Nesting birds need lots of calcium
We all need calcium — mammals, birds, even plants. Consider how many times as a kid you heard: “Drink your milk; it helps build strong bones.”
The calcium contained in milk is an important component of bones. Calcium in the form of calcium carbonate also is the main component of eggshells of birds, but unlike mammals, birds don’t drink milk.
At this time of year female birds often find it a challenge to secure enough calcium for a clutch of eggs. The need is great. This is especially important for birds such as ducks and quail, which lay large clutches that may consist of more than a dozen eggs. Some smaller birds also lay large clutches of six to eight eggs, including chickadees, titmice and bushtits.
The amount of calcium needed to produce a full clutch of eggs can exceed the amount of calcium in the skeleton of a female bird. The challenge is even greater if you consider how fast they must produce eggs. Females typically lay an egg a day until a clutch is complete.
So how do birds manage? Seeking out foods rich in calcium is one strategy. Seeking out bits of shell to consume is another. Feeding oyster shell to laying hens is a common practice known to those raising chickens.
We all know bird bones are hollow. Thin-walled bones, while still strong, help reduce the overall weight of a bird, helping to make flight possible. Yet this space is where females temporarily store much of the extra calcium needed for eggshells. Cut open a thigh bone of a female bird in breeding condition, and you likely will see bone deposits filling much of the space. This temporary supply is called medullary bone and is easily mobilized for the manufacture of eggshells.
Medullary bone comes with a cost. It is heavy just like other bone and puts a female at a disadvantage when flying. Extra weight requires extra energy. It also impacts flight capabilities, making them more vulnerable to predators when trying to escape.
Evidence of medullary bone has even been discovered in the fossils of some dinosaurs, including relatives of Tyrannosaurus. This is just one more piece of evidence that birds really are modern-day dinosaurs, as many scientists maintain.
The demands for calcium also may influence where birds live today. The Klamath Mountains, including the Siskiyous, are geologically diverse. Unlike the Cascades, which are composed almost entirely of volcanic rock, the Klamath Mountains are an eclectic mixture of rocks of all kinds. Some originated deep in the Earth beneath the crust.
One of these rocks from the mantle is called serpentinite, a glossy green rock common in parts of Josephine County and to the south near Yreka. The rock producing the soils in these areas contains very little calcium compared with the volcanic soils in the Cascades.
Calcium is important not only to birds but to plants. It takes a special set of plants to cope with the challenges of growing on serpentine soils. Given the challenges, plant productivity on serpentine soils is low. This lack of calcium also may help explain the relatively low populations of birds in these same areas.
Stewart Janes is a retired biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.