Sunday, June 16, 2013

A Family Group of Four Sandhill Cranes

The number of eggs a bird species lays (it’s “clutch size”) is genetically determined.  Some birds like ostriches and albatross lay only one egg during each nesting attempt.  Others like ring-necked pheasant and northern bobwhite lay up to 18 eggs in a clutch.  For an important ecological reason, most birds with a clutch size of two or more eggs do not begin to incubate their clutch until the last egg is laid.  This ecological adaptation is programmed into them genetically so that all young hatch at essentially the same time.  This has huge implications for species survival.

Sandhill cranes do things a tad differently.  Sandhills are a determinant layer at two which means that they are genetically programmed to lay no more than two eggs in a clutch.  Despite laying only two eggs when the first egg is laid the adults begin to immediately incubate that egg.  Two to three days later when the second egg is laid, the embryo in the first egg has a two to three day head start in development.  That means that a couple of weeks later when the young in the first egg hatches that first young has an advantage in growth and strength and survival over the second young.

After the first young (known as “colts” in cranes) is born it receives all the attention of the adults including all the food they can provide.  However two or three days later when the second colt arrives, suddenly the first colt receives half of the attention and more importantly half of the food it was receiving just a day earlier.  Human parents who bring their second or subsequent child home from the hospital are frequently familiar with the sibling rivalry that follows.  The same concept happens in cranes only with more deadly consequences.

The older colt senses that the younger colt poses a threat to its survival and almost 100 percent of the time the older colt kills the younger one.  It’s a practice called fratricide, it has been genetically programmed into cranes for tens of thousands of years, and as much as the PETA-types want to moan and cringe and turn their heads at the thought, it is a survival strategy that works for cranes.  If it didn’t they would have become extinct long ago.

The words “almost 100 percent of the time” in the previous paragraph are the reason for this post.

In the last several days I have noticed that my resident pair of adult sandhill cranes has with them two nearly-fledged colts!  This is the first time in my crane watching career that I have seen two nearly-fledged colts with an adult pair.  Once on Alaska’s Seward Peninsula about 105 miles north of Nome I watched a three day old colt kill a newly hatched young in the nest.  Never before however have I seen two colts this old with the same parents.  Granted when you are looking at a corn field filled with 20,000 sandhill cranes along Nebraska’s Platte River it’s highly likely that one or two family groups in that throng are made up of four birds but good luck in figuring that out.  In my local sandhill crane family it is a certainty that the adults are raising two colts to fledging.  After seeing millions of sandhill cranes in my life this is a pretty spectacular sight!

In the image above you can tell the adult birds from the colts by the color of their foreheads.  Adults have a deep cardinal red colored forehead while the colts have a rusty brown forehead.  For the colts it remains rusty brown until their second year when their forehead turns cardinal red and humans can no longer tell their age.

How and why this adult pair was able to raise two colts to near fledging is a mystery. Could it be that it’s because the colts were hatched in an urban environment?  Could it be that the adults were able to collect so much food for the oldest colt right after the birth of the youngest colt that the oldest was too sated to care and just left the young colt alone?  Could it be that these two adult cranes are passing along a gene that precludes fratricide from the genetic makeup of their offspring?  A friend of mine who lives six miles away told me just this morning that she has a family group of four near her home east of Interstate 75.  Maybe it is something peculiar to the Florida race of the sandhill crane (Grus canadensis pratensis) ? 

The possible explanations for this very unusual sighting are many.  The only entities that know the answer are the adult sandhill cranes and they are not saying much so it’s all speculation.  However it may have something to do with why sandhill cranes in Florida are able to survive in heavily urbanized areas.  Regardless, it’s sightings like this that make ornithology such a fascinating topic and why I remain excited that I chose it for a career long ago.  

I think I’m going to go back outside now to see if they adults want to give up a few more secrets. 

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