Aplomado falcon (Falco femoralis septentrionalis)
Armendaris Ranch, NM
The species has suffered a significant population decline throughout its historical range due to a variety of factors including habitat degradation and alteration (e.g. shrub encroachment and development of native grasslands).
- Listed as endangered under the ESA in 1986
Beginning in 1997, our goal was to collaborate with The Peregrine Fund and the USFWS on efforts to reintroduce falcons to restore a viable population to the Chihuahuan grasslands of the Armendaris Ranch and environs, which would count toward federal recovery of the species per the ESA. By 2013, it was apparent that ecological circumstances precluded restoration of such a population. Therefore, that year we terminated field efforts on behalf of the species.
The aplomado falcon inhabited the inland and coastal grasslands of Texas, New Mexico, and Arizona until about 1930 when populations began to decline. In 1986 the species was listed as endangered under the Endangered Species Act. In 2006, TESF helped to lead an effort to restore the species to its former range in New Mexico with the release of 11 birds at the Armendaris Ranch in the south-central portion of the state. Through 2011, 112 birds were released at the Armendaris. To promote their survival, TESF implemented an extended supplemental feeding program. From 2006 through 2008, supplemental food was provided daily. From 2009 through 2011, food was provided every other day. Providing food daily related to an increase in the short-term survival of falcons. Unfortunately, increased short-term survival did not seem to lead to long-term survival. Generally, very few falcons utilized the ranch for any notable period of time. Annual spring surveys revealed that only three nesting pairs established themselves on the ranch, fledging five offspring. No pair persisted for more than one breeding season. By 2013, there was no evidence that any aplomado falcons occupied the Armendaris or environs.
Despite the release of another 225 birds elsewhere in New Mexico between 2006 and 2012, there was no evidence that any of the animals or any progeny occurred in the state. No releases were conducted in New Mexico in 2013 and none were planned for 2014 or beyond. Reduction in the abundance and availability of prey (probably due to drought and shrub encroachment to the grasslands) and high rates of mortality from raptor predation appear to preclude the falcon’s restoration to New Mexico, including the Armendaris Ranch and environs.
TESF terminated field work on behalf of the aplomado falcon in 2013.
Armendaris Ranch comprises approximately 1,439 km² in Sierra and Socorro counties in south-central New Mexico, east of the Rio Grande. The ranch lies within the Jornada del Muerto basin in the northern Chihuahuan Desert. Topography consists mostly of an open valley plain, bordered by the Fra Cristobal Mountains to the west, the Jornada del Muerto Volcano and lava fields to the north, the White Sands Missile Range to the east, and the Jornada Experimental Range to the south. Ranch habitats include desert grassland and scrub, bajadas, lava flows, and riparian areas.
The Armendaris is privately owned and managed by Turner Enterprises, Inc. for livestock production of plains bison (Bison bison). The bison herd includes approximately 500 individuals, mostly cows and calves, which are managed to have minimal impact on the grasslands. To promote high quality hunting for several quail species, water and grain stations are common throughout portions of the ranch. A vegetative model showed the Armendaris offered highly suitable habitat for aplomado falcons.
While prey biomass was approximately an order of magnitude lower in the Chihuahuan desert than at the higher-rainfall areas of eastern Mexico and south Texas where reintroduced aplomados were thriving, avifauna surveys conducted at the Armendaris in the late 1990s, combined with a review of relevant literature, revealed that prey biomass at the Armendaris was significantly higher than other areas in the Chihuahuan desert.
In 2006, the TESF and Turner Enterprises partnered with the U.S. Fish and Wildlife Service, New Mexico Department of Game & Fish, and The Peregrine Fund to initiate a reintroduction project as part of a larger effort to restore the aplomado falcon to New Mexico. The ranch was chosen as the first release site because it offered secure and extensive suitable habitat that had historically supported a small breeding population of aplomados.
The Peregrine Fund, the entity principally responsible for the release phase of the reintroduction project, employed standard raptor hacking (reintroduction) procedures for releasing 112 aplomado falcons at the Armendaris Ranch from 2006 through 2012. This procedure involved holding the juvenile aplomado falcons in a hack box, for 7-10 days, on an elevated platform erected in suitable habitat, and then releasing them at an age when they would naturally fledge (Mutch et al. 2001). On release day, the box was opened remotely allowing the birds to emerge on their own thus simulating natural fledging conditions. To promote survival and encourage the aplomados to establish residency near the release site, hack site attendants provided supplemental food [freshly thawed Japanese quail (Coturnix japonica)], twice a day for approximately 40 days post-release.
Extended Supplemental Feeding Program
To improve survival of recently released birds at the Armendaris, TESF and staff from the ranch were responsible for extending the supplemental feeding program after the standard 40-day period and for extended, intensive post-release monitoring of the birds. Rationale for extending the supplemental feeding program rested on two facts: 1) even after 40 days of free-flying experience the aplomados were still relatively naïve owing to their captive origins, and 2) the cessation of the standard 40-day feeding program coincided with a reduction in the abundance of naturally occurring prey items due to the onset of fall and winter. It was hypothesized that extending the feeding program would promote the survival of the released aplomados until spring of the first year following releases when the abundance of naturally occurring prey items would increase. Intensive post-release monitoring was essential for determining the outcome of the releases and effecting management actions to promote restoration of a viable population of falcons.
Due to falcon attendance at the feeding stations, the extended feeding program varied in length from year to year. In an attempt to improve the cost-effectiveness of the program the schedule of feedings was changed from daily for 2006 through 2008 to every other day from 2009 through 2011. Very infrequently, scheduled feedings were missed due to weather or other circumstances. For each feeding, one less quail than the number of expected falcons was provided in the evenings. The extended supplemental feeding program continued as long as falcons visited the feeding station, there was no evidence of falcons being targeted by predators at the feeding station, or until late spring of the first year following the birds’ release.
In all years survival was obtained through recorded observations of birds attending feedings. In 2011, 10 aplomado falcons released at the Armendaris were equipped with VHF radio transmitters to document movements and mortality as part of a larger study by The Peregrine Fund. While most of those transmitters malfunctioned, three mortalities due to avian predators were confirmed.
Participation in Extended Supplemental Feeding Program
Of the 102 aplomado falcons released at the Armendaris through 2011, 56 birds participated in the extended supplemental feeding program. An additional nine falcons released at the White Sands Missile Range, about 48 km from the Armendaris feeding stations, took advantage of the supplemental feeding program. Out of the 65 birds that took advantage of supplemental feedings, 42 were involved in every day feedings and 23 were involved in every other day feedings.
In 2006 one bird attended 124 feedings out of 148 feeding opportunities. The largest number of birds to attend a single feeding was 17 in 2008. Also in 2008 up to nine birds regularly attended feedings through March of the year following their release. A female aplomado released in 2007 at the White Sands Missile Range attended 12 of 20 feeding opportunities at the Armendaris in 2009.
Effect of Frequency of Feeding on Survival
Because the length of the feeding programs varied a great deal from year to year we only considered the survival of birds for 30 days following the standard 40-day feeding period for the logistic regression and a test of two proportions analyses. For survival analysis, because we were comparing the amount of time each individual bird was known to take advantage of the feeding program we considered the survival of birds for 60 days following the standard 40-day feeding period.
Logistic regression revealed that the frequency of feeding was a significant predictor of whether the recently released aplomados were known to be alive at day 30 (p-value = 0.019). Specifically, the probability of a falcon known to have survived to day 30 increased by 3.78 if food was provided every day compared to feeding every other day.
A test of two proportions showed a significant difference between the proportion of birds known to be alive at day 30 when food was provided every day, compared to every other day (p-value = 0.009). Specifically, when food was provided daily 57% of the recently released aplomados were alive at day 30 compared to only 26% of the aplomados when food was provided every other day.
Survival analysis using the log-rank and Wilcoxon methods of comparison revealed a significantly greater percentage of birds survived to day 60 when fed every day compared to birds fed every other day (p-value = 0.000). Specifically, none of the birds that was subjected to feeding every second day survived to day 60, whereas about 50% of the birds fed every day survived to day 60.
Longer term monitoring revealed that while some birds persisted into the first and second years following their initial release, most disappeared within a few months of release.
Extended Supplemental Feeding and Falcon Reproduction
Annual spring surveys revealed that releases at the Armendaris led to the formation of nesting pairs at the ranch in 2007, 2009, and 2011. These were the only recorded nesting pairs in New Mexico despite 203 birds being released from 2006 through 2011 at sites separate from the Armendaris Ranch. All nesting individuals were regular attendees at extended supplemental feedings. Indeed, the one bird that attended 124 feedings out of 148 feeding opportunities was part of the pair that fledged two chicks in 2007.
The 2007 pair fledged two chicks, the 2009 nesting attempt failed, and the 2011 pair fledged three chicks. Because none of the fledglings were banded it is impossible to comment accurately on their survival. However, from 2007 to October 2010 an unbanded female aplomado falcon resided at the Armendaris and regularly attended extended supplemental feedings. It is believed that this female was one of the chicks fledged in 2007 and as an adult was a part of the failed nesting attempt in 2009.
Effect of Frequency of Feeding on Attendance at Feeding Stations
A weighted regression showed that the frequency of feedings was a significant predictor of feeding station attendance by falcons (p-value = 0.001).
Survival Trends as a Function of Feeding Frequency
The results show that when supplemental feeding was extended, following the close of the standard supplemental feeding protocol about 40 days after a bird’s initial release to the wild, recently released juvenile aplomado falcons were more likely to survive to at least day 60. Additionally, birds that were subjected to extended daily feeding programs were more likely to attend feedings than birds subjected to every other day feeding programs. Furthermore, when food was provided more frequently, more aplomados visited the feeding stations. All outcomes indicated that the extended supplemental feeding program promoted survival, retention, and reproduction of recently released juvenile aplomado falcons. Similar results have been reported for other captive-born carnivores that have been reintroduced including the red wolf (Canis lupus rufus), Mexican wolf (Canis lupus baileyi), and the black-footed ferret (Mustela nigripes). Unfortunately, and in contrast to these other species, improved short-term prospects for aplomado falcons did not seemingly translate into improved long-term survival or retention for population establishment.
Nearly all of the 112 birds released at the Armendaris, with the exception of the members of the nesting pairs and a few others, did not become permanent or even infrequent residents. Similarly, of the aplomados that used the ranch in the year following their release, only a few were known to inhabit the ranch during the following year. While the final fates of the released birds are largely unknown, there is no evidence that they established themselves elsewhere. It seems likely that most, if not all, did not survive to reproduce.
These results stand in contrast to efforts to restore aplomados in south Texas. Releases of captive-born juveniles began there in the 1990s and included 839 birds through 2004. These releases gave rise to two populations that collectively include over 30 breeding pairs along the south Texas coastal plain. Unfortunately, notable challenges exist to improving circumstances for both populations.
Because of the failure of released aplomados to persist at the Armendaris Ranch no falcons were released at there, or elsewhere in New Mexico, in 2013. No releases were planned for the state in 2014 and beyond.
It remains unclear why the release of 112 aplomado falcons did not lead to the establishment of at least a few birds on the Armendaris Ranch. Releases elsewhere in New Mexico were similarly unsuccessful. Hunt et al. (2013) emphasized the importance of drought and its impact on prey populations and predator activity as a principal cause for the lack of success in New Mexico and West Texas.
At the Armendaris and environs, similar to the aplomado falcon restoration area in coastal Texas, it may be that changes to the structure of habitat are sufficiently substantial to affect the hunting success of recently released aplomados and harbor avian predators such as the Great Horned Owl (Bubo virginianus).
Historically, the aplomado falcon inhabited open grasslands with prominent widely scattered tall woody vegetation. Most of the once open grasslands have, however, undergone significant vegetation changes in the last century. One study in southern New Mexico showed a decline in grass cover of 70% during the preceding century. Another study reported a proportional increase in shrubs to the decrease in grass cover, in the Jornada Basin, directly south of the Armendaris Ranch. Factors causing this vegetation shift include overgrazing by cattle, fire suppression, and changes in climate. Additionally, the once common prairie dogs that maintained large shrub-free areas are now extremely rare. It is noteworthy that observations of reintroduced aplomado falcons in coastal Texas indicated that they avoided even small clusters of shrubs perhaps because of the difficulty of detecting and avoiding large avian predators.
Regardless of the mechanism, grasslands at the Armendaris and elsewhere in the Chihuahuan desert now support a more obvious shrub component that may greatly hinder successful hunting by aplomado falcons and support large avian predators. Any future reintroductions scheduled for the area should be preceded by careful consideration of efforts to reduce the abundance of shrubs. This could be achieved through prescribed fire, mechanical removal, and/or chemical application. It is noteworthy that the Bureau of Land Management has treated over 400 km2 acres of shrub-infested land in the nearby Jornada Basin since 2007 through the application of herbicides, deferred grazing for 5 years, and prescribed fire.
Prospects are dim for restoring a population of aplomado falcons to the Armendaris Ranch and environs unless those factors responsible for limiting the survival of recently released birds are eliminated or substantially ameliorated. If future reintroductions are scheduled, our work supports the inclusion of an extended supplemental daily feeding program through spring following a falcon’s release year and probably a concerted effort to reduce the abundance of shrubs in the grasslands.