The lopsided tendency of evolution to produce many species in one group while preserving just a few in other groups applies quite well to birds. I called this tendency Haldane’s Rule. Is there something special about groups, like the perching (or Passerine) birds, that dominate our biodiversity? Or is their diversity a natural outcome from a lopsided process?
Mega-branching occurs several times in the Bird family tree. The early passerines, small animals suited to perching in trees, exploded into a wide variety of groups (82 families remain though there were probably more to begin with). This rapid branching happens several more times in the passerines, once among the tyrant flycatchers and then again with the especially colorful tanagers, each time forming 100 or more smaller groups . The sparrows (Passerine literally means “sparrow-like”) have their own branching event resulting in 72 groups. The same thing happens outside the perching birds. There are 128 genera (or kinds) of hummingbird and 80 genera of parrot. In the end, though, the perching bird species easily outnumber all the other 27 orders of bird combined – 6 out of 10 bird species are passerines.
There are few major divisions in the perching birds, one group of 4500 species is considered to have more advanced voiceboxes, and get a special designation as songbirds. The remaining birds sing simpler songs and are mainly based in Latin America.
The key to the diversity of perching birds appears to be that they started branching early in bird evolution and never stopped. Altogether, the remaining orders of birds- everything from ducks to eagles to ostriches produced 90 families that survive to this day. The passerines pressed their early advantage and averaged 15 genera to a family as compared to 10 genera in the average non-passerine family. By the time it came down to splitting off into individual species the perching birds slowed their hectic pace, producing an average of 5 species per genus. This is still more than the average non-passerine genus but no longer exceptional. They can only have done this by 1) being especially likely to branch into new species and then 2) being especially likely to survive against all competitors, including lizards, amphibians, mammals and other types of birds. The branching part, as we have discussed before, is especially difficult to predict. It relies on isolation of small groups to prevent breeding. This has been demonstrated several times for groups of island birds. It is, however, ironic to be talking about isolation in a group of animals which can overcome almost any physical barrier (e.g. they can fly over the Himalayas!). The type of isolation that we would expect in birds is behavioural, congregating in certain habitats. For reasons that I go into elsewhere, I doubt that sexual selection was a big factor in these branching events. What is really interesting is to consider what would make these fairly rare speciation events so common as to produce 70-100 branches in a relatively short space of geological time.
Raikow considers a couple of reasons why there are so many perching birds, concluding in the end that none of them offer us a way to predict future branching. Two intriguing reasons are size and diet (see Fundamentals of Ecology). Their size (and perching feet!) allow them to nest and feed throughout the forest canopy, adding a literal third dimension to the kind of habitat they can choose. The items in their diet, insects and seeds, belong to the most diverse groups of plants and animals on the planet. These groups were busy branching into new groups at about the same time as the perching birds and provided plenty of opportunities for coevolution and specialization. Despite these opportunities we are left wondering what it is about the perching birds that allowed them to diversify where other similarly-sized animals did not.
One possibility is that this group had an inherited trait that disposed it to forming groups within a species, a clique gene that makes individuals sensitive to differences in habitat, diet and appearance and causes them to be more conservative about trying new habits or associating with birds that look different from their parents and siblings. This cliquish behaviour forms breeding groups which, if allowed to continue for hundreds of generations, makes species fragile and likely to break apart into new groups.
More likely, there was nothing about the ancestral passerines or their opportunities that made the class of birds so lopsided. Instead, it was the lack of branching (or perhaps survival) among other bird groups that yielded the uneven result. The average number of families per order of birds is only 6, a number that is significantly lower than the branching that occurred both within and outside of the passerines after that stage of evolution (see “How lopsided is evolution?” for how I arrived at this). On the other hand, the maximum number of families in an order, the 82 passerine families, is completely consistent with later branching. What really stands out about the birds, then, is the large number of orders, 12, with only one family! The ostriches take the prize in this case, producing only one species for the entire order. Other oddities with one family include the mousebirds and the sandgrouse. Perhaps these groups were especially gregarious and inclusive in their breeding behaviour to keep their species intact and maintain low branching rates for great stretches of evolutionary time. Some of the single family groups got over their evolutionary bottleneck, however, including the respectably diverse parrots and pigeons.
So, the next time you are delighting in the songs and colors of your local group of perching birds, be aware that except for a trick of history, you could have been seeing an equally diverse groups of owls, ducks and woodpeckers!