Some ornithologists believe that the resemblances in wing structure are convergent and that the other shared characters are relatively widespread among several orders of birds and of little significance in indicating relationships.
The nightjars and related families date back as a group to the Eocene Epoch (about 56 34 million years ago) and are not as old as many other orders of birds.
In the second half of the book he summarizes the evolution of all the modern orders of birds, discussing such subjects as the evolution of filter-feeding in ducks and flamingos, the evolution of flightlessness, the evolution of birds of prey, and the rise of landbirds.
Of 27 orders of birds, only three--parrots, hummingbirds, and songbirds--have this talent.
First, most orders of birds do not have fossil evidence across the entire timespan of their existence.
Most vertebrates communicate acoustically, but few, among them humans, dolphins and whales, bats, and three orders of birds, learn this trait.
As it reviews the major orders of birds, the book highlights their wide diversity and critically evaluates ornithological concepts and theories.
Cooper and Penny looked at 16 orders of birds, using two small fragments of genes--600 base pairs of a protooncogene in the nucleus, and a 390-base pair region from the mitochondrion, a cellular organelle that has its own DNA.
The first edition was organized as follows into four "Orders" of birds.
Yet despite centuries of close coexistence, he said, "it's been very, very hard for people to work out the simple relationship between different orders of birds".
