Thursday, July 23, 2009

Hawks on the Wing . 5 | Bird News

Coupling completed and the nest built, the female lays a clutch of 2 - 3 eggs (more unusually, 1, 4 or 5). As with most birds clutch size is related to the current feeding conditions - more eggs are laid when food is abundant and fewer in lean times. The eggs are about 2.5-inches long, white or bluish-white and spotted with brown or unmarked.

Brooding, or incubating, begins after the first egg is laid rather than when the clutch is complete. This strategy allows the eggs to hatch asynchronously in the order that they they were laid. Incubation habits differ among bird species, but in most hawks both males and females seem to share the brooding duties equally as they do nest building.

Almost all birds brood, generally by sitting on their eggs, whose shells are specially designed to bear the weight of the parent's body without breaking, often with some support from the raised side of the nest. When brooding a bird transfers its body heat to the eggs to develop the embryo. Birds are warm blooded animals, of course, with normal temperatures of 104° F (they have to run hotter than us because they're smaller). Some heat is lost in the transfer, so it turns out that the ideal avian incubation temperature is actually around the human norm of 98.6° F. But feathers make the bird, so how is this heat transfer accomplished with all that high R-factor insulation in the way?

Just before brooding most birds develop a brood patch, a temporarily featherless area of bare skin lined with swollen blood vessels on the abdomen which will come in contact with the eggs. Hawks, both male and female, have one large patch, other species may have several strategically placed patches. In hawks the patch develops naturally before brooding due to hormonal changes. Other species such as ducks and geese pluck out the feathers to make the patch and use them to line their nests. In addition to brooding, the eggs are turned frequently to assure even development of the embryo.

Incubation takes about a month and the eggs hatch out in 30 to 35 days, one after the other following their 'laying order'. Red-tails are born with no means to walk or fly, covered in down, with eyes wide open and must be constantly fed by their parents. Following the lore of falconry, for which Red-tails are often the current raptor of choice, the young before they fledge - fly and leave the nest - are called eyasses. They quickly resemble cute little ruthless, hook-billed killers, which will soon be their instinctual career path.

Bird species vary widely in their stage of development at birth. The spectrum spans from the precocial to the altricial. Precocial species are, well, precocious - competent from birth, to various degrees. Altricial young take time to develop competence.

Image ... Red-tail with a pigeon.