|
|
Of all the vertebrate taxons: fish, amphibians, reptiles, birds and mammals, birds are the only class that every species lays eggs. Live bearing or egg laying members exist in fish, amphibians, reptiles and mammals. For example, all members of the fish family Poeciliidae give birth to their young. Amphibians such as the Alpine Salamander (Salamandra atra) and African Viviparous Toad (Nectophrynoides poyntoni) also give birth to their young. Reptiles such as North American Water snake (Nerodia sp) and Blue-tongued Skinks (Tiliqua sp.) give birth to their young as well. As far as mammals, only five species lay eggs: Platypus (Ornithorhynchus anatinus), Short-beaked Echidna (Tachglossus aculeatus), Western Long-beaked Echidna (Zaglossus brujinii), Sir David's Long-beaked Echidna (Zaglossus attenboroughi) and the Eastern Long-beaked Echidna (Zaglossus bartoni); all the others give birth to their young.
All birds except the Megapods (family: Megapodiidae) inclubate their eggs themselves. Some Megapods build mounds of decaying vegetation and lay their eggs in them; the heat from the decaying vegetation inclubates the eggs, but the parents still monitor the temperature at the mounds. If mounds get too hot, some of the debris will be removed, if too cold, the male will add more debris. Other megapod species will use solar heat to incubate the eggs. There are also species that will use either one, depending on environmental factors. Other than these 21 species of megapods, the other 10,000 species of birds inclubate their eggs with their own heat. The incubation time can be as short as 11 days (Yellow-billed Cuckoo) to 85 days (Wandering Albatross).
Baby birds come into this world in six forms: altricial, semialtricial (1 & 2), semiprecocial, subprecocial, precocial and superprecocial. The most helpless chicks are the altricial ones, which are born naked (no down), blind, unable to get around, and require food & attention from parents. On the other end of the scale, superprecocial chicks are completely independent once they hatch out of the egg. Songbirds are altricial, nighthawks are semialtricial, gulls & terns are semiprecocial, cranes are subprecocial, ducks & quails are precocial and the megapods are superprecocial.
In some birds, there are such intense sibling rivalries that it leads to the death of the younger chick. For example, in Great Egrets, the older chick will often kill the younger chick. This way, the older chick will get all the food and have a better chance to making it to fledging. This siblicide is so strong that it in their genes. In Cattle Egrets, the female actually injects the first two eggs with more androgens in them, thus making the chicks more aggressive. The third egg is given less androgens, and will most likely be killed by the older siblings. The female also starts incubation the minute the first egg is laid. This means the older chick will be a one to two days older than the next chick, and up to 4 days older than the third chick; thus giving it an advantage over the youngest chick (Alcock 2001). It seems cruel to us humans, but that's nature's way of ensuring that the healthiest and strongest chick makes it to reproductive age. In the south, Alligators usually hang out under rookery sites during nesting season; waiting for the losing chick.
As the altricial chicks get older, down develops, their eyes open, their feathers start to come in and push the natal down out. New feathers come out of these spiny looking structures. They make some baby birds look like little porcupines.
Some flight feathers start to come in and the chick is able to move better. Some birds stay in the nest until they fledge, others jump out and explore their surroundings. After they learn how to fly, some species keep their juvenile plumage until the second year. In other bird species, such as Brown Jays, the immature bird will have a different colored beak than from the mature adults. This different beak color tells other adults that this individual is immature.
Only a fraction of baby birds make it into sexual maturity. Many fall prey to foxes, bobcats, squirrels, crows, snakes and disease. As mature birds, they face a new set of challenges, such as establishing a territories, fighting for position in the flock, building nests, finding mates and a number of other duties. As the human population grows and advances, birds face new dangers. Deaths from flying into windows (kills over 1 billion birds a year in the U.S.), falling prey to the domestic cat (around 6 billion deaths per year) and flying into communication towers (around 200 million bird deaths a year) take their toll on avian populations (Weisman 2007). These factors along with increased habitat loss make it a hard life for avian species.
Hopefully with luck and good conservation practices, endangered bird species will recover. Species such as the Bald Eagle, Brown Pelican and Osprey have all made a comeback because of the DDT ban, conservation programs and federal protection. However, birds as a whole face some serious problems in the next 50 years. Many common species, such as the American Goldfinch, may become threatened species in the next few decades because of Global Warming. The last individuals of a species are sometimes seen in zoos, and when they are gone, they are exhibited in natural history museums in preserved forms. Aldo Leopold would calls these birds bookbirds, just like how he called the Passenger Pigeon "book-pigeons" (Leopold 1966). Hopefully that won't be the case with one of the most endangered birds in North America, the Attwater's Prairie Chicken. |
 |
