Do Ducks Eat Rocks?

Yes, on purpose. Ducks have no teeth, so they swallow small stones and sand — grit — which lodge in the gizzard, a muscular stomach that grinds food against them. Wild ducks pick grit up as they feed. A kept duck on pellets does not need it, a duck eating whole grain does, a duck that ranges finds its own, and grit is not the same thing as oyster shell. The habit has one dark chapter: for decades, ducks mistook spent lead shot for grit, and it killed them by the million.

No teeth, so a stomach that chews

A bird’s digestion is arranged in two stomachs. The University of Kentucky’s extension guide explains that “The proventriculus (also known as the ‘true stomach’) is the glandular stomach where digestion begins”, and that “The gizzard, or ventriculus, is a part of the digestive tract unique to birds. It is often referred to as the ‘mechanical stomach.'” The national poultry extension site, written by the same author, adds that the gizzard is “made up of two sets of strong muscles that act as the bird’s teeth and has a thick lining that protects those muscles.” Ducks Unlimited says the same of ducks: “This thickly muscled organ essentially functions as a duck’s ‘teeth.'” and “The ventriculus often contains grit (sand or small stones), which aids in mechanically breaking down large food items.” The organ even resizes itself to the job: “Waterfowl species that consistently feed on harder food items like mollusks and hard-cased seeds will have a larger gizzard than species that consistently forage on softer food items.” What passes for teeth at the front of a duck is a different story, told in do ducks have teeth.

How long a stone stays

Grit is consumed, not kept. The poultry extension site: “The acidic environment in the proventriculus softens the stones, and then the strong muscles of the gizzard grind them into tiny pieces. The stones remain in the gizzard until they are ground into pieces small enough to pass to the rest of the digestive tract.” In ducks that has been timed. A 2000 study of 24 captive mallards in Preventive Veterinary Medicine found that “Calcareous grit used in the experiments had a shorter half-life (1.4 days) in the gizzard than values reported for siliceous grit (3.1 days)” — limestone grit is gone in days, flint or granite lasts longer. A duck is therefore always looking for more, which is why grit-eating never stops.

Which ducks need grit from you

Metzer Farms, a California duck hatchery, gives the rule: “If they are fed a mash, crumble or pellet, grit is not needed as the feed mill has ground the feed for them”, but “if they are fed whole corn, milo, wheat or other whole grains, grit is needed.” Birds that forage find their own — “If your chickens are free-range and have access to dirt, grit is not necessarily needed” — and the Open Sanctuary Project agrees: “If birds are free range, they’ll take care of their grit needs on their own. If in confinement, you’ll have to provide them with hard grit at least once a month.” Offer it on the side: “Do not mix the grit in with their feed. Provide the grit in a separate feeder or bowl so they can eat it at will.” A duck given corn as a treat is a duck that needs grit, and the reasons are in can ducks eat corn.

Ducklings are a special case. Metzer: “Do not provide grit to day-old birds. First you are providing starter feed that has already been ground”, and “Normally whole grains are not fed until birds are at least four to six weeks old.” Once they eat anything but crumble they need it — the Open Sanctuary Project: “As ducklings have no teeth, they will need duckling-appropriate insoluble grit to help them break down any food other than their starter food” — and size matters: “a duckling can’t handle anything much bigger than sand!” Hatcheries sell grit in trade grades — Murray McMurray Hatchery lists “#1” as “a smaller size for baby chicks and juvenile birds” and “#3” as “a larger size for adult birds” — which is as precise as the sizing gets. Feeding by age is in raising ducklings week by week.

Grit is not oyster shell

The two are sold side by side and do opposite jobs. Metzer: “Do not confuse grit with oyster shell. Oyster shell is provided to dissolve over time in the bird’s gut and provide necessary nutrients for shell building. Grit is to grind feed.” Grit is insoluble by design; oyster shell is calcium for laying hens, which NC State’s extension guidance offers as “free choice ground oyster shell” alongside a layer diet. Ducklings must not get the second: “DO NOT offer grit that contains oyster shell or additional forms of calcium as too much calcium can result in health issues for ducklings.” What else goes in the bowl is in what to feed ducks, and how much, in the duck feed calculator.

The lead-shot lesson

A duck cannot tell a pebble from a pellet. Minnesota’s Department of Natural Resources explains the consequence: “Because grit is essential for the digestive systems of waterfowl (and most upland game birds) and birds do not differentiate between lead shot and grit of a similar size, wildlife feeding and gathering grit in these wetlands also pick up lead shot.” The gizzard grinds the pellet as it would a stone, and the lead is absorbed. Cornell’s Wildlife Health Lab: “Waterfowl have very muscular gizzards for grinding plants in their diet and ingest small rocks to facilitate this grinding”, and the signs of poisoning “include incoordination, weakness, drooped wings, anorexia, reduced activity, and green watery diarrhea.” The US Geological Survey’s field manual estimates that “Annual losses from lead poisoning prior to the 1991 ban on the use of lead shot for waterfowl hunting were estimated at between 1.6 and 2.4 million waterfowl”. The Fish and Wildlife Service records the remedy: “The ban on the use of lead shot for hunting waterfowl was phased-in starting with the 1987-88 hunting season. The ban became nationwide in 1991.” A behaviour as small as swallowing gravel changed a national law.

Can a duck eat too much grit?

We found no documented case of a duck harmed by eating too much grit. The Poultry Site’s guide to gizzard impaction blames “litter, grass, string etc.” and large stones swallowed by free-range birds, and states that “Grit would not be classed as a foreign body”. The gizzard grinds grit down and passes it, and every keeper source above offers grit free-choice rather than by the measure. The one genuine hazard is size: stones a duckling cannot pass, or shot. Offer grit that suits the bird’s age, in its own dish, and let the duck decide.

About that walnut

Old textbooks pass down a story that a turkey’s gizzard “could crush 24 walnuts in their shells in under 4 hours”, and a figure for grinding pressure of more than 585 kilograms per square centimetre. Both appear only as textbook quotations, without the original experiment attached, and neither was measured in a duck; enjoy them as anecdote. What is certain is what the gizzard does daily to snails, mussels and hard seed, which is why a duck’s menu is longer than most people expect — see what do ducks eat.