Socorro mockingbird
The Socorro mockingbird (Mimus graysoni) is an endangered mockingbird endemic to Socorro Island in Mexico's Revillagigedo Islands. The specific epithet commemorates the American ornithologist Andrew Jackson Grayson. Mimus graysoni shows its close relationship to the northern and tropical mockingbirds rather subtly. It is a much stouter bird, resembling some thrashers in habitus. It also has a distinct juvenile plumage, more rufous above and has a heavy pattern, especially below. This uncannily resembles, e.g., the gray thrasher (Toxostoma cinereum) from Baja California, but is apparently a case of convergent evolution. Systematics and taxonomy This is a rather distinct Mimus mockingbird and was for some time placed into a distinct genus, Mimodes. This was revealed to be incorrect based on analysis of mtDNA NADH dehydrogenase subunit 2 sequences. Rather, the present species is closely related to the northern and tropical mockingbirds. Its distinctiveness is the result of the strong selective pressure on its island home, which enforced the evolution of conspicuous adaptational autapomorphies. The juvenile plumage might also have been the result of genetic drift enforced maybe by resource partitioning in this aggressive bird. The standard model of molecular clocks cannot be applied for mimids as their rates of mutation seem to vary much over time. While it is the most phenotypically distinctive bird on Socorro, it also has the strongest ecological change from its ancestors; therefore its distinctiveness is not informative except supporting the theory that it is one of the older Socorro endemics. Thus, and because the adaptation to the peculiar conditions on Socorro may even have accelerated not only morphological but also molecular evolution – see also founder effect -, it cannot be said with any certainty whether or not among Mimus, the Socorro species is a quite recent island offshoot of either of the mainland species. In any case, the three taxa are very close relatives. This serves to show that evolution does not move on at a constant speed; certainly not on a morphological level, and often neither on a molecular level either. Rather, the rate of evolutionary change varies, sometimes considerably, depending on the circumstances and the strength with which natural selection acts upon a founding population. Ecology and status The Socorro mockingbird today lives mainly in unmodified low forest above 600 m (2000 ft), where it prefers groves of Oreopanax xalapensis and the endemic Guettarda insularis, with an understorey dominated by Triumfetta socorrensis and the endemic Eupatorium pacificum. As late as March 1953 it was still "common" at lower elevations, foraging in arid open areas of the Croton masonii-prickly pear shrubland. In November of the same year, the birds had retired to the more humid forest in the uplands and were busy singing and defending territories. Today, they mainly remain in Ficus cotinifolia stands when visiting the lower elevations. The breeding season is extended, with nests in attendance between November and July, with the peak laying occurring in March and April. Three eggs are laid, which take no more 15 days to hatch. The birds