what is night ?

what is Grebe ?

Grebes are small to medium-large in size, have lobed toes, and are excellent swimmers and divers. However, they have their feet placed far back on the body, can run for a short distance, but often fall over.

Grebes have narrow wings, and some species are reluctant to fly; indeed, two South American species are completely flightless[1]. They respond to danger by diving rather than flying, and are in any case much less wary than ducks. Extant species range in size from the Least Grebe, at 120 grams (4.3 oz) and 23.5 cm (9.3 inches), to the Great Grebe, at 1.7 kg (3.8 lbs) and 71 cm (28 inches).

However, the North American and Eurasian species are all, of necessity, migratory over much or all of their ranges, and those species that winter at sea are also seen regularly in flight. Even the small freshwater Pied-billed Grebe of North America has occurred as a transatlantic vagrant to Europe on more than 30 occasions.

Bills vary from short and thick to long and pointed, depending on the diet, which ranges from fish to freshwater insects and crustaceans. The feet are always large, with broad lobes on the toes and small webs connecting the front three toes. The hind toe also has a small lobe. Recent experimental work has shown that these lobes work like the hydrofoil blades of a propeller[1]. Curiously, the same mechanism apparently evolved independently in the extinct Cretaceous-age Hesperornithiformes, which are totally unrelated birds.

Grebes have unusual plumage. It is dense and waterproof, and on the underside the feathers are at right-angles to the skin, sticking straight out to begin with and curling at the tip. By pressing their feathers against the body, grebes can adjust their buoyancy. Often, they swim low in the water with just the head and neck exposed.

In the non-breeding season, grebes are plain-coloured in dark browns and whites. However, most have ornate and distinctive breeding plumages, often developing chestnut markings on the head area, and perform elaborate display rituals[1]. The young, particularly those of the Podiceps genus, are often striped and retain some of their juvenile plumage even after reaching full size.

When preening, grebes eat their own feathers, and feed them to their young. The function of this behaviour is uncertain but it is believed to assist with pellet formation and to reduce their vulnerability to gastric parasites.

Grebes make floating nests of plant material concealed among reeds on the surface of the water. The young are precocial, and able to swim from birth[1].

Quantum properties of molecular water

Heavy Water and isotopologues of water

There are several isotopes of both hydrogen and oxygen, so several isotopologues of water are known. Hydrogen has three naturally occurring isotopes. The most common, making up more than 99.98% of the hydrogen in water, has 1 proton and 0 neutrons. A second isotope, deuterium (short form "D"), has 1 proton and 1 neutron. Deuterium oxide, D2O, is also known as heavy water and is used in nuclear reactors as a neutron moderator. The third isotope, tritium, has 1 proton and 2 neutrons, and is radioactive, with a half-life of 4500 days. T2O exists in nature only in tiny quantities, being produced primarily via cosmic ray-driven nuclear reactions in the atmosphere. D2O is stable, but differs from H2O in that it is denser - hence, "heavy water" - and in that several other physical properties are slightly different from those of common, Hydrogen-1 containing "light water". Water with one deuterium atom HDO occurs naturally in ordinary water in very low concentrations (~0.03%) and D2O in far lower amounts (0.000003%). Consumption of pure isolated D2O may affect biochemical processes - ingestion of large amounts impairs kidney and central nervous system function. However, very large amounts of heavy water must be consumed for any toxicity to be apparent, and smaller quantities can be consumed with no ill effects at all.

Oxygen also has three stable isotopes, with 16O present in 99.76 %, 17O in 0.04% and 18O in 0.2% of water molecules.[15]

[edit] Transparency

Main article: Water absorption

Water is relatively transparent to visible light, near ultraviolet light, and far-red light, but it absorbs far ultraviolet, infrared light, and microwaves. Most photoreceptors and photosynthetic pigments utilize the portion of the light spectrum that is transmitted well through water. Microwave ovens take advantage of water's opacity to microwave radiation to heat the water inside of foods.