What is causing the level of fluid in my hummingbird feeder to go up and down?

July 18, 2012 6:19 PM

The amount of hummingbird food in my feeder is going up and down on its own. What's going on? Is it possible that it's acting as a thermometer or a barometer?

This morning my wood-and-glass hummingbird feeder was empty. At 5 PM it was 1/8 full. Now it's empty again.

My best guess was that this is resulting from changes in temperature or barometric pressure. At 11 AM it was 985 hPa and 88°F. At 5 PM it was 983 hPa and 89°F. And now it's 983 hPa and 82°F. So that doesn't seem to explain things.

Short of a hummingbird regurgitation, I'm out of ideas. Any suggestions as to what's going on here?

posted by waldo to science & nature (30 answers total)

Is it windy enough that gusts could be re-distributing the feed?
posted by 6550 at 6:22 PM on July 18, 2012

D'oh. I forgot to explain that hummingbird food is liquid, not a solid. The feeder looks like this, with an enclosed, opaque hopper in the bottom.

It was dead calm today.
posted by waldo at 6:25 PM on July 18, 2012

Is anyone else around that likes hummers and might be filling it up in your absence?
posted by Burhanistan at 6:42 PM on July 18, 2012

Good Lord, I hope not. I live on 30+ acres on the top of a mountain with a half-mile-long driveway with a couple of dogs who alert me to any visitor. :) I was here all day!
posted by waldo at 6:45 PM on July 18, 2012 [4 favorites]

If the feeder in the sun in morning or afternoon, such that its temp could be higher than the outside air temp during those times?
posted by LobsterMitten at 6:49 PM on July 18, 2012

That is, if it's in the sun from say 1-4, it could be cool in the morning, heat up in the afternoon, then cool off again in the evening -- so it could indeed be acting like a thermometer, with the liquid rising up the column as it expands in the warm sun and then lowering when it cools.
posted by LobsterMitten at 6:52 PM on July 18, 2012 [1 favorite]

didn't rain did it?
posted by edgeways at 6:54 PM on July 18, 2012

If the feeder in the sun in morning or afternoon, such that its temp could be higher than the outside air temp during those times?

Clever suggestion! The feeder is in the sun in the morning, and then again in late afternoon, which I know the opposite of what explains things, but I still think it's possible that you're onto something. It's hanging from the eastern edge of the eave of a covered porch, and it gets sun throughout the morning and again when the sun gets below the porch's roof on the other end. Maybe the heating in the morning sort of primes the pump? Perhaps water saturated with sugar holds its temperature particularly well, and the heat of the day finishes it off?

didn't rain did it?

I'm afraid not, though I sure wish it had. :)
posted by waldo at 7:37 PM on July 18, 2012

If it only happened once, the most logical explanation is that you made a visual mistake. You may have seen something like a red-breasted hummingbird behind the clear plastic that made it appear 1/8 full. The scientific thing to do is see if it happens again, then walk over and investigate.
posted by weapons-grade pandemonium at 9:27 PM on July 18, 2012

The feeder is in the sun in the morning, and then again in late afternoon, which I know the opposite of what explains things

Seems to me that warming that feeder would tend to expand the air in the enclosed part above the liquid and drive the liquid level downward, while cooling it would make that air contract and suck liquid back up out of the base.

My guess is that you are indeed looking at a low-precision thermometer.
posted by flabdablet at 9:32 PM on July 18, 2012 [2 favorites]

The thing worth noting here is that gases expand much more than liquids do in response to temperature. Take water that is near freezing and bring it to a boil and you'll see a slight change in volume. But water vapor, over the same range has a 36% change in volume. If it goes from say 60° at night to oh, maybe 120° if it's sitting in direct sunlight. The volume changes in proportion to the temperature change in Kelvin, so that's 288° K to 322° K - which accounts for a 12% change (which is right about 1/8).

Since the bottle is bigger at the top then the bottom, if you're talking 1/8 up from the bottom, rather than in terms of total volume, it's possible the temperature change was quite a bit smaller.
posted by Kid Charlemagne at 9:55 PM on July 18, 2012 [2 favorites]

Incidentally, the reasons that mercury and alcohol thermometers work the opposite way around to this is that the heat source whose temperature being measured with one of those gets applied directly to the fluid reservoir at the bottom of the thermometer, and the resulting tiny expansion or contraction is made visible by virtue of the extreme narrowness of the indicator tube. Also the indicator tube itself is evacuated so that the liquid expansion doesn't have back pressure to work against.

In your feeder there is no indicator tube, and most of the assembly is functioning as the reservoir for the thermometer's working fluid, which is air; the whole arrangement is kind of upside-down with respect to a conventional fluid thermometer.
posted by flabdablet at 9:56 PM on July 18, 2012 [1 favorite]

I think a hummingbird feeder would act as a kind of water barometer or storm glass, so it would respond to air pressure changes as well as temperature changes. (Apparently they're quite sensitive to both; if you want to use one as a barometer you need to keep it at a constant temperature.)
posted by hattifattener at 11:09 PM on July 18, 2012 [1 favorite]

This seems like it has to be a visual illusion. Did you walk right up to it when it appeared to have fluid in it?

Changes in temperature, barometric pressure, or relative humidity simply won't change the volume of fluid much at all. Thermometers, hygrometers, and barometers all go to great lengths such as using special materials and mechanical advantage to make the effect visible to the naked eye. The feeder has none of these. Glasses of water don't magically fill when the weather changes (well, the do when it rains, but...) so neither will the feeder.

My guess is that the combination of viewing angle and direction of sunlight the red from the base was reflected off and/or through the bottom facets of the reservoir making it apear that it contained fluid.
posted by Ookseer at 11:35 PM on July 18, 2012 [1 favorite]

Changes in temperature, barometric pressure, or relative humidity simply won't change the volume of fluid much at all.

That's true, but barometric pressure and especially temperature variations will change the volume of the gas trapped above the fluid inside the top of the feeder. And if the feeder is apparently empty, that volume is relatively large.

In fact the feeder can't have been completely empty when it appeared to be so; it must still have contained some liquid inside the base where the beak holes are, so that when it cooled down again and the gas in the top part shrank some, there was fluid available for the top container to suck back up.
posted by flabdablet at 2:48 AM on July 19, 2012

If it only happened once, the most logical explanation is that you made a visual mistake.

That's what I told my wife when it happened the day before. :) It's happened at least a few times.

This seems like it has to be a visual illusion. Did you walk right up to it when it appeared to have fluid in it?

Yup. When my wife reported this happening, I patiently explained that the fluid level couldn't possibly go up and down independently, pointed out that there was none to be seen in the glass reservoir, and informed her that it must have been an optical illusion. This was received...poorly. (New AskMeFi: How do I apologize to my wife for strongly implying that she's insane?)

In fact the feeder can't have been completely empty when it appeared to be so; it must still have contained some liquid inside the base where the beak holes are

This is an important point that I omitted, a result of me foolishly assuming that the whole world is well aware of how a hummingbird feeder works and what hummingbirds eat. :-/ Yes, you are absolutely right.

Seems to me that warming that feeder would tend to expand the air in the enclosed part above the liquid and drive the liquid level downward, while cooling it would make that air contract and suck liquid back up out of the base.

This is very clever. Combined with Kid Charlemagne's point that "gases expand much more than liquids do in response to temperature" and flabdablet point about the method of construction of a hummingbird feeder, I believe that we have an explanation here. As with so many good AskMeFi answers, I learned a lot about science in the process. :)

I'm glad to know that there's an explanation here that doesn't involve my wife and I hallucinating due to buildups of carbon monoxide in our house. :)
posted by waldo at 5:22 AM on July 19, 2012 [1 favorite]

In retrospect, I realize the second sentence in my comment was pretty garbled, so let me clear things up a bit.

The relevant equation is: V1/T1 = V2/T2

Or, if you want to work pressure in to it, P1∙V1/T1 = P2∙V2/T2

If pressure remains constant (It tends to stay within a few percent in the great outdoors. If you are in the eye of a hurricane or other extreme weather system that might not be the case.) then volume is directly linked to absolute temperature. You have to use the Kelvin (absolute) scale for this though. To get to the Kelvin scale you take the temperature in Celsius and add 273 to it. So 32°F or 0° C (where water freezes) is 273° and 100°F or 38° C is 311°

The shift to Kelvin matters because if you want to double 32° F you have to heat to 523° F, not 64°.
posted by Kid Charlemagne at 7:38 AM on July 19, 2012

If it goes from say 60° at night to oh, maybe 120° if it's sitting in direct sunlight. The volume changes in proportion to the temperature change in Kelvin, so that's 288° K to 322° K - which accounts for a 12% change (which is right about 1/8).

This is talking about a 12% increase in gas volume from morning (60 °F) to late afternoon (120 °F). That change in gas volume would have pushed some liquid out (the feeder is not a closed system) which might make the liquid volume look lower in the afternoon and then slightly higher (the gas has compressed) in the morning (though still lower than the previous day).

That is the opposite of what was reported.

This morning my wood-and-glass hummingbird feeder was empty. At 5 PM it was 1/8 full.

And as Ookseer noted these changes won't affect the volume of the liquid significantly, which is what we would need for the apparent liquid volume to have increased.

I would love to see some precise data on all this but at this time I'm leaning strongly towards optical illusion.

I would also note that optical illusions do not require hallucinations and that being fooled by them is not a sign of insanity.
posted by mountmccabe at 9:54 AM on July 19, 2012

I would love to see some precise data on all this but at this time I'm leaning strongly towards optical illusion.

I would also note that optical illusions do not require hallucinations and that being fooled by them is not a sign of insanity.

Well, when the fluid level is high, I am able to walk around it and observe the bright-red fluid level quite clearly. If I tilt the feeder, the fluid levels out accordingly. When the fluid level is low, it appears that there is no fluid in it whatsoever, although I imagine that if I turned it 90°, fluid would emerge from the base. In short, it behaves precisely as one would expect a 1/8 full hummingbird feeder to behave.

Given this set of facts, what test do you suggest that I apply to determine whether this is an optical illusion (and what sort of optical illusion do you believe it to be)?
posted by waldo at 10:55 AM on July 19, 2012

That is the opposite of what was reported.

You're right - and now I'm truly puzzled.

Can you tell what, exactly, is going on in that base? The humming bird feeders I'm most familiar with are like little water bottles for flying hamsters, but I was assuming that there was some kind of reservoir that fluid could be pushed into and pulled back out of hidden inside the base.

Is the bottle actually that red color or do you have the version with a more transparent reservoir (I found this while looking for an image explaining the internal workings of the thing.)

If it's an optical illusion (which may not be quite the right word) I'd bet on some version of total internal reflection due to the angle of the light where you have the feeder placed.
posted by Kid Charlemagne at 11:44 AM on July 19, 2012

Can you tell what, exactly, is going on in that base? The humming bird feeders I'm most familiar with are like little water bottles for flying hamsters, but I was assuming that there was some kind of reservoir that fluid could be pushed into and pulled back out of hidden inside the base.

Yup, your assumption is correct—there is a reservoir of fluid down there.

Is the bottle actually that red color or do you have the version with a more transparent reservoir (I found this while looking for an image explaining the internal workings of the thing.)

The bottle is clear glass, and the fluid is red.

If it's an optical illusion (which may not be quite the right word) I'd bet on some version of total internal reflection due to the angle of the light where you have the feeder placed.

If so, then I'd really want to patent this and start producing copies for Hammacher Schlemmer, because it is stunningly identical to a 1/8 full bottle of fluid. It looks 1/8 full from all angles and it sloshes properly. Note that it's only looked full when it's not in sunlight, but shaded by the whole of my house.

Maybe I need to dump out the fluid when it's high and when it's low, to both weigh it and measure its volume.

Or maybe a prankster hummingbird is hiding in the bushes, laughing his tiny ass off.
posted by waldo at 12:05 PM on July 19, 2012

If I really wanted to get to the bottom (or at least 1/8) of this

-Mark volumetric graduations on the cylinder with divisions a minimum of 1/10 of the full volume (the finer the better).
-Take the mass the empty feeder on a balance with a precision of at least +/- 1 g.
-Place the feeder and the liquid in sun and in the late afternoon fill it back to ~1/8 (but right on one of the graduations).
-Track properties over at least the next couple days, recording (in this order) the time, apparent volume (standing in the same spot each time), mass (feeder and liquid), air temperature and feeder surface temperature (if possible). Barometric pressure may be useful too.
-The more readings the better but I would aim for every hour during the most exciting times (when the sun hits the feeder in the morning, minimum and maximum temperatures) and probably let it slip to every two or three hours other times.
-I'd probably also put netting around the feeder to keep those pesky hummingbirds from drinking any of the liquid and skewing the data.

I'd also consider repeats of the test with higher initial liquid volumes.

I'd have to look at the data to know what it meant but I'd look for things such as:

-How much of a volume change is there? Are the graduations fine enough to get distinct readings?
-Does the mass drop more than would be expected due to evaporation? (Which may require reference tests).
posted by mountmccabe at 12:06 PM on July 19, 2012

I wish I could photograph my wife's face as I explain this to her. :)
posted by waldo at 12:14 PM on July 19, 2012 [1 favorite]

And I really don't have any guesses for what may be producing the illusion, if it is an illusion. I am only leaning towards that because I cannot think of a physical explanation.

If so, then I'd really want to patent this and start producing copies for Hammacher Schlemmer, because it is stunningly identical to a 1/8 full bottle of fluid.

Video/time-lapse photography would be a good start towards selling/documenting this, assuming the wind cooperates.
posted by mountmccabe at 12:15 PM on July 19, 2012

mountmccabe, perhaps you're missing this part of the setup.

Note that the container as shown is made of colored glass, meaning that its temperature (and very quickly that of the air inside it) will depend quite sensitively on how much direct sunlight falls on it.

The temperature of the liquid food, and that of the feeder base, won't matter much at all.

So there's really no need to go all Science! technician about this. To work out whether the "gas thermometer" model is plausible, all you need to do is feel the glass container. If it's always warm when the fluid level is low, and always cool when the fluid level is high, the model is most likely correct.
posted by flabdablet at 5:57 PM on July 19, 2012 [3 favorites]

If pressure remains constant (It tends to stay within a few percent in the great outdoors. If you are in the eye of a hurricane or other extreme weather system that might not be the case.) then volume is directly linked to absolute temperature.

In theory the air pressure inside the top chamber of the feeder won't be the same as atmospheric pressure except when the fluid levels inside and outside that chamber are equal. Any time there's fluid visible inside the top chamber, the air pressure inside will be lower than atmospheric pressure by enough to support a column of fluid that high.

You can work out the required pressure difference by multiplying the height of the fluid column (that is, the difference in fluid levels between the inside and outside of the top chamber) by the density of the fluid and the acceleration due to gravity.

The density of water is one gram per cubic centimetre; maple syrup is about 1.3g/cm³. I expect hummingbird food would be somewhere in between; let's say 1.1g/cm³. The acceleration due to gravity is 9.8m/s² = 980cm/s². So for a fluid column height (measured relative to the level outside the top chamber) of say 4cm, the pressure inside the chamber would need to be (4cm × 1.1g/cm³ × 980cm/s²) = 4300g/cm/s² = 4.3kg/cm/s² = 4.3hPa below atmospheric.

This is small compared to the atmospheric pressure of 983hPa measured at your site, so it's perfectly reasonable to ignore it and work on the basis of gas volume being directly proportional to absolute (Kelvin) temperature.
posted by flabdablet at 6:56 PM on July 19, 2012

One last point and I'll stop machine gunning this question: measuring the mass of the fluid in the feeder is likely not going to reveal much, as the fluid is not going away - it's simply moving back and forth between the base of the feeder and the top reservoir chamber.
posted by flabdablet at 7:03 PM on July 19, 2012

...and it sloshes properly.

That pretty much rules some sort of illusion out - I figured you were looking at it through a window or some such.
posted by Kid Charlemagne at 9:44 PM on July 19, 2012

Maybe I have misunderstood the ambient conditions the OP is dealing with.

OP:
This morning my wood-and-glass hummingbird feeder was empty. At 5 PM it was 1/8 full. Now it's empty again.

If "morning" here means "11 AM, after the feeder has been in direct sunlight for four hours" and "5 PM" means "5 PM, after the feeder has been cooling in the shade for five hours" and "Now" means "a little bit ago when I was outside, just after sunset so the feeder had been in direct sun for three hours" and the air temperature hasn't changed much all day then I can see how it could be working as suggested, flabdablet.

Now that I look at it like this the information was there, I just missed it, assuming different conditions.
posted by mountmccabe at 5:51 AM on July 20, 2012

One last point and I'll stop machine gunning this question: measuring the mass of the fluid in the feeder is likely not going to reveal much, as the fluid is not going away - it's simply moving back and forth between the base of the feeder and the top reservoir chamber.

And I think that's what it would demonstrate, too. If the mass of the fluid remains the same, then I'll know that it really is moving back and forth. But if the mass changes, then I'll know...well, I'd be terribly confused, actually, but I'd definitely know it's not moving back and forth. :)
posted by waldo at 9:40 AM on July 20, 2012