What Is Crop Milk? How Pigeons and Doves Feed Chicks

What is the crop?

The crop is a stretchy pouch at the bottom of a bird’s throat, part of the esophagus. Many birds use it to store food before digesting it, so they can eat a lot quickly and then fly off to safety. In pigeons and doves, the crop has a second job during the breeding season: it turns into a milk factory.

A pigeon’s crop has two lobes. Outside the breeding season its wall is thin. As the eggs near hatching, the lining thickens dramatically, from about 0.15 mm to as much as 3 mm, and gains a richer blood supply.

How pigeons make crop milk

Crop milk is made in a different way from mammal milk. In mammals, glands release milk as a liquid. In pigeons, the cells of the crop lining multiply rapidly, fill up with fat and protein, and then break away into the crop. This mass of loose cells is the crop milk, a pale yellow paste often compared with cottage cheese.

The process is controlled by prolactin, the same hormone that triggers milk production in mammals. Pigeons played a part in its discovery: in the 1930s, scientists identified and named prolactin partly by testing how extracts made a pigeon’s crop lining grow, and the “pigeon crop test” was used for years to measure the hormone.

Both parents start producing crop milk a few days before the eggs hatch. In the last days before hatching, parents may eat less, so that the first meals are pure milk without seeds.

How chicks are fed

Pigeon chicks, called squabs, hatch blind and helpless. To feed, a chick pushes its bill into the corner of the parent’s mouth, and the parent pumps crop milk up from the crop. You can often see the chick’s head almost inside the adult’s bill.

The diet changes as the chick grows:

On this diet squabs grow astonishingly fast, roughly doubling their weight within the first couple of days. Pigeons normally lay just two eggs, and research suggests that a pair cannot make enough crop milk to raise three chicks well, which may help explain why the clutch size is so small. The fast growth also means that, by the time young rock pigeons leave the nest, they look almost like adults.

Crop milk vs mammal milk

Crop milk contains more protein and fat than cow or human milk. It also carries antibodies, antioxidants and helpful bacteria that support the chick’s developing immune system and gut. Unlike mammal milk, it contains essentially no carbohydrates.

Mourning doves and other doves

All members of the pigeon and dove family, Columbidae, produce crop milk. In North America, the mourning dove feeds its two chicks crop milk in the same way. Because both parents can feed the young and crop milk does not depend on insects being available, mourning doves have a long breeding season and can raise several broods a year in warmer regions.

Crop milk is one reason pigeons and doves can breed at almost any time of year. Their chicks do not need caterpillars or other seasonal insects, which most songbirds depend on.

Other birds that make “milk”

Two unrelated groups of birds produce similar feeding secretions, which shows that the trick evolved more than once.

Flamingos. Both flamingo parents produce a liquid “milk” from glands lining the upper digestive tract. At first it is bright red, colored by carotenoid pigments rather than blood, and the color fades as the weeks pass. Parents can feed it to their chick for many weeks, until the young bird can filter food for itself. Learn about the American flamingo.

Emperor penguins. Male emperor penguins incubate the egg through the Antarctic winter while fasting. If the chick hatches before its mother returns from the sea, the father can feed it a protein- and fat-rich secretion from his esophagus, keeping it alive for a few days until she arrives with food.

Myths about pigeon milk

People sometimes say “pigeon’s milk” to mean something that does not exist, an old joke for sending apprentices on a fool’s errand. In fact, pigeon milk is real, although it is not a true milk in the mammal sense.

Another misunderstanding is that only mother birds feed chicks. In pigeons, the male takes a full share, both making crop milk and feeding the young, and in emperor penguins only the male makes the secretion at all.