|6/19/2014 11:03:00 AM|
Great Egg-spectations -
by Don ReimerMid-June is an opportune time to take stock of the current nesting season, as over 200 species nest across Maine landscapes. In recent decades about 25 southern nesting species have pushed northward into New England as indicated by an influx of Carolina Wrens, Red-Bellied Woodpeckers and Cardinals that now call Maine home. This southern trend is expected to continue in the future.
Seasonal timing of nesting activity varies between species. Great Horned Owls began their nesting campaigns in January, and Bald Eagles have occupied nest sites since late March. New families of Chickadees, European Starlings, American Robins, Wild Turkeys and Hairy Woodpeckers are now being seen.
In terms of individual egg-laying practices, two biological principles drive the hatching process. These biological stratagems relate to the chicks' degree of development at their time of hatching. Spring hatchlings fall into two distinct categories: Precocial ("ripened beforehand") and Altricial ("requiring nourishment"). Before these dry-sounding scientific terms dampen our true enthusiasm for fuzzy baby birds, let's consider real-life examples from these two groups.
A member of the plover family, the familiar Killdeer lays sets of precocial eggs. The four well-camouflaged eggs are placed in a shallow, gravelly scrape out in open territory where any form of organized, structured nest would betray the location to predators. The Killdeer's large, nutrient-rich eggs (twice the size of a robin's egg) require a lengthy hatching period of 24 to 26 days. Since incubation of the eggs does not commence until the egg clutch is completed, the four chicks hatch simultaneously. Precocial embryos remain inside the shell about twice as long as those of altricial birds, and this extended development phase pays immediate dividends for the active, vigorous hatchlings.
Clothed entirely in downy feathers at hatching, the agile Killdeer babies begin life with their eyes wide open. Under vigilant parental supervision, the chicks scurry about in search of food. With any sign that danger is approaching, the parent may vocally summon and assemble the chicks temporarily beneath its protective feathers.
Eastern Bluebirds are altricial hatchlings. Like the majority of other songbirds, the female Eastern Bluebird incubates her clutch of three to six eggs for about 12 to 14 days. The Bluebird chicks are born blind, helpless and essentially naked. Incapable of independent movement, the chicks are fed by both parents as they strengthen and grow into their first set of feathers. Upon leaving the nest cavity a couple weeks later, the Bluebird parents continue to provide food and nearby support for a while longer.
Regardless of their particular hatching techniques, most birds share some common traits during incubation. Female birds (and some males) develop a brood patch, an area of bare swollen skin on the lower breast that transfers direct body heat onto the egg surfaces. Due to hormonal changes, some breast feathers loosen and drop out. Species such as ducks pluck out some breast feathers to line the nest during construction. Gulls and shorebirds have three brood patches to accommodate their sizable eggs. Sensitive skin muscles open the brood patch when the bird settles onto the nest to create efficient contact with the eggs.
The other shared trait involves the turning of nest eggs on a frequent basis. The rate of turning varies between every eight minutes for American Redstarts to about an hour for Mallard Ducks. Turning presumably helps to warm the egg evenly and perhaps helps prevent the embryo from sticking to the inner shell.
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