> [Birds] are ... characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.
Notice that when you put all of those traits together, you get the ability to lead a lifestyle centered around long-range migratory flight.
One or a few species evolved this particular set of traits, which gave them the ability to forage incredibly long ranges in search of food, in a landscape that was—at the scales other dinosaurs could travel—increasingly inhospitable to life. This small set of species then took over the habitats of every other dinosaur species as they died out, and then speciated into all modern birds. It was a sort of evolutionary bottleneck point.
In other words, "birds" are essentially defined as the set of theropod species that survived the K-T extinction. Birds happen to all have many unique traits in common, because these traits were adaptive through the rapid change of environment that killed off the other dinosaurs.
All birds had a common ancestor species. Anything that's a descendant of that ancestor species is a bird.
The actual ancestor hasn't been found, but it can be determined by genome studies, and by comparing known details (usually from the skeleton, because that is the only thing available from most fossils).
Essentially they collect a lot of traits of many species. If you assume that more closely related species have more traits in common a computer program can then figure out the most likely evolutionary tree.
In that tree, anything in the bird branch below the "bird ancestor" node is a bird.
The change happened very gradually over millions of years. The answer to that question is similar to the Sorites paradox, or the paradox of the heap of sand.
If you have a heap of sand, and remove grains of sand one at a time, at which point does the heap stop being a heap?
Well according to many people birds today are dinosaurs. (they are indisputably members of "Dinosauria")
But as with pretty much everything, there would have been a long period where they were on the border of being birds. Categories in biology always have blurry boundaries. And those boundaries will of course adjust as we get more fossils and other information.
Precocial young don't necessarily imply a difficult time, it's simply one of several strategies for successful reproduction. Do you minimize the chances your young are preyed on in the nest by A) Building a very well hidden nest and guarding it well or B) Ensuring the nest is only needed for a few days?
Additionally, altricial species tend to be much more intelligent so that's another factor to weigh in on which evolutionary strategy is optimal for a given species.
https://en.wikipedia.org/wiki/Ancient_DNA - it looks like scientists gave up hope of retrieving dna from amber, but there are recent examples of extracting dna from ~ 1 million year old specimens...
[+] [-] colordrops|8 years ago|reply
[+] [-] derefr|8 years ago|reply
> [Birds] are ... characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.
Notice that when you put all of those traits together, you get the ability to lead a lifestyle centered around long-range migratory flight.
One or a few species evolved this particular set of traits, which gave them the ability to forage incredibly long ranges in search of food, in a landscape that was—at the scales other dinosaurs could travel—increasingly inhospitable to life. This small set of species then took over the habitats of every other dinosaur species as they died out, and then speciated into all modern birds. It was a sort of evolutionary bottleneck point.
In other words, "birds" are essentially defined as the set of theropod species that survived the K-T extinction. Birds happen to all have many unique traits in common, because these traits were adaptive through the rapid change of environment that killed off the other dinosaurs.
[+] [-] nn3|8 years ago|reply
The actual ancestor hasn't been found, but it can be determined by genome studies, and by comparing known details (usually from the skeleton, because that is the only thing available from most fossils).
Essentially they collect a lot of traits of many species. If you assume that more closely related species have more traits in common a computer program can then figure out the most likely evolutionary tree.
In that tree, anything in the bird branch below the "bird ancestor" node is a bird.
[+] [-] teekert|8 years ago|reply
[+] [-] partycoder|8 years ago|reply
The change happened very gradually over millions of years. The answer to that question is similar to the Sorites paradox, or the paradox of the heap of sand.
If you have a heap of sand, and remove grains of sand one at a time, at which point does the heap stop being a heap?
[+] [-] robbrown451|8 years ago|reply
But as with pretty much everything, there would have been a long period where they were on the border of being birds. Categories in biology always have blurry boundaries. And those boundaries will of course adjust as we get more fossils and other information.
[+] [-] LordKano|8 years ago|reply
[+] [-] nickhalfasleep|8 years ago|reply
[+] [-] j9461701|8 years ago|reply
Additionally, altricial species tend to be much more intelligent so that's another factor to weigh in on which evolutionary strategy is optimal for a given species.
[+] [-] unknown|8 years ago|reply
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[+] [-] majestik|8 years ago|reply
[+] [-] aerodog|8 years ago|reply
[+] [-] pvaldes|8 years ago|reply
[+] [-] howfun|8 years ago|reply
[+] [-] rnprince|8 years ago|reply