Most people think of Darwin and Birds in relation to Darwin’s Finches. Unless, of course, you’ve heard a bit more about Darwin than the first-order mythology we tend to attach to our scientific leaders and know that Darwin ignored the finches and the whole bird evolution thing was figured out later by someone else. But of course, that is not true either. Rather, it is quite complicated but no matter how we interpret what happened for Darwin on his route from Potential Country Pastor to First Evolutionary Biologist, the mocking birds figured in because they impressed young Charles when he first saw them.
In the Voyage monograph on birds, in the section on the genus Mimus, Darwin interrupts John Gould …
It will be seen, that the three last species of the genus Mimus, were procured from the Galapagos Archipelago; and as there is a fact, connected with their geographical distribution, which appears to me of the highest interest, I have had these three figured. There are five large islands in this Archipelago, and several smaller ones. I fortunately happened to observe, that the specimens which I collected in the two first islands we visited, differed from each other, and this made me pay particular attention to their collection….
So, the mockingbirds have gotten his attention …
I found that all in Charles Island belonged to M. trifasciatus; all in Albemarle Island to M. parvulus, and all in Chatham and James’s Islands to M. melanotus. I do not rest this fact solely on my own observation, but several specimens were brought home in the Beagle, and they were found, according to their species, to have come from the islands as above named….
So mixed in with the description of the biogeography we see one of the pieces of evidence Darwin scholars use to reconstruct his thinking …
Charles Island is distant fifty miles from Chatham Island, and thirty-two from Albemarle Island. This latter is only ten miles from James Island, yet the many specimens procured from both belonged respectively to different species. James and Chatham, which possess the same species, are seventy miles apart, but Indefatigable Island is situated between them, which perhaps, has afforded a means of communication.
… by “communication” he means migration or movement across distance between islands …
The fact, that islands in sight of each other, should thus possess peculiar species, would be scarcely credible, if it were not supported by some others of an analogous nature, which I have mentioned in my Journal of the Voyage of the Beagle. I may observe, that as some naturalists may be inclined to attribute these differences to local varieties; that if birds so different as O. trifasciatus, and O. parvulus, can be considered as varieties of one species, then the experience of all the best ornithologists must be given up, and whole genera must be blended into one species.
Yes, Darwin was capable of being passive aggressive ….
I cannot myself doubt that M. trifasciatus, and M. parvulus are as distinct species as any that can be named in one restricted genus.
And, Darwin’s assertion was confirmed at the time by experts and stands today.
The habits of these three species are similar, and they evidently replace each other in the natural economy of the different islands; nor can I point out any difference between their habits and those of M. Thenca of Chile; I imagined, however, that the tone of their voice was slightly different.
And there we see Darwin getting the inkling that species vary over time indepenantly in different population which, if they remain separate long enough, drift apart and give rise to new species; Two of Darwin’s main evolutionary ideas, that things change and that species rise, are embodied in the case of the mockingbirds. More importantly, they are embodied in the case of the mockingbirds as separate from Natural Selection. Darwin is not suggesting that these birds diverged into different habitats. This way of thinking kept those concepts … the rise of species, change over time, and Natural Selection … as separate and independant processes (that can be linked). Darwin undrstood this quite early, even though many people today don’t understand this, or if they ever learned it, seem to forget it. Evolution is not Natural Selection, though Natural Selection is part of evolution.
Back to Darwin….
They are lively, inquisitive, active birds, and run fast; (I cannot assert, positively, that M. Thenca runs).
Darwin avoids saying things he’s not pretty sure of.
They are so extremely tame, a character in common with the other birds of this Archipelago, that one alighted on a cup of water which I held in my hand, and drank out of it. They sing pleasantly; their nest is said to be simple and open. They seem to prefer the dry sterile regions nearer the coast, but they are likewise found in the higher, damper and more fertile parts of the islands. To these latter situations, however, they seem chiefly attracted by the houses and cleared ground of the colonists. I repeatedly saw the M. melanotis at James Island, tearing bits of meat from the flesh of the tortoise, which was cut into strips and suspended to dry, precisely in the same manner as I have so often observed the M. Orpheus, in La Plata, attacking the meat hung up near the Estancias.
Source: Darwin, C. R. ed. 1841. Birds Part 3 of The zoology of the voyage of H.M.S. Beagle. by John Gould. Edited and superintended by Charles Darwin. London: Smith Elder and Co. Online here.