Birds do not actually produce milk as mammals do. However, some bird species do produce a milky fluid in their crop to partially digest and feed their young. This “crop milk” is made up of different nutrients depending on the species.
What is crop milk?
Crop milk is a secretion from the lining of the crop of parent birds that is regurgitated to young birds. The crop is a pouch near the throat where birds can store food before digesting it. Crop milk contains protein, fat, carbohydrates, minerals and antibodies to help young chicks grow and fight disease.
Which birds produce crop milk?
Some examples of birds that produce crop milk include:
- Pigeons
- Doves
- Flamingos
- Penguins
- Pigeons
However, the best studied producers of crop milk are pigeons and doves in the Columbidae family. The production of crop milk allows these birds to breed without an external source of food or water. The milk provides all the nutrients the squabs need in their first week of life.
What is pigeon milk made of?
The crop milk produced by pigeons consists of:
- Proteins – 60% of the milk by weight. Caseins make up over 90% of the proteins.
- Fats – Around 35% fat, mostly long chain polyunsaturated fatty acids.
- Carbohydrates – Around 1.2% carbohydrates, mostly glycoproteins and free glucose.
- Minerals and vitamins – Such as calcium, phosphorus, sodium, potassium, magnesium and vitamin A.
- Antibodies – Immunoglobulins to transfer passive immunity to the squabs.
The concentrations of fat, protein and carbohydrates vary over the course of feeding. The crop milk becomes progressively more concentrated over time, providing nutrients tailored to the squab’s growth.
Comparison of Pigeon Milk and Cow Milk Composition
Nutrient | Pigeon Milk | Cow Milk |
---|---|---|
Protein | 60% | 3.5% |
Fat | 35% | 3.7% |
Carbohydrate | 1.2% | 4.8% |
As you can see, pigeon milk is much higher in fat and protein than cow’s milk. It needs to meet all the nutritional needs of growing squabs.
What is flamingo milk made of?
Flamingo milk also contains high levels of fat and protein for their chicks. Exact amounts vary between species, but a typical composition is:
- Protein – Around 17%
- Fat – Around 10%
- Carbohydrates – Around 1%
- Ash – Around 2%
- Water – Around 70%
The milk is red or pink in color due to carotenoid pigments from the shrimp and algae the parents eat. These pigments are thought to play a role in coloring the chicks’ feathers.
What is penguin milk made of?
Penguin milk also contains high levels of protein and fat, but very little carbohydrate. An analysis of Emperor Penguin milk found it contains:
- Protein – Around 14%
- Fat – Around 11%
- Carbohydrate – Less than 1%
Penguin milk fat is unique in being mostly monounsaturated fatty acids, compared to other bird crop milks that are higher in polyunsaturated fats. The milk also contains vitamin A, B, C, D, E along with various minerals.
How do birds produce crop milk?
Birds have special cells in the lining of the crop called “crop sac cells” that secrete crop milk. In pigeons, these cells start proliferating and secreting milk within 2-5 days of the eggs hatching.
The secretion of crop milk is stimulated by prolactin. This hormone surges right before hatching and remains high as long as the parents are feeding. It stimulates the crop sac cells to swell and proliferate, starting milk synthesis.
Both male and female pigeons can produce crop milk to feed their young. However, the composition of the milk differs between sexes. Females produce more fat and protein-rich milk than males.
How do bird chicks drink milk?
Parent birds produce crop milk on demand multiple times a day to feed their chicks. When it’s time to feed:
- The parent regurgitates the milk from its crop, where it was stored.
- The chick puts its head inside the parent’s open mouth and drinks directly from the crop.
- This stimulates further milk production and secretion.
Young chicks have special feeding spots on the floor of their mouths and tongue that milk drips into. This triggers them to swallow. They can consume up to 10% of their body weight in crop milk per feeding session.
How long do birds feed crop milk?
Birds produce crop milk temporarily while their chicks are young. The duration of feeding varies by species:
- Pigeons – Feed their squabs crop milk exclusively for the first 5-7 days after hatching. Then slowly wean them onto regurgitated food.
- Flamingos – Feed milk for the first 2-3 weeks as chicks transition to filter feeding.
- Penguins – Feed milk for around 3 weeks until chicks are ready to venture to sea to hunt.
The crop milk provides vital nutrients and antibodies while chicks are too young to digest solid food and lack mature immune function.
Conclusion
While birds do not actually produce true milk like mammals, some species have evolved the ability to synthesize a nutrient-rich fluid called crop milk. Pigeon milk is the best studied and contains very high levels of fat and protein compared to cow’s milk. Flamingos, penguins and other birds also rely on crop milk to quickly nourish their chicks after hatching until old enough to feed themselves.