In the past three columns we have look at and explored many aspects of bird nests and bird eggs. Now in part four I want to look at the next logical topic–incubation. All birds lay external eggs that need to be kept warm throughout their development. This process is called incubation. It may not be an easy process but it is a process that has stood the test of time and seems to work well for the avian world.
One interesting aspect of incubation is why at least one or two species of bird have not evolved to give birth to live young. As I mentioned last time, there are a couple mammal species which lay and incubate external eggs, so it reasonable to think that at least one species of bird may give birth to live young. Snakes and other reptiles, which reproduce mainly with external eggs, have many species that give birth to live young. So why not birds?
Apparently the advantages of laying eggs have out-weighed the advantages of reproducing with live young. For one thing, with eggs, both parents (or at least in some species) can help incubate or at least keep the eggs warm, thus reducing the burden of incubation. In addition reproducing with eggs would allow the parents to abandon their eggs in the face of deadly danger such as floods, fire, or predators. This would allow the parents to survive and reproduce another brood in another location at a later date.
The temperature needed for the development of the embryo within the egg varies across the species. In general, the temperature for incubation in most birds runs between 95 and 100 degrees F. Since air temperatures rarely remain a constant at these levels in the northern states addition heat needs to be provided by the incubating adult. There are some birds such as the Wilson’s Plover which nests near the Gulf of Mexico or the Black-necked Stilts in the Salton Sea of Southern California actually need to cool their eggs during the day but that is another story.
Incubating eggs seems like a simple enough process. You just sit on the eggs to keep them warm. Right? Wrong. I think we all know the fabulous insulating properties of feathers–down feathers in particular. Down feathers are the small fluffy inner feathers of a bird that you usually cannot see. If you have ever seen a bird during a snow storm you may have noticed that the snow flakes pile up on the bird without melting. This is another testament to the insulating values of feathers. No body heat is lost through the feathers. So will just sitting on the eggs be enough to transfer enough body heat from the parent to the eggs? NO!
A few days before the female (and in some species the male) begins incubating her clutch of eggs her body undergoes a great transformation. The small down feathers on the breast and belly begin to fall out. In most species of duck and goose, the mother pulls or plucks out the down feathers in this region creating an incubation patch (also called a brood patch). In addition to the loss of down feathers the skin becomes swollen through the retention of water in the tissues and more importantly the expansion of the blood vessels that feed blood to the skin.
The brood patch takes up much of the area of the belly and breast but is actually concealed by the contour feathers which cover the bird. In other words, the larger, often colored feathers that cover a bird’s body hide the bare skin beneath where the down feathers once were. So the inner layer of a birds two layers of feathers has been removed to expose the brood patch.
To incubate a bird spreads apart the contour feathers to expose its brood patch beneath. This allows direct contact to the eggs with the mother’s body thus allowing direct transfer of body heat. Most birds have a body core temperature somewhere between 98-105 F which allows the mother to rapidly warm her eggs and maintain the warm temperatures over a long period of time.
How long a bird must sit on there eggs will be the subject next time. Until then…