Specimen of the Month: Double Clutching
Another search for a new Specimen of the Month led me to a lower corner of our specimen storage room, where fragile eggs rested in an assortment of boxes. Very carefully, I picked up one of the larger cardboard boxes and looked through its cutout window at two rather large bird eggs nestled in some cotton bedding and scattered nesting debris. Comparably sized, one was a creamy white with minimal speckling while the other was a pale buff irregularly marked with darker brown shades. Black embossing tape on the box read, "Crane eggs. From the International Crane Foundation. Baraboo-Wisconsin." Intrigued by their appearance and source, I carried these eggs out of storage and investigated.
Two eggs typically adorn a sandhill crane nest per season. This is actually an adaptation to increase the likelihood of successful reproduction. Occasionally a pair will only produce one egg in a season, but a second egg works as a sort of insurance against failure should one be infertile. Known as "double clutching," many crane species can continue to lay eggs until two are safely in the nest.
After learning this, I wondered if there were any crazy numbers of known eggs laid in one season. The International Crane Foundation, who had gifted us these sandhill crane eggs, provides excellent information for those hoping to educate themselves on anything crane related. The Foundation works to conserve crane populations, which includes breeding all 15 crane species in captivity. To my delight, their website stated that "as many as 19 eggs have been produced by one [captive] female during a breeding season." Well, I personally find that quite impressive!
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| Sandhill crane eggs, like eggs of many other bird species, may vary in coloration or pigmentation. |
Some of my initial interest in these specimens came from the fact that they were drastically different in color. I cannot be certain that our two eggs came from the same nest or season, but I do know that intraclutch eggshell coloration does vary with many species of birds. Interestingly, studies on this phenomenon have found that color variation seems to be systematic--the coloration can indicate laying order with less pigmented eggs arriving first or towards the end of a breeding season. Some even hypothesize that females may use egg color to identify individual eggs. Perhaps what I observed could be similarly explained. While one could delve much further into the topic, there are many other intriguing habits associated with sandhill crane nesting.
Crane mothers work hard to ensure the survival of a new generation, but fathers put in considerable work too. Nesting begins with mates gathering nest materials together. Sandhills prefer to nest in small wetlands or gravitate towards the edges of larger ones. Both parents will gather surrounding vegetation for their nest site--drier materials are used earlier in the season with greenery added as spring stretches toward summer. Parents will throw that which they gather over their shoulders, piling up the nesting mound for the mother to arrange later.
Once nests are constructed and eggs laid, incubation will last for roughly one month. While parents share that duty, the female generally takes on a bit more than the male and tends to take the overnight shift. Unfortunately, their two large eggs may be preyed upon if left unsupervised. Coyote, wolves, foxes, raccoons, bobcats, and larger birds of prey all enjoy a good crane egg when available. The best case scenario for a season would be two fluffy chicks hatching, covered in down and active with eyes already open. Thankfully for cranes, dedicated organizations like the International Crane Foundation work to keep populations going. Because southern central Wisconsin is a more common breeding area for sandhills, the Cable Natural History Museum has been fortunate to obtain such specimens for its collection.