Thursday, October 20, 2016

Bird Eggshell Color; Complex Patterns and the Avian Control System

For a busy inland birder like myself, a trip to the coast is a rare and special treat. When I do visit the coastline, I often choose the off-season, when most vacationers have long since gone.

This allows me to wander at will, combing the sands for bird tracks, feathers and sometimes the occasional eggshell fragment.

My last trip to the coast yielded a number of avian discoveries, including two whole eggs. I wondered, how on earth does something this delicate and appetizing avoid being detected by local predators? I took a few close-up photos of the eggs for identification purposes and returned home for the evening.

At home, I was able to confirm the identity of these eggs as belonging to a Least Tern and an American Oystercatcher, two species that are known to nest at this coastal location.  

But my curiosity with the eggs did not end with their positive identification. Despite a large discrepancy in size, the eggs were remarkably similar to one another in ground color and spotting. So much so that they perfectly matched the sand that they were still resting on.

Aside from nest site selection and egg coloration, the two are very different species. The Least Tern forages for fish throughout the day over shallow-water bays, estuaries and tidal marshes. Habitually plunge-dives and grasps prey with open mandibles. The American Oystercatcher on the other hand uses its knife-like bill to exclusively open bivalves, mollusks and crustaceans that inhabit intertidal areas (Source: Bird of North America Online Edition).

So how do two bird species, from two separate families, end up being able to nest side-by-side along the same coastal sand spit? How did they manage to create identically patterned eggs?

Least Tern Eggshell Pattern
Photo Credit: Paul Cianfaglione
American Oystercatcher Eggshell Pattern
Photo Credit: Paul Cianfaglione

Natural selection provides the most logical explanation here. A key mechanism in evolution, the Least Tern and American Oystercatcher eggs are proven to be better adapted than other species of Charadriiformes to nest in this particular coastal environment, which leads to an increase in survivorship and the production of more offspring.

Yes, natural selection does help us to understand why the eggs are colored this way, but it doesn’t tell us how. How does this coloration happen? Below is the short and sweet answer to this question.

In the uterus the ovum receives its outer shell and, in most species of birds, its pigmentation. As the egg moves through the uterus region it may twist and turn. Pigment-secreting areas of the uterus mark the eggshell as it passes, making the egg patterns a visual record of egg movement within the uterus. Rapid movement results in streaks or elongated tracks on the egg surface. Slow periods are marked with round spots, or even bands around the egg. It takes about twenty hours for the final deposition of the hard outer eggshell. The egg then passes through the vagina area of the oviduct and into the cloaca and is immediately laid. (Source: Proctor, N.S. and Lynch P.J. (1993) Manual of Ornithology).

Did natural selection permanently fix these tern and oystercatcher eggs to be the color that they are today? So what if their nesting habitat had suddenly disappeared? Can intricately colored eggs, like the Least Terns, ever keep up with swift environmental changes? If their nesting strategies were anything like the Japanese Quails, probably not.

A new study out in Current Biology suggests that female quails select a nesting area that is the best camouflage for her specific egg appearance (sort of encoded). Because females tend to lay similarly patterned eggs over time, the findings suggest that the quails are choosing a laying location rather than controlling the appearance of their eggs based on their environs.

This may work well for the quails and other species in more stable environments, but in the terns and oystercatchers ever shifting world, not so much.

As was just mentioned, the important word here is control. Control in the short-term may be the key to a birds continued existence.   

In a book that I just finished reading titled, The Rise of Birds, 225 Million Years of Evolution (2015), author Sankar Chatterjee takes us to a deeper understanding of how some birds may control the outcome of their own egg color prior to entering the uterus. Chatterjee nicely explains the avian control system here;  

The central nervous system of a bird consists of the brain, the sense organs, the spinal cord, and the nerves. Nerves carry messages from the eyes or ears to the brain. A bird’s sensory apparatus assesses the environment and conveys impressions through electric pulses, which stream to the spinal cord for immediate reaction or are transferred to the brain for central processing. The forebrain is considerably enlarged in birds for processing sound, touch and sight, and to adjust behavior to new situations.

He goes on to say this;

Birds live in a world that is dominated by sight and sound. Their sense of vision is so highly developed that for most birds, three of the four senses – touch, smell and taste – are irrelevant. Birds have enormously large eyes that provide keen sight, sharp images, and superb color perception. Most birds have flat eyeballs with large retinas, which are excellent for scanning the landscape. A bird can gain more information about its surroundings through its eyes than through all its other sense organs combined.

This gathering of information was obviously important for the species that eventually went on to lay the perfectly adapted egg. But this was only one of many adaptations that allowed these birds to successfully nest at this coastal site. The adaptation of flight to obtain food from afar, an adapted bill to secure local foods, the ability to nest colonially with other like-minded species and the aggressive behavior to prevent predation, were all necessary prior to the final product which was an egg that fully matched the shoreline substrate.

Black Skimmer Eggs
Photo Credit: Wikipedia

In another study involving the avian control system, researchers at the Harvard Department of Genetics have discovered a molecule in the famous Darwin Finches (Geospiza) that controls and regulates the length of the bird’s bills, enabling them to adapt rather swiftly (evolutionarily speaking) to available seeds and insects. Remarkably, researchers identified the protein Calmodulin, which activates certain enzymes, which triggers a signal that eventually turns specific genes on or off. These signals alter the behavior of cells responsible for beak sculpturing.