Crop Milk

While most birds that hatch altricial (”helpless”, needing the care of their parents to survive) chicks have crops which they use to store and subsequently regurgitate food for their young, some birds go one step farther!

All of the doves and pigeons (family Columbidae) and a few of the flamingos (genus Phoenicopterus) all have similar glands in their digestive tracts, which produce a substance known as crop milk. Emperor penguins (Aptenodytes forsteri) don’t have crops, but have similar cells in their esophageal tract which produce a substance with very similar qualities.

In flamingos and pigeons, both sexes produce crop milk, but in penguins, only the males produce it, and only when the female takes a particularly long time returning with food after the chick has hatched. In other words, it’s more of a survival mechanism for penguins, rather than a primary food source for their chicks.


Crop milk is quite different from mammalian milk. While mammalian milk is formed in modified apocrine glands, and is largely water and proteins, with many other essential nutrients, crop milk is closer to sloughed crop lining than a true “secretion”. The consistency is closer to cottage cheese than water, and it’s largely protein and fat globules. It does not contain much water, and has no essential minerals (like calcium and phosphorus) which are found in mammalian milk.

However, both crop milk and mammalian milk contain protective immune cells (IgA antibodies), and both are stimulated by the hormone prolactin.

John James Audubon. 1835-1849.

De-Extinction and the Passenger Pigeon

The last lonely passenger pigeon died in 1914. Her stuffed body is on display at the Smithsonian Institution.  I’ve seen her.  It’s a sad exhibit.

But what if passenger pigeons could be reincarnated?

That’s the idea behind de-extinction. Take DNA harvested from museum specimens and figure out which genes are most important to the species’ identity.  Then use genetic engineering to edit in the DNA of a closely related species (in this case the band-tailed pigeon).  If all goes well, a chimera pigeon (with both passenger and band-tailed genes)  that looks and acts like a passenger pigeon could be hatched by a band-tailed mother.  

When and if chimera passenger pigeons are produced, there will initially be very few.  In a very small population, closely-related individuals must mate.  This inbreeding can be lethal in the short term but might be good for the population in the long term.

To predict the effects of inbreeding on the chimera passenger pigeons, it’s important to know whether the species in question went through at least one “population bottleneck" as humans probably did 70,000 years ago.  Historical records suggest that the original passenger pigeon population did this several times, as their populations were subjected to "boom and bust” cycles.

That’s a good news, because inbreeding can purge lethal genes from a species’ gene pool. This happens when individuals carrying especially bad gene combinations die before they can reproduce, thus lowering the incidence of deleterious genes. And the fewer deleterious/lethal genes in the pigeons’ gene pool, the better the chances of “re-creating” a passenger pigeon.

After a bottleneck purge, it’s possible for the a tiny population to develop a healthy gene pool.  Pigeons are profligate breeders, making them  good candidate for de-extinction.  One day flocks of passenger pigeons may again grace the skies of North America!


References: Scientific American and Wikipedia.

Image: Passenger Pigeons by John James Audubon

Copyright 2014 by Ann Marcaida.

Magnificent Frigatebird (Fregata magnificens), John James Audubon

I viewed this painting at the New York Historical Society’s Audubon Part II exhibit last spring. Listening in on the docent-led school groups coming through the gallery, I learned:

-Audubon wanted to paint the frigatebird’s body swooping down in a big dramatic pose, but knew this position would hide its feet. Since this was supposed to be a science illustration, showing all of the bird’s anatomy, he decided to paint the feet separately in the corner, a top and bottom view

-To capture the sheen of the dark plumage, Audubon used charcoal pencil to draw in every feather. Watercolor purists considered this blasphemy, but since Audubon was more interested in conveying information about his birds, he took a practical approach in his use of mixed media for his art

-Audubon ate many of the birds he hunted for his paintings

(this last fact is questionable)


Our next feature from the Adopt a Book program is Audubon’s Quadrupeds of North America.  

Audubon, John James, 1785-1851. The quadrupeds of North America. New-York, V. G. Audubon, 1854-55.  MERLIN library catalog

Before conservation work, these volumes were practically unusable.  The spines were cracking, and a misguided person had at some point covered the spines in clear tape.  Here’s a before shot:

The conservator removed the tape and repaired the spines and text blocks, and now we can see all the great images inside. How could anyone resist those cute bunnies?