Why is my hummingbird feeder leaking in the sun?

The short answer

• An inverted bottle feeder that drips on sunny afternoons is working as designed. The sun warms the air trapped at the top of the bottle, the air expands, and it pushes nectar out of the ports.
• Air grows about 0.19% for every 1 °F of warming. From 70 °F to 110 °F that is about 7.5%. The nectar itself grows less than 1%.
• So the drip depends on how much air is in the bottle. A half-full 16 fl oz bottle pushes out about 0.68 fl oz (4 teaspoons). The same bottle filled to the top pushes out about 0.14 fl oz (1 teaspoon).
• Fill the bottle to the top, fill with nectar at outdoor temperature, and hang it in afternoon shade. A saucer feeder holds nectar below the ports and does not drip from heat at all.
• A drip in shade, at night or on a cool still day is not heat. Look for a cracked bottle, a missing or unseated gasket, cross-threading, or a feeder hanging off level.

Because the sun is warming the air trapped at the top of the bottle. Warm air takes up more room, and in an inverted feeder the only way it can grow is by pushing nectar out of the ports. Warm that air from 70 °F to 110 °F and it grows by about 7.5%. The nectar grows by less than 1%. So the drip depends on how much air is in the bottle, and the fix is to leave less of it.

Most pages on this stop at "heat makes it leak." The useful part is the size of the drip, because it tells you why a full feeder stays dry and a half-empty one makes a puddle.

What an inverted feeder is, and why it holds nectar at all#

An inverted bottle feeder is a bottle filled with nectar, sealed, and turned upside down over a base with feeding ports. The nectar sits above the ports. A saucer feeder is the other design: a shallow basin with the ports in the lid, so the nectar sits below them.

The bottle does not empty because air cannot get in to replace the nectar. When you turn it over, a little nectar runs out and the air left at the top, the headspace, stretches to fill the gap. Its pressure falls a little below the air outside. Outside air pushing up at the ports then holds the nectar in. Our hummingbird feeders page compares it to a water cooler.

That balance has a weak spot. Anything that makes the headspace bigger pushes nectar out. A 1995 feeder patent names the main ones: warming of the air in the reservoir, "causing the air to expand and expel the solution," a feeder "rocked by the wind," and a bird's bill at the port.

The arithmetic of a sunlit bottle#

Air expands in step with its absolute temperature. NASA's Glenn Research Center states the rule, known as Charles's law: at constant pressure, volume equals a constant times temperature. The pressure in the headspace stays close to constant here, because nectar can leave as the air grows.

"Absolute" matters. Temperature has to be counted up from absolute zero, not from 0 °F. The Fahrenheit version of that scale is called Rankine (R): add about 460 to the °F reading. NASA's own example uses kelvins, the Celsius version.

Now the numbers. 70 °F is about 530 R. 110 °F is about 570 R. Divide one by the other:

• 570 ÷ 530 = 1.075, so the air grows by about 7.5%.
• Each 1 °F of warming adds about 1/530 of the volume, or 0.19%.

That is our derivation from NASA's rule. It is close to the site's saucer vs inverted figure of about 7% for 68 °F to 104 °F.

The nectar also expands, but very little. The U.S. Geological Survey lists water at 0.99802 g/cm³ at 70 °F and 0.98870 at 120 °F. That works out to under 1% growth even across 50 °F, and less over the 40 °F used here. Nectar is four parts water to one part sugar, per our nectar recipe, and the cases below use water's figure, rounded up to 0.9%.

A worked case: one 16 oz bottle, half full#

Take a 16 fl oz bottle feeder with 8 fl oz of nectar in it. That leaves about 8 fl oz of air. Suppose it hangs on a west wall, the morning is 70 °F, and the sun brings the bottle to 110 °F by mid afternoon.

1. The air. 8 fl oz grows by 7.5%, so about 0.6 fl oz of extra air.
2. The nectar. 8 fl oz grows by at most 0.9%, so about 0.07 fl oz.
3. Together. About 0.68 fl oz has to go somewhere. It goes down into the base and out through the ports. That is about 4 teaspoons.

Worked the same way with the bottle filled to the top, there is almost no air to grow. Only the 16 fl oz of nectar expands: about 0.14 fl oz, or 1 teaspoon. That is why "fill it to the top" works. The same bottle drips nearly five times as much at half full.

Here is the whole range for that bottle and that warming, worked from the same two figures:

Read down the table and you see the second thing it shows. A feeder gets worse as the week goes on. It drips a little on the day you fill it and more each day after, as the birds drain it and the air takes their place.

There is a daily cycle too. As the bottle cools in the evening, the air shrinks, and outside air bubbles in through the ports to fill the gap. The next sunny afternoon, that air expands and drips again.

Why "leave room at the top" is the wrong advice#

You will read that you should never fill a feeder to the brim, so the nectar has room to expand. That is true of a sealed jar, where the air is a cushion. It is backward for an inverted feeder. Here the air is above the nectar and the ports below it are open. When the air grows, it acts as a piston and pushes the nectar out. More air means more drip, as the table shows.

The advice persists because it sounds like common sense and it is right for most containers. It is only wrong for this one. The real limit on filling up is freshness. Nectar in heat needs changing often, on the schedule in summer bird feeding. If a full bottle would outlast that schedule, a smaller feeder filled to the top beats a big one half full. The nectar calculator sizes a batch to your feeder.

The other causes, ranked#

Work down this list. The first question is always when it drips.

1. Sun on the bottle. It drips on warm afternoons, on the sunny side, and worse as the bottle empties. Fill it to the top and move it to afternoon shade.
2. Nectar warmer or colder than the air. Warm nectar heats the headspace after you seal it, so the bottle drips in the first hour. Nectar straight from the fridge does the reverse. It chills the air, more air bubbles in, and the bottle drips as it all warms to the day. Our nectar tool says to bring nectar to about outdoor temperature before filling. This is the reason.
3. Wind or tilt. The 1995 patent names a feeder "rocked by the wind." A drip only on windy days points here. Shorten the hanger or move the feeder out of the wind. Also look at it from the side. A bent hook can leave it hanging off level all the time, which is a tilt that never stops. Straighten the hook so the base hangs level, then test again.
4. A broken seal. Air has to stay out of the bottle for the vacuum to hold. A hairline crack, a gasket that is missing or out of place, or a bottle that is cross-threaded or not screwed down lets air in. Then the bottle drains toward the base level, in shade, at night, any time. The maintenance calendar checks for perished seals each April.
5. Birds at the ports. The patent also notes that a bird's bill breaks the film of nectar at the port. A few drops around busy ports are normal. Wipe them at each refill on the cleaning routine.

Saucer feeders do not drip from heat at all. The warm air leaves through the ports, which sit above the nectar, and takes nothing with it. If a saucer is wet, it is filled up to the ports or the basin is cracked. The fill rule in saucer vs inverted keeps it two thirds full.

Tell them apart in one afternoon#

To run the test, wash and dry the outside of the feeder. Fill it to the top with nectar at the day's temperature. Hang it in full shade for two hours. Dry in shade but wet in sun means heat. Wet in shade means the seal or the hang, and no shade will fix it.

Why the drip matters#

A drip is not just a sticky deck. The 2017 feeder patent notes that leaking vacuum feeders create "an increased risk of insect contamination." The trail down the outside is what recruits ants and draws bees and wasps to the ports. It also throws off any count based on nectar used, as how many hummingbirds explains. Stop the drip first and those problems shrink with it.

What this page does not cover#

The 110 °F bottle is a supposed case, not a reading. We found no source that measured how hot a sunlit feeder gets. It will vary with glass or plastic, color, wind and where it hangs, so the table shows the shape of the drip, not your yard's exact amount. The nectar figure is plain water's, rounded up; no source here tested sugar water. Some bases hold a little overflow before it reaches the ports, so a small drip may never show. We also leave out changes in air pressure with the weather, which none of our sources sizes. And this page does not cover nectar spoiling in heat, which matters more to the birds than a puddle. If your feeder drips in shade after you have reseated and leveled it, the seal is gone and the part needs replacing. The rest of the troubles that bring you to a feeder are matched to their cause on the problems hub.

Frequently asked questions#

Why is my hummingbird feeder leaking?

If it is an inverted bottle feeder and it drips in the sun, the air trapped at the top of the bottle is warming up. Warm air takes up more room, and the only way it can grow is by pushing nectar out of the ports. If it drips in the shade or at night, the cause is a leak in the seal: a cracked bottle, a gasket that is missing or out of place, threads that are crossed, or a feeder hanging off level.

Should I fill my hummingbird feeder all the way?

For an inverted bottle feeder, yes, if you will use the nectar before it needs changing. The drip comes from the air in the bottle, so less air means less drip. A half-full 16 fl oz bottle warmed from 70 °F to 110 °F pushes out about 4 teaspoons. Filled to the top it pushes out about 1. If a full bottle would sit past its change date, use a smaller feeder rather than a half-full big one.

Doesn't leaving air space at the top stop the leak?

No. That advice is right for a sealed jar, where the air is a cushion the liquid can push into. In an inverted feeder the air sits above the nectar, and the ports at the bottom are open. When the air expands, it pushes the nectar down and out. More air space means more drip, not less.

Why does my hummingbird feeder bubble in the evening?

It is the drip running backward. As the bottle cools, the trapped air shrinks and the pressure in it drops. Outside air then pushes in through the ports and rises through the nectar as bubbles. The next warm afternoon, that extra air expands and drips again. Bubbles at dusk are normal. Bubbles all day long point to a leak in the seal.

Do saucer hummingbird feeders leak?

Not from heat. A saucer keeps nectar in a basin below the ports, so warm air escapes out of the ports without carrying nectar. If a saucer is wet, check that it is not filled up to the ports, and check the basin for a crack.

Sources and further reading

1. Charles and Gay-Lussac's Law NASA Glenn Research Center, Beginner's Guide to Aeronautics At constant pressure and amount of gas, volume equals a constant times temperature; worked in kelvins (4.0 m3 at 300 K falls to 3.0 m3 at 225 K)
2. Water Density U.S. Geological Survey, Water Science School Density of water by temperature: 0.99802 g/cm3 at 70 °F, 0.99318 at 100 °F, 0.98870 at 120 °F
3. US Patent 5,628,277: Hummingbird feeder (filed 19 Sep 1995, issued 13 May 1997) U.S. Patent and Trademark Office, via Google Patents Names leak causes in gravity feeders: warming of the air in the reservoir that makes it expand and expel the solution, a feeder rocked by the wind, a bird's beak breaking the surface film at the port, and handling at refill
4. US Patent 9,700,022: Top-fill hummingbird feeder with vertically operative base sealing mechanism (issued 11 Jul 2017) U.S. Patent and Trademark Office, via Google Patents Background: vacuum feeders can permit nectar to leak, creating an increased risk of insect contamination