Socorro mockingbird

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.[3]

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.[4] 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.[2]

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.

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.[1] As late as March 1953 it was still "common" at lower elevations, foraging in arid open areas of the Croton masonii-prickly pear[note 1] shrubland.[2] 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.[2] Today, they mainly remain in Ficus cotinifolia stands when visiting the lower elevations.[1] The breeding season is extended, with nests in attendance between November and July, with the peak laying occurring in March and April.[1] Three eggs are laid, which take no more 15 days to hatch.[1]

The birds are generally reluctant to fly and as late as the mid-20th century were still fatally unwary; if pressed they will rather hop away than fly and if they take wing, it is usually for a few meters only.[2] This may be an adaptation to the fact that Socorro has no native terrestrial predators, but red-tailed hawks and great frigatebirds that not infrequently prey on mockingbird-sized birds.

This species feeds on small invertebrates, the remains of land crabs (Gecarcinus planatus)[2] and fruit, namely of the endemic shrubs Ilex socorrensis and Sideroxylon socorrense, the latter of which has also become rare.[1] Flies are pecked up but do not seem to be snatched out of the air.[2] Like many Mimidae, the Socorro mockingbird is an aggressive, solitary species living alone or with its partner. When they come together at a plentiful food source – e.g. blowflies on a carcass – there is a marked social hierarchy between birds and rarely are more than two or three actively feeding; the less dominant birds hang around nearby, waiting for their turn.[2]

The call is two medium followed by one lower whistle. Less often, a full song is given, consisting of a variable warbling tune, repeated several times as in many mockingbirds. It is not clear whether this species imitates other birds like its relatives do; in any case only the Socorro tropical parula, Socorro towhee and Socorro wren would seem to possess songs that might serve as models for the mockingbirds, and except the parula these are not often found in the mockingbird's core habitat.[2]

• BirdLife Species Factsheet.