SAAS March/April 2021

Aplomado Falcons By Mike Scully Some day I should drive down along the Gulf Coast in search of an Aplomado Falcon (Falco femoralis). I have never seen one, but then I don’t have all that many Texas life birds left so I’m not in any particular hurry—who would want to see them all? Larger than a kestrel, smaller than a peregrine, these colorful (Aplomado means lead-colored in Spanish) falcons with blue- gray plumage occupy open areas interspersed with scattered trees and patches of forest. Within the United States, the northern subspecies (F.f. septentrionalis) originally occurred in the grasslands of South and West Texas, southern New Mexico and Southeast Arizona. It still is found across much of Mexico south to Guatemala, with the highest densities occurring along the coastal plain from Veracruz to the Yucatan. Two other subspecies collectively occupy much of Central and South America from Panama south to Tierra del Fuego. The total population across this extensive range is such that the Aplomado Falcon is actually considered a Species of Least Concern by the International Union for Conservation of Nature.

Aplomado Falcon by Peter K. Burian. As is common among tropical raptors, over most of its range, mated pairs of Aplomado Falcons occupy permanent territories year-round with some altitudinal migration from the high Andes and movement from southern South America during their winter. Some contemporary authors stated that our original Texas population moved south in winter; however, the present Texas population, descended from birds originally trapped in Mexico, appears to be non-migratory. The pair bond remains strong year-round, with the pair roosting and often hunting together. A pronounced reverse sexual dimorphism exists, with males averaging less than two-thirds the mass of the females. The usual prey of Aplomado Falcons consists of small to medium-sized birds, large insects, small rodents and reptiles. In flight and mode of attack, Aplomado Falcons SAAS March/April 2021 resemble accipiters, often perching quietly inside a tree while hunting, or alternatively flying low above tall vegetation seeking to surprise birds into flight. Their long legs add to the accipiter-like impression, and like those hawks they may run along the ground after prey. After a kill is made the pair often share it, to the extent of sometimes feeding each other. Aplomado Falcons do not construct a nest, using instead the abandoned nests of other raptors, corvids or other large birds, and occasionally they pirate occupied nests. Alternatively, man- made structures offering a suitable surface may be used. The availability and nature of nest sites may be a limiting factor, as population densities in Mexico are greatest where bromeliads provide suitable nest sites. Here in Texas, the exact design of nesting platforms provided for them has proven crucial. Three eggs are the norm, incubated primarily by the female, with the male provisioning her and their hatched young. Young reared together have been observed hunting cooperatively after the manner of adult falcons. When, exactly, these young of the year depart their parent territory awaits further study. In Texas, the Aplomado Falcon existed and at present does exist on the very northern edge of the historic range of this widespread species. In the absence of reliable data, the present status of the species across much of contiguous northern Mexico is unclear. In the second half of the 20th century the advent of chlorinated hydrocarbon insecticides followed by an ongoing drought certainly reduced populations there. The exact status of this falcon in Texas prior to the 20th century remains undetermined. Contemporary authors left contradictory accounts and few verified Texas specimens exist. In recent decades, over a 20-year period, nearly 1,000 captive-bred Aplomado Falcons were released into the wild along the Texas coast from Mustang Island to Laguna Atascosa before our present small population became established. A similar introduction of more than 800 birds in West Texas, Arizona and New Mexico has thus far proved unproductive. The causes of this species’ original extirpation from Texas remains uncertain. Persecution by humans was likely a factor, and possibly losses of eggs and young to predators. While the disappearance of native grasslands has often been assumed as a primary cause, elsewhere in its range the species has adjusted to human alterations to the landscape, even benefitting from the clearing of tropical forests. The present Texas population, consisting of perhaps 60 established pairs along the lower Texas Gulf Coast, also occupies a landscape altered since pre- settlement days.