why does sugarwater level in hummingbird feeder change, day vs. night?

September 5, 2013 4:55 PM

This has been asked before with many responses, none satisfactory. The level changes every day in the conical shaped liquid reservoir, due I presume to temperature changes, but the system is open, with four feeding tubes that should but do not overflow in hot sun. What is happening?
posted by fkeese to Home & Garden (8 answers total)

I was one of the people who asked this here, with no responses that proved to be correct, so I'll be watching this closely. It's a heck of a mystery.
posted by waldo at 5:20 PM on September 5, 2013

Discussed previously here--

Barometric pressure and temperature change of the air inside the feeder seem like the obvious explanation. How exactly did these get ruled out?
posted by mullingitover at 5:24 PM on September 5, 2013

Because, as my wife pointed out to be, our hummingbird feeder hangs under our covered porch, facing north—it's never in direct sunlight. For the fluid levels to change dramatically between 11 AM (when the pressure was 985 hPa and the temperature 88°F) and 5 PM (when it was 983 hPa and 89°F), something other than pressure and temperature must be at work, presumably. Direct sunlight was presumed to be that other thing, but we can see that's not the case.
posted by waldo at 6:45 PM on September 5, 2013

I've wondered this about mine and the other people in my house thought it might be condensation. I notice it more on hot days and I do see as the day heats up a lot of condensation at the top of the glass feeder. I am not science-y enough to know whether I should believe that this explains it.
posted by hilaryjade at 7:17 PM on September 5, 2013

11 AM (when the pressure was 985 hPa and the temperature 88°F) and 5 PM (when it was 983 hPa and 89°F)

Hmm, yeah, the direct sunlight would help explain a lot. But . . .

Your other measurement was "983 hPa and 82°F" at about 8:20pm. That is a 7 degree temperature swing and in the dead of night the temp drops another 10 degrees or so.

So you have the feeder getting down to, let's just say, 70 degrees at night and say 95 in the middle of the day..

A swing of say 20 or 25 degrees up and down over the course of 24 hours is plenty to explain the phenomenon you saw. (And note that the temperature swing in Charlottesville on the day in question was actually 23 degrees.)

All that makes me think of these things:

1. When the feeder is nearly empty of liquid, that means it is nearly full of air. That air is what will expand and contract with the temperature & barometric changes, so when the feeder is nearly empty of liquid is when you'll see the largest swings of this type.

2. The feeder will not be the exact same temperature as the ambient air. Especially in that northern/shaded position it is likely to lag behind ambient air in its temperature changes. How much? You'd have to measure the actual feeder/water to know for certain.

3. When the temperature rises, the air will expand, driving the liquid down. But if the feeder is nearly empty of liquid, that means it is nearly full of air. So in that situation, it is quite possible that that air will expand enough to drive all the liquid out and in addition keep expanding more and burp some of the air out. So then when it cools down some, the air will contract some and that will suck liquid in again (keep in mind that liquid hasn't really gone anywhere--it's just in the lower dish). Now, because some of the original air has now been lost (burped out), when you get back to your original temperature, you will have less air volume and thus more liquid than you had the first time around.

It would go something like this:

6am: 1/16 full
11am: near empty (air has warmed & expanded some, driving out most of the water)
1pm: temp still rising, air continues to expand and now some of it burps out due to the expansion cause by the heat
3pm: 1/16 full, temp has dropped some, that air volume has contracted some and this sucks some water back in from the dish
5pm: 1/8 full, temp has dropped more, back to the 11am level, but with some of the air burped out at 1pm, there is less air left in the feeder, meaning that it must suck in more water to fill the space.

4. At 8pm, the temperature continues to go down, contracting the air and sucking more water into the main container at the feeder. Since there isn't much water in the feeder to start with (1/8 full, which isn't much in that style of feeder), eventually it sucks essentially all the water from the bottom dish of the feeder up into the main container.

But at some point, the bottom dish is sucked dry, the seal at the bottom of the main container is lost, and now it's sucking in air instead of water. So eventually a big glurp of air enters, allowing most or all of the water to drain from the main container down into the bottom dish.

So something like this:

7pm: temperature continues to drop, air volume contracts, so now the main container 3/16 full, and the dish below nearly sucked dry
8pm: dish is sucked dry, loss of seal means a nice air bubble enters the main container, allowing all the water in the main container to empty into the dish. Glurp!

8:20pm: You check again and find the level is zero or close to it.

All this is a bit speculative but definitely possible. To find out for certain you'd have to carefully observe the feeder for an extended period. A lot of it would be slow, steady, almost unnoticeable change over a long period of time and then a rather large, sudden change (glurp!) that might take just a second. So even if you're trying to pay attention, you would still almost certainly miss it. So maybe a webcam focused on it all day long and then review the footage later? Maybe fill the feeder 1/8 full, put it in the oven, and dial the temp up and down so you can see what happens as the temperature goes up or down 25 degrees or so?

At any rate, I'll wager all this 'strange' behavior happens when the water level is very low, down around the 1/8 level, and it doesn't really happen when the main container is, say, more than half full. When the water level is low is when you can get the 'burps' both going out & coming in.

The other bit of 'secret' is that the '1/8' amount of water isn't just appearing and disappearing from nowhere. The dish at the bottom serves as a reservoir where this amount water can be stored and later retrieved if conditions are right.
posted by flug at 8:03 PM on September 5, 2013

The heat causes the water to evaporate during the day, the concentrated sugar water absorbs moisture during the damp night.

Barometric pressure isn't going to make a difference. What makes barometers work is the weight of the liquid pulling down on a vacuum, and the pressure of the air counteracting that. If there is no vacuum, there is no barometer.
posted by gjc at 8:03 PM on September 5, 2013

The dish at the bottom serves as a reservoir where this amount water can be stored

Btw the style of feeder described by fkeese is different, with the feeder tubes rather than the reservoir at the bottom. (It would help us, fkeese, if we could have a photo of the feeder in question.)

But I think the same general dynamic is at work:

- It likely happens only when the amount of water in the main container is fairly low
- The 'tubes' in fkeese's feeder servers the same general function as the 'lower dish' in waldo's - as a reservoir where the water from the main container can go to or come from, without gaining or losing any water overall.

With four feeder tubes, there is actually quite a lot of storage space there.

Now if the feeder tubes were all simply full of water--no air whatsoever--what you're describing would be impossible. Expansion of the air in the main container would drive water into the feeder tubes. Since those tubes were full to begin with, that would force water out the end of them.

But that's not how the feeder tubes work. What happens is, as the hummingbirds drink some of the water in the feeder tubes, an air bubble forms in the tube. As they drink more, the bubble gets bigger. Finally the bubble gets big enough, it (the air in the bubble) glurps up into the main container, that adds a bit more air to the top of the main container, which releases some of the vacuum in the main chamber, allowing water down into the feeder tube to refill it.

So at any given time the feeder tubes are not 100% full of water, but have some water and some bubbles. Or maybe they are mostly bubbles and just a little water . . .

So as the temperature rises, the air in the main container expands, and drives some water down into the feeder tubes. But instead of driving water out, it drives the bubbles out and you don't notice a lot of water being forced out.

In short, the pressure changes of the air in the top of the main container of the feeder can almost completely fill and empty the four feeder tubes, which amounts to a fair amount of up & down of the liquid level in the main container without gaining or losing any significant amount of the sugar water.

You could do the same experiments as waldo to verify (videotape the feeder over a 24 hour period, or put it in the oven and experiment with temperature changes). But the first thing I'd do is inspect the feeder tubes to see if you can see the air bubbles I'm talking about. I'll bet you'll find them . . .
posted by flug at 8:25 PM on September 5, 2013

Response by poster: flug, I like your answer. I think though that it requires active hummingbirds eating for it to work. Soon they will be leaving and the level should be unchanged. I'll see in 6 weeks or so.
posted by fkeese at 12:53 PM on September 8, 2013