Moldy birdseed: what grows in it and how to stop it

The short answer

• Damp seed grows Aspergillus and related molds. Aspergillus causes a respiratory infection in birds that is usually fatal, and it is a hazard to you as well when you empty a clogged feeder.
• Corn and peanuts carry the extra risk of aflatoxin, a liver toxin produced by some Aspergillus species that can be present with no visible mold and is not destroyed by heat.
• The bottom inch of a tube feeder is where it always starts: water wicks in at the lowest ports, condensation runs down the inside, and the seed below the last port never gets eaten.
• Bin moldy seed in a sealed bag. Never put it on an open compost heap, over the fence or in the woods, because that moves the problem to ground level where doves, juncos and sparrows feed.
• Prevention is a quantity rule, not a cleaning rule: fill 3 to 5 days' worth, empty rather than top up, and dry the feeder completely before it is refilled.

Seed is a dry storage organ with a lot of oil in it. Add water and warmth and it becomes an excellent growth medium, which is exactly what happens in the bottom of a feeder in October.

What is actually growing in there#

Several things, and they are not equally dangerous.

• Aspergillus. The one that matters most. It grows readily on damp grain and seed, and birds inhale the spores while feeding. The result is aspergillosis, a respiratory infection that is hard to treat and usually fatal in wild birds. It does not spread bird to bird, so it is not an outbreak in the usual sense: every affected bird is breathing your feeder.
• Penicillium and Fusarium. Common spoilage molds. They ruin the food value of the seed and signal that conditions are wet enough for worse.
• Bacteria, chiefly Salmonella. Not a mold, but it thrives in the same wet, fouled conditions and it is spread through droppings at a crowded feeder. This is the pathogen behind most feeder die-offs, and it is covered in bird diseases at feeders.
• Pantry moth larvae. Webbing and small caterpillars in stored seed. Unpleasant, not a disease risk, and a storage problem rather than a feeder problem. See storing birdseed.

Aflatoxin: why corn and peanuts get the strict rule#

Some Aspergillus species produce aflatoxin, a potent liver toxin. It concentrates in corn and peanuts, it is not destroyed by heat or by drying the seed out, and it can be present in grain that shows no visible mold at all. There is no home test.

That gives corn and peanuts three rules of their own:

1. Buy small and turn it over fast. Quantities you will use within a few weeks, not a season.
2. Never put out more than a day or two of peanuts, and never leave corn in a tray that has been rained on. Details in peanuts for birds and cracked corn.
3. Reject anything dusty, discoloured, caked or musty at the point of purchase. Black sooty powder on corn kernels is a stop sign.

Catching it before the birds do#

The order of appearance is consistent, and the first sign is mechanical rather than visual.

Stage 1 is the useful one. A feeder that looks full for a week while birds are still using it is almost always a feeder with a plug in it, not a feeder nobody wants. If you are working through no birds at my feeder, that is a check worth doing early.

Smell is the most sensitive test you own. Tip a handful into your palm outdoors and put your nose near it, not in it. Fresh sunflower smells faintly nutty and oily. Anything cellar-like, sour or sharply chemical goes in the bin.

Why the base of a tube is always first#

Three mechanisms converge on the same inch of seed:

• Water gets in at the ports. Rain runs down the outside of the tube and wicks in at every opening. The lowest ports collect what the ones above shed.
• Condensation runs down the inside. A tube cools faster than the air overnight. Moisture forms on the inner wall and gravity does the rest.
• The seed below the last port never moves. No bird can reach it. It sits there for months while you top up above it, which buries it deeper and keeps it damp.

That is why the standard advice to buy a tube with a removable base is a hygiene rule and not a convenience. Other designs fail differently: hoppers rot in the reservoir corners, solid trays hold rain like a dish, and suet goes rancid rather than moldy in heat. Weather protection varies enormously by design:

Read the weather column alongside your own rainfall. In the Pacific Northwest, the Deep South in summer, or anywhere with long humid spells, a design scoring low on weather protection needs a dome over it or needs replacing. Tube-specific failure modes are in tube feeders.

Getting rid of it safely#

The disposal rule: seal it in a bag and put it in the household bin.

What not to do, and why:

• Not the compost heap. An open heap is at ground level, in a yard where doves, juncos, towhees and sparrows forage. You have not removed the mold, you have served it. A fully enclosed tumbler, well away from any feeding area, is acceptable for spent hulls but not the place for a moldy plug.
• Not over the fence, in the brush or in the woods. Same problem, larger audience.
• Not scattered for the ground feeders, which is the instinct people have when they are reluctant to waste seed. Spoiled seed at ground level is the worst version of this problem.
• Not into the animal feed chain. See the aflatoxin warning above.

Protect yourself while you do it. Aspergillus spores are a recognized human respiratory hazard, particularly if you have asthma, a lung condition or a suppressed immune system, and emptying a plugged tube releases a cloud of them.

Cleaning after mold is a full clean, not a rinse: every internal surface has to be brushed, and the feeder has to be dried right through before it is refilled. Damp is the whole problem, so the drying step is the one that decides whether it comes back. The method is in how to clean a bird feeder.

The prevention routine, with numbers#

Cleaning is the backstop. The routine below is what actually stops mold forming.

• Fill 3 to 5 days' worth. The practical version: watch how much your feeder gets through in a week, then fill half that. In a busy yard a 40 oz tube often means filling it a third full. Seed that is eaten in four days cannot spoil in the feeder.
• Empty, do not top up. Every refill starts from an empty feeder. This one habit removes the single most common cause.
• Within 48 hours of sustained rain, empty, check the base, and dry the feeder before refilling. This is more important than the calendar interval.
• Clean every 2 weeks, and weekly in heat, humidity or heavy traffic. Work out your own interval with the feeder cleaning schedule.
• Nectar on its own clock: change every 2 days from 80 °F, daily above 90 °F, every 3 days in the seventies, and scrub the ports each time. Recipe and ratios in the hummingbird nectar recipe.
• Suet comes down in heat. It goes rancid rather than moldy, and rancid fat is its own problem, so pull it above roughly 80 °F unless it is a no-melt formulation. See suet for birds.
• Store the bag properly. Sealed metal or hard plastic container, off the concrete, somewhere cool and dark, in quantities you will use within four to eight weeks.
• Drill drainage. Any solid tray or dish that holds rain gets 1/8 in holes in the low points. A weather dome over an open feeder pays for itself in a single wet autumn.

When to shut the station down#

Two situations justify taking everything down rather than cleaning harder.

Repeat mold. If a feeder grows mold twice in a month despite small fills and a fortnightly clean, the design is wrong for your climate. Replace it with something that sheds water or move it under cover. You will not clean your way out of a feeder that collects rain.

A local outbreak. If your state, provincial or federal agency reports salmonellosis or avian influenza in your area, or if you find more than one sick or dead bird in the yard, take the feeders down, clean everything, and leave them down for the period the agency recommends. That decision, and the signs that trigger it, are in bird diseases at feeders.

Frequently asked questions#

Is moldy birdseed dangerous to birds?

Yes. Damp seed supports Aspergillus and related molds, and inhaled Aspergillus spores cause aspergillosis, a respiratory infection that is difficult to treat and usually fatal in wild birds. Wet seed also supports Salmonella and other bacteria spread through droppings at a crowded feeder. Corn and peanuts add aflatoxin, a liver toxin produced by some Aspergillus species. None of this is a reason to stop feeding. It is a reason to fill small, empty rather than top up, and clean on a schedule.

How can I tell if bird seed has gone moldy?

Look for clumping first, since seed that has bonded into a solid plug is already wet enough to support growth. Then look for white or gray fuzz, black sooty powder on corn kernels, dark shrivelled seeds, and a slimy film on the inside wall of a tube. Smell it: a musty cellar odor or a sharp sour ferment both mean bin it. Fine silk webbing is a different problem, pantry moth larvae in stored seed, which is unpleasant but not a disease risk.

Can I compost moldy bird seed?

Not on an open heap that birds can reach. Composting concentrates spoiled seed at ground level exactly where doves, juncos, towhees and sparrows forage, which is worse than leaving it in the feeder. A fully enclosed tumbler well away from any feeding area is acceptable for spent hulls, but the safe default for anything visibly moldy is a sealed bag in the household bin. Do not feed it to poultry, livestock or pets either, because aflatoxin is a real hazard to them.

Why does the bottom of my tube feeder always go bad?

Three things happen at once down there. Rain runs down the outside and wicks in at the lowest ports, warm humid air condenses on the inside wall overnight and runs to the base, and the seed below the lowest port never gets eaten, so it sits in that moisture for months. Topping up buries it deeper. Choose a tube with a base you can remove, empty it completely at every clean, and drill drainage if the design allows it.

Is moldy bird seed dangerous to me?

It can be. Aspergillus spores are a recognized human respiratory hazard, particularly for anyone with asthma, a suppressed immune system or an existing lung condition. Emptying a plugged tube releases a cloud of them. Wear gloves and a fitted particulate mask, work outdoors and upwind, never blow or knock the dust out where you will breathe it, and wash your hands and the tools afterwards. Bag the seed before you carry it through the house.

How do I stop seed going moldy in the feeder?

Quantity first: put in three to five days' worth rather than filling to the top, so seed never sits long enough to spoil. Then empty rather than top up, so old seed is not buried under new. Add a weather dome or choose a covered design, drill drainage holes in any solid tray, and empty and dry the feeder within 48 hours of sustained rain. Clean fortnightly, weekly in heat or humidity, and dry the feeder completely before refilling.

Sources and further reading

1. Bird Feeder Maintenance and Cleaning Cornell Lab of Ornithology, Project FeederWatch Cleaning method, bleach dilution and spoiled seed removal
2. National Wildlife Health Center US Geological Survey Wildlife disease information including aspergillosis and salmonellosis
3. Animal and Veterinary: feed safety US Food and Drug Administration Aflatoxin as a regulated contaminant of corn and peanut-based feed
4. Bird Feeding and Care National Audubon Society General feeder hygiene guidance