Silent Spring

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United States Fish and Wildlife Service
Special Wildlife Survey
1996
==Abstract==
During the years 1992-1996, it was observed that the local birds in and around Faulkner Wildlife Refuge in southern Missouri had been unusually quiet. In 1996, the US Fish and Wildlife Service conducted a qualitative wildlife survey of the refuge. The goal of the survey was to determine any probable causes of this phenomenon, and what, if any, steps should be taken in response.
==Summary==
Faulkner Wildlife Refuge is an 8600-acre FWS property in Ozark County, Missouri, that is closed to the public. The refuge consists of temperate woodland and savanna and is bordered by a state park and a state conservation area. The refuge has no major rivers, but it does have several creeks, springs, and lakes as well as two limestone caves.
In 1992, FWS employees working at Faulkner Wildlife Refuge reported that the population of all types of birds at the area seemed to be vocalizing far less often than normal. Throughout the year, they reported hearing far less birdsong than during previous years, including the vocalizations of nocturnal birds such as owls. These reports continued until 1996, when a study was authorized to address this information.
The objectives of the wildlife survey were as follows:
1. To determine if birdsong in the area had indeed diminished, or if the employees were mistaken.
2. If there was in fact less birdsong, to determine if the cause was fewer birds in the area or less vocalization per individual specimen.
3. To determine the specific cause of 2, if 1 was found positive.
4. To recommend corrective action, if appropriate.
The wildlife survey was conducted from February of 1996 until November of 1996.
==Methodology==
This wildlife survey was primarily qualitative, but some quantitative methods were included. Stationary microphones and cameras were placed in various points around the Refuge, and field teams were sent out. Sound-file stimulation was also used, particularly for owl species.
The first objective was to determine if the local bird populations had decreased. After a few weeks, it was determined that bird populations in and around the Refuge had remained relatively stable compared to past surveys. The next objective was to determine if bird vocalizations had diminished.
No wildlife survey focused on vocalizations had previously been conducted at Faulkner Refuge, so vocalizations at the site were compared with vocalizations at neighboring areas to establish a baseline. Using a combination of microphones and subjective determinations, it was determined that birdsong in the area was indeed abnormally absent.
One notable exception to this was an apparent increase in vocalizations from corvids, including crows, ravens, and blue jays.
Owl vocalizations were also less frequent, though this may actually have been related to a decrease in the local owl population. Sound-file stimulation, which is usually an easy way to locate owls (especially barred owls [Strix varia] and great horned owls [Bubo virginianus], both of which are common in Missouri) produced infrequent responses. Combined use of microphones, cameras, night-vision, and FLIRs revealed the population density of local owl species to be lower than normal at Faulkner Refuge.
Several causes for the reduction in birdsong were speculated. Air quality monitors were set up to discover if the cause could be a chemical leak. Additionally, live specimens from several avian species were captured to study their behavior.
==Results==
The survey concluded that overall bird vocalizations were less frequent than normal at Faulkner Wildlife Refuge, with the exception of corvids. With the exception of owls, this decrease did not appear to be related to any changes in local bird populations.
Several factors were identified as possible causes of this phenomenon. Poachers may have been a contributing factor. During the survey, dozens of illegal tree stands were found on the Refuge, and one poacher was arrested. Another contributing factor may have been increased construction and development near the refuge.
While the results of this survey were inconclusive, it was determined that there was no need for immediate action.
==Addendum==
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Classifying Authority:Justin Kenny, Deputy Secretary of the Interior
==Abstract==
During the years 1992-1996, it was observed that the local birds in and around Faulkner Wildlife Refuge in southern Missouri had been almost completely silent at all times. In 1996, the US Fish and Wildlife Service conducted a qualitative wildlife survey of the refuge. The goal of the survey was to determine any probable causes of this phenomenon, and what, if any, steps should be taken in response.
==Summary==
In 1992, FWS employees working at Faulkner Wildlife Refuge in Ozark County, Missouri reported that the population of all types of birds at the area seemed to have ceased vocalizing almost completely. Throughout the year, they reported hearing almost no birdsong, including the vocalizations of nocturnal birds such as owls. These reports continued until 1996, when a study was authorized to address this information.
The objectives of this survey were as follows:
1. To determine if this silence was due to a sudden depopulation of birds, or if individual birds had simply ceased vocalizing.
2. To determine the cause of 1.
3. To recommend action.
The wildlife survey was to be conducted from February of 1996 until November of 1996, but was terminated in August of 1996.
==Methodology==
This wildlife survey was primarily qualitative, but some quantitative methods were included. Stationary microphones and cameras were placed in various points around the Refuge, and field teams were sent out. Sound-file stimulation was also used, particularly for owl species.
The first objective was to determine if the local bird populations had decreased. After a few weeks, it was determined that bird populations in and around the Refuge had remained relatively stable compared to past surveys. The next objective was to determine why the birds had stopped vocalizing (one notable exception to this absence was an apparent increase in vocalizations by corvids, such as crows, ravens, and bluejays).
Owl vocalizations were also less frequent, though this may actually have been related to a decrease in the local owl population. Sound-file stimulation, which is usually an easy way to locate owls (especially barred owls and great horned owls, both of which are common in Missouri) produced infrequent responses. Combined use of microphones, cameras, night-visions, and FLIRs revealed the population density of local owl species to be lower than normal at Faulkner Refuge.
Several reasons for these phenomena were speculated. Air quality monitors were set up to discover if the cause could be a chemical leak. Additionally, live specimens from several avian species were captured to study their behavior.
==Results==
It was determined that the cause of silence among birds of different species at Faulkner Wildlife Refuge was a constant fear of predation. This is probably also the explanation for the increased noise of corvids, which typically vocalize in response to the presence of predators. This explanation had previously been ruled out, since silence in response to predators is not known to affect such large areas for such long periods of time. Following this realization, the research team then set about attempting to determine what actual or perceived predators were causing this phenomenon.
Faulkner Refuge is home to several birds of prey species, which are the main predators of birds in Missouri. Common diurnal predators include the red-tailed hawk (Buteo jamaicensis), the red-shouldered hawk (Buteo lineatus), and the Cooper's hawk (Accipiter cooperii). Nocturnal predators include the great horned owl. Mammalian predators include red foxes (Vulpes vulpes) and coyotes (Canis latrans). In recent years, black bears (Ursus americanus) have also been sighted in Ozark County, due to Arkansas's black bear reintroduction program. While black bears are not known to take songbirds with any significant frequency, birds could still perceive them as a threat.
However, none of these predators are present in sufficient quantities to cause such total and lasting silence among birds throughout the refuge. However, the remains of several larger animals, such as deer, were found, scattered in such a way to suggest having been eaten despite there being no natural predators in Missouri large enough to take such species.
While there was clear evidence of poaching, this should not have caused such widespread silence. It was speculated that aircraft may be violating the no-fly zone above the refuge and frightening birds, but this was found not to be the case.
Use of cameras, microphones, and night vision was increased to find any new predators in the area.
A lead was produced during a routine sound-file owl survey. Two researchers were using audio files to draw responses from barred owls and great horned owls. They received a response to a great horned owl call from an unknown animal, which sounded like a call from some bird of prey. As the call grew closer and louder in response to the owl calls, they recorded it and started playing it back in an attempt to draw in the unknown bird.
The researchers did not see the bird, but reported that it seemed to land in the boughs of a nearby red oak. The tree was over 80 feet tall, and the researchers reported that many branches were moved at once near its canopy, indicating a bird of large size. Additionally, several large branches fell from the canopy at the same time.
They were not able to spot the bird using flashlights, but one reported catching a glimpse of it in the dark as it flew off. This researcher said the bird was very large, at least the size of a bald eagle (Halieetus leucocephalus), which is not a nocturnal species (although bald eagles have been known to be infrequently active at night near their nests). According to the researcher, the bird may have been even larger, even up to as big as a California Condor, or there may have even been two birds.
The recordings of the bird's vocalizations could not be matched with any known bird species in the world. The calls had a shrill and piercing quality like those of a red-tailed hawk (and unlike those of a great horned owl, making it confusing that the bird responded to the owl calls) but were louder and longer. Speakers playing the recordings were set up around the Refuge with cameras.
The cameras never captured footage of the unknown bird, but did pick up response calls to the audio files. During this time, corvids at the site also stopped vocalizing. 
Responses to the recorded calls were more common at night. One night, a camera recorded footage of a great horned owl giving territorial calls in response to the unknown bird. The other bird could not be seen, but the volume of its calls suggested close proximity to the camera. As the calls grew closer, the owl began to retreat, until flying off entirely. It should be noted that great horned owls are notoriously aggressive and territorial and known for killing and eating other large birds of prey, and few bird species are able to take them without the advantage of numbers.
During the summer, calls seemed to increase during thunderstorms. During especially violent night storms, the calls were so frequent that it would have been difficult for any hearing-capable person on the premises at the time to fail to notice them.
After several weeks, responses to the recorded calls lessened. This tends to happen with over-use of audio files to observe birds of prey; eventually, a raptor realizes there is no real danger and stops responding to the perceived territory invasion. In response, new files were added to the rotation, sourced from the bird's responses to the original files. This caused the bird to begin responding more, but it was still never seen on-camera or in-person.
In one experiment, recorded calls from this bird were played back in the presence of captive bird specimens. It was found that the calls triggered an intense fight-or-flight response from each bird's amygdaloid complex, resulting in not simply the loss of desire to vocalize, but a virtual neurological impairment preventing birds from being able to vocalize, with parts of the avian brain related to vocalization possibly shutting down for hours at a time; the longer the exposure to the recordings, the longer this effect.
One day in July, the fresh carcass of an adult female deer was found hanging in the canopy of a white oak tree that was over one hundred feet high. The carcass appeared partially eaten. There is no predator naturally occurring in Missouri known to cache such large kills in trees with the possible exception of mountain lions (Puma concolor), which have been sighted on rare occasion in the state.
In an attempt to record footage of the unknown bird, researchers constructed an artificial nest for it in June 1996. The nest was situated in a tall oak tree. Multiple cameras were set on the nest, but they went offline during a nighttime thunderstorm. After the cameras came back online, the artificial nest showed severe damage. Close inspection revealed that the damage was not caused by the storm; the nest seemed chewed in many places.
During the survey, several trespassers were arrested at the refuge. While most were poachers, one man who was arrested was a self-proclaimed cryptid-hunter who claimed to be tracking a bird of enormous size, possibly rivaling that of a small aircraft.
==Memo==
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Classifying Authority:Anthony J. Peters, Deputy Secretary of Defense
RE:"Project Thunderbird"
Per joint agreement of the Department of Defense and the Department of the Interior, this wildlife survey has been terminated. All researchers have been sworn to secrecy regarding portions of the survey. Following DoD intervention, the phenomenon of avian silence has been rectified.
-Maj. Gen. Matthew Harris, Commander, United States Air Force Advanced Bioweapons Systems Research Group, Nellis Air Force Base, Nevada.
==Wildlife Image Credits==
1. Bill Thompson
https://www.flickr.com/photos/43322816@N08/6789310741/
Public Domain
2. Courtney Celley
https://www.flickr.com/photos/49208525@N08/41971112452/
Public Domain
3. Frank Miles
https://www.flickr.com/photos/43322816@N08/5277661543/
Public Domain
4. Ray Fetherman
https://www.flickr.com/photos/51986662@N05/41437064424/
CC-BY 2.0
5. Tom Koener
https://www.flickr.com/photos/51986662@N05/14499857199/
CC-BY 2.0
6. Tracey Rock
https://www.flickr.com/photos/41464593@N02/26905320846/
Public Domain
7. Photographer unknown
https://www.flickr.com/photos/54430347@N04/5057204597/
CC-BY 2.0
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