have interests in woodworking and history. So the single-minded clearly doesn’t apply. In my 6 years of studying physics as an undergraduate and graduate student, I never took a course that discussed aerodynamics or water-dynamics. But, as Dave says, your thinking about the cormorants splash seems solid to me.
And I’m certainly not an expert on the physics of water, Nancy!
We’ll just have to see how all this plays out. Thank you for the kind words.
I’m going to postulate an alternative theory. The angle of the cormorant’s feet is such that the water being splashed is being directed forward at a low angle by the bottom of the feet, causing splashed/disturbed water to move more laterally than upward. As for the Pelicans,
since their forward motion is much slower upon landing due to flaring, they don’t propel the water forward.
Thanks for providing the alternate theory, Bernie. And you could be right but I don’t believe that’s what I see with cormorants during the process. The water just doesn’t seem to be propelled that far forward by the feet while I’m watching it. I wish I had slow motion video of a landing cormorant so I’d know for sure. I looked for one online but struck out.
I wonder if both processes might be happening…
Ron, your theory sounds a little pushy but I’m buying it all the way.
Thanks for clearing the Thanksgiving cobwebs out of my brain.
“A little pushy” is an apt description in this case, Dave. Thanks.
This was a great read, it wasn’t too “nerdy” as you put it. I sometimes see Mallards and Canada geese do the same thing, but I never questioned exactly how they did it until now! Great photos as per usual!
Thanks, Xavier. It’s always piqued my curiosity when I’ve seen it happen.
Oh, and I think I can see a reason the water moves forward in these shots. When the pelican hits the water, there’s still some open space behind it, due to the short tail and the way its body is positioned. The cormorant however, has its tail and lower body in the water, so instead of going around the bird, it just moves forward. Only a guess though, I’d welcome 2nd opinions as well!
Xavier, See the third note from Jim DeWitt I’ve just added to my post. I think it’s related to your comment.
Yeah I hit post right before seeing it. Still very interesting though!