PROJECT TITLE: Working toward success in returning juvenile golden eagles to the wild
JUSTIFICATION
The golden eagle (Aquila chrysaetos) in western North America ranges from the arctic tundra to central Mexico. It forages primarily on medium-sized mammals in open grassland, and nests on cliffs and in trees. After a long period of intense human persecution owing to a general misunderstanding and dislike of predators, the species received legal protections in the 1960s. Thanks to science, films, and celebratory articles, our society now values golden eagles, and they have become a focus of conservation. There remains, however, a variety of human-related mortality factors that annually take their toll on populations, the most significant being electrocution on distribution lines. Other mortality agents include lead poisoning, wire-strikes, collisions with vehicles, wind turbine blade strikes. Despite these problems and others, however, golden eagle nesting territory occupancy and reproduction remain at healthy levels over most of their western range.
Every year, veterinarians and wildlife rehabilitation facilities receive injured golden eagles of various ages, as well as young eagles not yet able to fly. In summer 2021, for example, well-meaning people delivered at least ten of the latter to California wildlife facilities. There are several reasons why fledglings are so often encountered on the ground, including their being blown from the nest by high winds. There is also the problem of fledgling golden eagles bolting from the nest when a human approaches. It has been my experience and that of my coworkers that most of these grounded eagles survive because they remain guarded and provisioned by their parents. If generally true, it would be far better to leave these young eagles alone than to “rescue” them, contrary as that might seem to human nature. Grounded eaglets tend to walk uphill, presumably to make themselves more visible to their parents and to find updrafts as aids to flight. Occasionally, however, a fledgling may find itself encumbered and immobilized by vegetation or under a continuous tree canopy, invisible to its parents. Such a bird starves unless it can move to open ground.
One might speculate that this “flushing” adaptation relates to predation by large climbing predators over evolutionary time. For example, a fledgling might be better off bolting when an advancing bear is still on the ground than when it is elevated and able to see where the fledgling has landed.
It is desirable to return eagle “patients” to the wild, the alternative being to commit them to life in confinement a dismal fate for such remarkable beings whose life expectancy may exceed thirty years. Not much is known, however, about the extent to which current rehabilitation practices promote golden eagle survival after release. As I understand it, such methods have focused only on the development of flight muscles in aviaries or through quasi-falconry techniques simply to make sure the bird can fly, followed by release. If applied to an inexperienced young golden eagle, the bird would have no chance of survival for reasons stated below. Meanwhile, California Department of Fish and Wildlife does not permit the use of telemetry to monitor the fates of released birds, so no feedback has been available on the efficacy of such releases in our state. It is my understanding that few, if any, immature golden eagles released by rehabilitators are known to have survived anywhere in the wild. What is needed then are fresh ideas.
For eagles that were competent foragers prior to injury, and whose health can be fully restored, the task of retuning them to the wild may involve conditioning through falconry techniques, primarily to redevelop strength and stamina. Restoration proceeds from lure training to soaring on slopes and thermals, the primary means by which golden eagles travel and find food. Another method worth considering with these older, experienced eagles is to place them in large, tall flight pens for extended periods where they can redevelop strength and endurance sufficient to their survival post-release. Neither method has as yet generated performance data sufficient to provide proof of efficacy, but both are credible candidates for experimentation and refinement.
It is essential that inexperienced fledglings or juvenile golden eagles, the focus of this document, must learn to forage prior to their ultimate release. So let us first consider how wild eagles might acquire that capability and what it entails.
LIFE-HISTORY CONSIDERATIONS
Fledgling golden eagles normally take their first flights in late spring or early summer at about ten weeks of age (Watson 2010). They usually remain in or near their natal territories for most or all of the summer and sometimes beyond, supported and protected by their parents. Such behavior might suggest that learning to catch prey is initially difficult. But it is important to know that carrion is a major source of food for golden eagles and, if available, is far more easily obtainable by juveniles in their first fall and winter (Watson 2010, Watson et al. 2019). Live prey become particularly vulnerable in the subsequent spring when young-of-the-year mammals first emerge; meanwhile, juvenile eagles are thought to be mostly dependent on carrion (Jenny et al. in press). A high-soaring eagle, with its prodigious visual acuity, can see magpies, ravens, vultures, and raptors on carcasses at great distances over vast areas of landscape. Not surprisingly, studies using “trail cameras” are finding that, although ravens are normally the first to visit ungulate remains, golden eagles often appear soon thereafter (see Watson 2019).
The very large spatial scales at which eagles are known to travel the western landscape in a typical day (thanks to satellite-reporting telemetry) suggests that access to carrion can, at least in some regions be sustaining (Katzner et al. 2020). Sources include livestock carcasses, placentas, deaths within waterfowl concentrations, the remains of gun-killed animals, vehicular casualties, and other fatalities. Much depends, of course, on the regional densities of animals that eagles use as food. Prolonged droughts periodically reduce mammal populations, including livestock. Another consideration involves cyclic prey, hares being an important food over much of the range of golden eagles. Jackrabbit populations tend to fluctuate in boom-and-bust cycles over large areas. Releasing eagles in a region where food is scarce for any reason would be ill- advised, irrespective of whether the issue is carrion or live prey.
Choosing a favorable region for release is thus an important consideration in golden eagle rehabilitation. A multi-year radio-tracking study of golden eagles in west-central California, for example, revealed unexpectedly high survival of juveniles (Hunt et al. 2017). Among a sample of 101 radio-marked juveniles, 84.2% survived from fledging to one year past fledging, and 89.3% or more might have done so in the absence of human-caused mortality. Only one juvenile was known to have starved after leaving its natal territory, and it was not tested for lead- poisoning, a factor causing starvation despite food access.
ASSUMPTIONS
There are no reliable published observations that golden eagle parents “teach” their progeny to hunt. The departure of juveniles from their natal territory mainly follows the development of soaring behavior, strength, stamina, and the ability to find and appropriate carrion (see Watson 2010 for review). The transition is thus a matter of parents ensuring that their young do not starve while learning on their own to acquire food.
Following the natural way of things, the rehabilitation of young golden eagles might be achieved by releasing them in an area where they can be reliably provisioned with carrion until they leave on their own, a process known as “hacking” that has been highly successful in a variety of other raptors (Simmons et al. 1988). The procedure involves placing flightless nestlings in an elevated structure known as a “hack-box” with an open side, often fitted with vertical bars preventing early escape (and predator access) but allowing a view of the landscape. Food is typically delivered through a plastic pipe from the back to prevent the youngsters from associating humans with food. The front bars are removed as the young get older, and eventually take their first flights, and so on. Food provisioning continues until the birds no longer return to the site.
Unfortunately, the age at which the young eagles considered in this document enter such a program presents a problem. Raptors being hacked under normal conditions are too young to fly when placed in the hack box. They therefore behave toward the latter as if it were their nest and accordingly remain in its vicinity and return to it as a source of food once they are on the wing. During their weeks of flight, they remain near the nestbox and are protected and provisioned by their parents. The hack-box “nest” is home base. The juvenile golden eagles that typically arrive at rehabilitation facilities, however, are older and unsuitable for placement in a hack box. The problem is that they may abruptly leave it before associating it with food. Being capable of flight, they may quickly lose contact with the hack site as a food source, ultimately starving unless located and returned. Another problem is they vary in age.
This is why I return to falconry methodology as a solution to the problem. The idea would be to familiarize each bird, prior to final release, to the hacking area where food is continuously available until no longer revisited (see below). A reviewer of an earlier draft of this document (see below) has rightfully expressed concern that the level of human contact normally associated with falconry may cause young eagles to associate humans with food. It is true that golden eagles raised by hand from the downy stage are apt to imprint upon humans and may later become aggressive if food-stressed. Older fledglings obtained within a few weeks of first flight are well past the imprinting stage and are fearful of humans. To accept training, they must be kept at a precise “response weight” such that they may be expected to be hungry at that particular time of day when training will take place. If “flown” at a substantially higher weight, they will usually ignore the falconer, and at higher weights, they may depart and remain unapproachable.
There is no reason to believe that golden eagles are of a temperament unsuitable to being hacked in groups, even groups of mixed age, so long as all are fully feathered and capable of flight. I base this supposition on comments by Dr. Peter Bloom, a recognized raptor expert, with considerable knowledge of golden eagle behavior in the wild. He has observed golden eagles waiting their turn to access ungulate carrion, and indeed, multiple individuals held together in flight pens typically display mutual tolerance. There is one paper I have seen (Jenny et al. in press) that reports aggression between golden eagles on ungulate remains in Europe, but I believe it to be a special case, i.e, limited habitat space for non-breeders. The main task, as I see it is to make sure that eagles do not become separated from the area of provisioning before acquiring foraging competence. To minimize the chance of associating food with humans, I am recommending that eagles in training be fed on a lure rather than on the glove.
I have pondered the efficacy of bypassing falconry conditioning and releasing fully feathered young golden eagles after prior confinement in an on-site holding pen containing a group of eagles. This method is regularly successful in the California Condor reintroduction program. Condors conditioned in this way watch previously liberated condors flying in close vicinity and feeding at carrion stations near the pen. Released condors, being social, readily join the flock and survive well. Golden eagles, however, are of a solitary nature in the wild. Again, without some reliable way to retrieve eagles that prematurely leave the hack site area, the procedure would unreasonably risk their lives. CERE will continue to reflect on such alternatives, and even now, some variation of the flight-pen method might serve older, already competent foragers.
PROJECT GOAL
We aspire to the experimental development of a hacking methodology that will substantially increase the survival of young golden eagles released to the wild. This prospectus focuses on rehabilitating eagles obtained as post-fledging juveniles, but what is learned may, with modification, apply to older eagles as well.
PROPOSED EXPERIMENT
The idea is to release eagles from an area, identifiable at great distance when soaring, and where food can be proffered for an indefinite period while monitoring their whereabouts with radiotelemetry. Staff would include two people, at least one of whom would be a highly experienced falconer.
Training. Appropriate subjects would include any golden eagle coming into captivity within about 4 months post-fledging. Each will be minimally “manned” and lure-trained over long distances (~300m) by a falconer, this implying that the bird can be flown free and recovered, with confidence, when suitably hungry. The eagle must therefore be flown at response-weight (“flying weight”) during the training period and wear a radio-transmitter (see below). From the point of acquisition, staff will take all necessary steps to avoid imprinting and will reject from the program any eagle showing such signs. The falconer will fly the eagle near the facility for at least a month and concentrate on the development of slope-soaring and the ability of the eagle to identify the hack station as a source of food from increasing distances. We predict that, for many birds, such training will progress rapidly to the point of release and passive monitoring.
Choice of hack site. We think there is no justification for requiring that golden eagles be liberated in the regions where they were obtained (see Response to Critic-A, below). Releases of this highly vagile generalist should instead target a safe environment with lots of terrain updrafts. First and foremost, would be to minimize the effects of drought and prey cycles by choosing an area with a mild climate and a variety of perennially available food, especially carrion. Costs permitting, however, there is no reason not to deploy an additional hack site in more xeric circumstances with the realization that there may be years of its unsuitability. Sites should not be near a windfarm, dangerously constructed electrical distribution lines, or within a golden eagle nesting territory. The terrain should be hilly, in open grassland or savanna, providing widespread and substantial, large-scale updrafts for soaring. It is also important that the hack site be associated with a mountain that can be seen and recognized by a soaring eagle from all quadrants.
Design of hack station. We think that affixing carcasses to the ground at the hack site on a continual basis, as typical of “vulture restaurants,” would function well. A VHF telemetry data-logger could record visits by individual eagles, and an attractive perch could be designed to monitor the eagle’s weight. Attendants would be housed in a nearby travel trailer (?) or other domicile for the duration of the hacking procedure.
RELEASE
Tracking equipment. Each eagle will wear a lightweight, ergonomically comfortable, satellite-reporting, gps-equipped transmitter in backpack configuration using 1.3-mm Teflon ribbon held together with cotton embroidery thread over the carina, a procedure designed to allow the transmitter to eventually fall off (Hunt et al. 1992. J Raptor Res. 26:243). Solar powered models soon stop transmitting when inverted, whereas our experiment absolutely requires the recovery of fatalities and timely assessment of the cause of death. A second transmitter minimizes this possibility. Therefore, a standard, battery-powered, two-stage, tail-mounted VHF transmitter with a one-year battery life and a tip-switch mortality-sensor will be applied to a single tail feather as insurance. Attendants will maintain regular contact with eagles by means of standard programable receivers, an omnidirectional fixed-base antenna at the hack site, yagi antennas for searches, and an appropriately equipped airplane, available when needed.
Emergency procedures. It is essential to the success of the project that the eagles remain in contact with the hack site long enough to recognize it as a reliable source of food and so return to it when hungry. Were attendants to lose contact with an eagle before it can be relied upon to feed itself or return for food, then it must be quickly recovered and brought back to the hack-site by a falconer.
An effective way to recover wandering birds without the necessity of having to maintain them at “response-weight” might be to train them to capture themselves within a portable plywood pyramid, each side being 4 or 5 feet in width (at its base). One side would offer a door consisting of a fixed row of “pigeon-bobs” serving as a one-way, one-time entrance, locking after entry and activating a transmitter to immediately alert the falconer. Each pyramid would be well-ventilated and lined with smooth plastic sheeting (Coroplast©) to prevent feather damage. All pyramids would be identical in appearance, all with the same colorful design and perhaps festooned with some odd shapes and even a flag, together rendering the presentation like no other on the planet. Inside the box would be a “bow-perch” draped with a rabbit haunch or something similarly enticing. The sides of the pyramid would be detachable so that several units could be stacked in a pick-up bed, thus deployable in multiple locations.
Another type of emergency might occur with an eagle that becomes so tame that it approaches people when food-stressed. Such eagles (“imprints”), should be removed from the program. Presumably, the falconer would detect such inclinations prior to release and strive not to inadvertently create them.
Determining success. The field portion of the hacking project can be concluded each year when all eagles being hacked are behaving normally in the wider landscape and their visits to the hack station, if any, are infrequent enough to verify their independence.
CRITICAL REVIEW
This draft research plan is a work in progress and needs rigorous criticism of its content in the spirit of refinement or rejection. It is desirable that it work well on its first application, but we should keep in mind the military axiom that “No plan survives first contact with the enemy.” Our approach, even after implementation, should be one of continual adaptive refinement based on experimentation, the measure of success being eagle survival in the wild.
PREPARER’S QUALIFICATIONS
I received my doctorate in Zoology in 1970 from the University of Texas at Austin with a specialty in evolutionary ecology. I offer this document as my expert opinion based on 50 years of experience conducting field studies of raptor populations, including those of three endangered species. My knowledge of eagles includes three multi-year field studies of bald eagles, and a seven-year study of a resident population of golden eagles in California (1994-2000), followed by periodic field evaluation through 2014. The study included a sample of 257 individuals with VHF transmitters, each with a 4-year duty cycle regularly monitored by airplane and often from the ground. The latter sample included 103 individuals tagged as fledglings, the subsequent survival of which being subject of the current document. Before retiring, I spent 14 years as Senior Scientist for The Peregrine Fund (2001–2015) during which time I oversaw the science aspects of its California Condor and Aplomado Falcon restoration programs.
I have written this document as my best attempt to find a workable solution to a problem with a substantial record of failure. I have no interest in its implementation other than a desire to help devise a way for young individuals of this remarkable species to reenter the wild. I am aware that annual numbers of candidate eagles are very small relative to the wild population, e.g., ten individuals in 2021. For example, my roughly 5,000 km2 study area in the northern portion of the Diablo Mountains alone contained an estimated 280 pairs (Wiens et al. 2015), suggesting a minimum of 1,000 pairs in the state, those roughly 2000 adults being only a fraction of the overall indigenous population. The project I have outlined offers a proactive methodology that, if properly refined and implemented, may successfully restore young golden eagles to the wild. And it is more than kindness. Developing a reliable way to rewild them might well be valuable should the species someday decline.
ON RELEASING EAGLES CLOSE TO THEIR NATAL TERRITORIES
Concerns from some members of the California Department of Fish and Wildlife (referred to as the department) having read and commented on an earlier draft of this plan, is opposed to the idea that multiple fledgling/juvenile golden eagles from a variety of nests might all be released from one or two hack-sites. The suggestions from the department are that each eagle should be released in or near (within 10 miles of) its natal territory. Four reasons are given, including:
Reason 1: the need to “…preserve the genetic make-up of the population” and preserve “…local adaptation to habitat, weather, prey, and hazards.”
My response: the department appears to believe that, if golden eagles from two different localities (i.e, greater than 16 km apart) were to form a pair bond, mate with each other, and produce young, the genes of their offspring and possible descendants thereof, may disrupt the coadapted integrity of that local portion of the gene pool where said reproduction takes place. I maintain that, despite a general tendency to philopatry, studies of the average distances golden eagles move before pairing have been shown to be much greater in this highly vagile species. Millsap et al. (2014), for example, reported the following: “Bayesian analysis of banding data suggest a median natal dispersal distance of … 46.4 (HDI = 36.0–55.2) km for Golden Eagles. In an analysis of radio telemetric data, Murphy et al. (2019) found that “…sixteen golden eagles (6 males, 10 females) tagged with satellite transmitters as ∼8-wk-old nestlings in the southwestern United States during 2010–2013 dispersed a mean of 55.3 km (SD = 29.7, median = 64.5)… between their natal nest sites and nests where they first bred.” And consider how such degrees of natal dispersal compound and magnify gene flow each year across the landscape. For example, a maturing female settling in an area 50 km from her natal area might well pair with a male originating from an area comparably distant, but from the opposite direction. As for the disruption of local adaptations, I doubt that any could develop at such a scale, and particularly in landscapes without physical impediments to gene flow. To assess my qualifications for addressing this issue, see Hunt and Selander (1973).
Reason 2: …minimizing “…the chances of introducing pathogens to new areas.
My response: The normal and considerable range of movement of juvenile golden eagles after leaving their natal territories indeed extends the potential reach of whatever pathogens they might carry far and wide. Pathogens can also be acquired at rehabilitation facilities, but the CERE document includes a section on screening for these and others. Golden eagles are asocial but are nevertheless exposed to a variety of pathogens and parasites in preying upon such a large spectrum of vertebrates. One might therefore expect an innately competent immune system, particularly among raptors with diverse diets like golden eagles. I am not aware of any literature on epidemics in wild raptors. External parasite infections occur in golden eagles, including Knemidocoptes mites (Mete et al. 2014), Mexican chicken bugs (Haematosiphon inodora; Morales-Yañez and Rodríguez-Estrella 2019), and others, but I know of no evidence of eagle-to-eagle transmission. It is conceivable that an eagle could acquire knemidocoptic mange from a carcass that another eagle had fed upon, but avian prey, e.g, wild turkey, is a far more likely source. A virulent intestinal protozoan Sarcocystis falcatula reported in a golden eagle was to attributed to opossum prey (Wünschmann et al. 2010). Golden eagles are susceptible to viruses like Avian pox, West Nile virus, and falcon herpesvirus, but again, modes by which wild eagles might infect one another seem doubtful. A review article on raptor diseases by Morishita et al. (1997) lists a number of pathogens and parasites, but provides no insight on modes of intraspecies infection.
Reason 3: …reducing “…the chances of intraspecies conflict“
My response: I cannot address the foregoing because I do not understand its meaning. It appears that the department discounts the natural extent to which free-ranging golden eagles travel the landscape.
Reason 4: “…disrupting the already-established social structure“
My response: the department is essentially declaring that, if any particular eagle is attracted to an area further than 10 miles from its natal territory, it can disrupt “…the already-established social structure.” While remotely conceivable, such events must happen all the time as a result of normal dispersal. But to what consequence? Territorial conflict, for example, is normal in golden eagle populations. In any case, the potential negative influence of so small a number of released juveniles, not to mention their attrition prior to adulthood, requires an unreasonable stretch of the imagination.
LITERATURE CITED
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